Patterns of Inheritance

Patterns of Inheritance
Chapter 9 Patterns of Inheritance © 2016 Pearson Education, Inc.

In this chapter we will learn
Chapter 9: Concepts In this chapter we will learn how genetic traits are passed from generation to generation and how the behavior of the chromosomes account for these rules Study different types of inheritance pattern Predict the ratio of resulting offspring for a particular trait Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet, Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question. How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP  pp and (b) Pp  pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. Many students have trouble with basic statistics. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 8. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 9. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. 10. As a simple test of comprehension, ask students to explain why lethal alleles are not eliminated from a population. Several possibilities exist: The lethal allele might be recessive, persisting in the population due to the survival of carriers, or the lethal allele might be dominant, but is not expressed until after the age of reproduction.

Why Genetics Matters Figure 9.0-1 Figure Why genetics matters

Biology and Society: Our Longest-Running Genetic Experiment
People have selected and mated dogs with preferred traits for more than 15,000 years. Over thousands of years, such genetic tinkering has led to the incredible variety of body types and behaviors in dogs today. The biological principles underlying genetics have only recently been understood. © 2016 Pearson Education, Inc. 4

Chapter Thread: Dog Breeding
Figure 9.0-2 Figure Dog breeding: breeding a best friend

Genetics and Heredity Heredity is the transmission of traits from one generation to the next. Genetics is the scientific study of heredity. Gregor Mendel worked ( ) “Father of Genetics ” monk, worked in the abbey garden in the 1860s and argued that parents pass on to their offspring discrete genes (which he termed “heritable factors”), genes are responsible for inherited traits, and genes retain their individual identities generation after generation, no matter how they are mixed up or temporarily masked. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 6

The structure of a pea flower
Mendel studied garden peas because they Petal are easy to grow come in many readily distinguishable varieties of traits are easily manipulated can self-fertilize normally Stamen (makes sperm- producing pollen) Carpel (produces eggs) Figure 9.2 Figure 9.2 The structure of a pea flower 7

In an Abbey Garden Perhaps the most important advantage of pea plants as an experimental model was that Mendel could strictly control their reproduction. The petals of a pea flower almost completely enclose the egg-producing organ (the carpel) and the sperm- producing organs (the stamens). Consequently, pea plants usually self-fertilize because sperm-carrying pollen grains released from the stamens land on the tip of the egg-containing carpel of the same flower. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 8

In an Abbey Garden Each of the characters Mendel studied occurred in two distinct forms. A character is a heritable feature that varies among individuals (flower color). A trait is a variant of a character (purple and white) Each of the characters Mendel studied occurred in two distinct traits. Mendel created purebred (true-breeding ) varieties of plants and crossed two different purebred varieties. Mendel worked on garden peas for his inheritance studies because they came in many readily distinguishable varieties – one variety produced purple flower and other white flower. A heritable feature here is flower color True breeding: varieties for which self-fertilization produced offspring all identical to the parent. What would happen when two different true-breeding varieties are cross-fertilized?

Mendel’s work ? Bred pea plants cross-pollinate true breeding parents
Pollen transferred from white flower to stigma of purple flower Bred pea plants cross-pollinate true breeding parents raised seed & then observed traits allowed offspring to self-pollinate & observed next generation all purple flowers result self-pollinate ? P = parents F = filial generation

Mendel's technique for cross-fertilizing pea plants
When Mendel wanted to fertilize one plant with pollen from a different plant, he pollinated the plants by hand and was always sure of the parentage of his new plants. Mendel created purebred varieties of plants and crossed two different purebred varieties Removed stamens from purple flower. Stamens Parents (P) Transferred pollen from stamens of white flower to carpel of purple flower. Carpel Figure 9.3-s1 Figure 9.3-s1 Mendel’s technique for cross-fertilizing pea plants (step 1)

Parents (P) Removed stamens from purple flower. Stamens
Transferred pollen from stamens of white flower to carpel of purple flower. Carpel Pollinated carpel matured into pod. Figure 9.3-s2 Figure 9.3-s2 Mendel’s technique for cross-fertilizing pea plants (step 2)

Parents (P) Removed stamens from purple flower. Stamens
Transferred pollen from stamens of white flower to carpel of purple flower. Carpel Pollinated carpel matured into pod. Planted seeds from pod. Offspring (F1) Figure 9.3-s3 Figure 9.3-s3 Mendel’s technique for cross-fertilizing pea plants (step 3)

In an Abbey Garden Hybrids are the offspring of two different purebred varieties. The cross-fertilization itself is referred to as a genetic cross. The parental plants are the P generation. Their hybrid offspring are the F1 generation. A cross of the F1 plants forms the F2 generation. Pure breeding or True breeding - varieties that produce identical offspring upon self fertilization Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 14

Mendel’s Law of Segregation
Mendel performed many experiments in which he tracked the inheritance of characters, such as flower color, that occur as two alternative traits. The results led him to formulate several hypotheses about inheritance. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity.

The seven characters of pea plants studied by Mendel
Dominant Recessive Dominant Recessive Flower color Pod shape Inflated Constricted Purple White Pod color Flower position Green Yellow Stem length Axial Terminal Seed color Yellow Green Tall Dwarf Seed shape Round Wrinkled Figure 9.4 Figure 9.4 The seven characters of pea plants studied by Mendel

Monohybrid Crosses Mendel performed a monohybrid cross between purebred parent plants that differ in only one character (Flower color) and found that the F1 plants all had purple flowers. P Generation (purebred parents) Purple flowers White flowers Figure 9.5-s1 Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 17

P Generation (purebred parents) Purple flowers White flowers All plants have purple flowers F1 Generation 100% (1st filial generation) Was the factor responsible for inheritance of white flowers now lost as a result of the cross? By mating the F1 plants with each other, Mendel found the answer to this question to be no. Figure 9.5-s2 Figure 9.5-s2 Mendel’s cross tracking one character (flower color) (step 2)

(1st filial generation)
P Generation (purebred parents) Purple flowers White flowers F1 Generation All plants have purple flowers (1st filial generation) Fertilization among F1 plants (F1 × F1) F2 Generation 2nd filial generation) 3 4 1 4 of plants have purple flowers of plants have white flowers Figure 9.5-s3 Figure 9.5-s3 Mendel’s cross tracking one character (flower color) (step 3)

Monohybrid Crosses Mendel figured out that the gene for white flowers did not disappear in the F1 plants but was somehow hidden or masked when the purple-flower factor was present. He also deduced that the F1 plants must have carried two factors for the flower-color character, one for purple and one for white. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 20

Monohybrid Crosses: What did Mendel’s findings mean?
From these results and others, Mendel developed four hypotheses: There are alternative versions of genes that account for variations in inherited characters. The alternative versions of genes are called alleles. For each inherited character, an organism inherits two alleles, one from each parent. An organism that has two identical alleles for a gene is said to be homozygous for that gene. An organism that has two different alleles for a gene is said to be heterozygous for that gene. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 21

Monohybrid Crosses: What did Mendel’s findings mean?
If the two alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele, and the other has no noticeable effect on the organism’s appearance and is called the recessive allele. Some traits mask others purple & white flower colors are separate traits that do not blend purple x white ≠ light purple purple masked white Geneticists use uppercase italic letters (such as P) to represent dominant alleles and lowercase italic letters (such as p) to represent recessive alleles. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 22

Monohybrid Crosses Gametes (sperm or egg) carries only one allele for each inherited character because the two alleles for a character segregate (separate) from each other during the production of gametes. This statement is called the law of segregation. When sperm and egg unite at fertilization, each contributes its alleles, restoring the paired condition in the offspring. Figure 9.6 illustrates Mendel’s law of segregation, which explains the inheritance pattern shown in Figure 9.5. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 23

Monohybrid Crosses Do Mendel’s hypotheses account for the 3:1 ratio he observed in the F2 generation? A Punnett square highlights the four possible combinations of gametes and offspring that result from each cross. each square represents an equally probable product of fertilization Geneticists distinguish between an organism’s physical appearance and its genetic makeup. An organism’s physical traits are its phenotype (3 : 1 ratio) An organism’s genetic makeup is its genotype (list of the organism’ genes and their exact DNA position) (1:2:1 ratio) Hypothesis: When alleles segregate during gamete formation in the F1 plants, half the gamete will receive P allele and other half p allele. During pollination among F1 plants, they reunite randomly. Mendel observed that all the 7 characters he studied had the same inheritance pattern: one parental trait disappeared in the F1 only to reappear in 1/4th of F2 population – this explains the mechanism involved in law of segregation

The law of segregation Reginald Punnett (1875 - 1967)
P Generation Genetic makeup (alleles) The law of segregation Purple flowers 2n PP White flowers pp 2n Alleles carried by parents MEIOSIS Gametes All P n All p n F1 Generation (hybrids) Purple flowers All Pp 2n Alleles segregate 2 1 2 1 Gametes P p F2 Generation (hybrids) Sperm from F1 plant Reginald Punnett ( ) An explanation of Monohybrid results using a Punnett square P p P Eggs from F1 plant PP Pp p Pp pp Phenotypic ratio 3 purple : 1 white Genotypic ratio 1 PP : 2 Pp : 1 pp Figure 9.6 The law of segregation 25

Practice A true-breeding plant that produces yellow seeds is crossed with a true-breeding plant that produces green seeds. The seeds of all of the offspring are yellow. Why? A) The yellow allele is recessive to the green allele. B) All of the offspring are homozygous yellow. C) The yellow allele is dominant to the green allele. D) The alleles are codominant. E) Yellow is an easier color to produce.

Genetic Alleles and Homologous Chromosomes
A gene locus is a specific location of a gene along a chromosome Homologous chromosomes Have genes at specific loci. have alleles (alternate versions) of a gene at the same locus. two chromosomes may bear either identical alleles or different ones at any one locus Homologous chromosomes P Genotype: Gene loci a aa b B Dominant allele Recessive Bb PP Homozygous for the dominant allele recessive allele Heterozygous with one dominant and one recessive allele Recall ch. 8 that every diploid individual has chromosomes in homologous pair. The matching colors of corresponding loci highlight the fact that homologous chromosomes carry alleles for the same genes at the same positions along their lengths

Practice An individual who is homozygous _____________.
A) expresses the dominant trait B) carries two different alleles for a gene C) is a carrier of a genetic disorder D) carries two copies of the same allele for a gene E) expresses the recessive trait

Practice An allele is _________________________. a type of chromosome
the dominant form of a gene a variety of pea plant used by Mendel an alternative version of a gene the recessive form of a gene

Genotype vs. phenotype Difference between how an organism “looks” & its genetics phenotype - description of an organism’s trait genotype - description of an organism’s genetic makeup F1 P X purple white all purple Explain Mendel’s results using …dominant & recessive …phenotype & genotype

PP pp Pp x Making crosses Can represent alleles as letters X
flower color alleles  P or p true-breeding purple-flower peas  PP true-breeding white-flower peas  pp F1 P X purple white all purple PP x pp Pp

phenotype & genotype can have different ratios
Punnett squares Aaaaah, phenotype & genotype can have different ratios Pp x Pp 1st generation (hybrids) % genotype % phenotype P p male / sperm PP 25% 75% Pp 50% P p female / eggs PP Pp Pp 25% 25% Pp pp pp 1:2:1 3:1

The Rules of Probability
Mendel’s strong background in mathematics helped him understand patterns of inheritance. For instance, he understood that genetic crosses obey the rules of probability—the same rules that apply when tossing coins, rolling dice, or drawing cards. For each toss of the coin, the probability of heads is ½ every time – the outcome of any given toss is unaffected by what has happened on previous attempts (toss is an independent event). If 2 coins are tossed simultaneously, the outcome for each is independent, unaffected by the other coin. What is the chance that both coins land heads-up ? the probability of dual event is the product of the separate probabilities of the independent events – ½ x ½ = ¼. The rule of multiplication states that the probability of a compound event is the product of the separate probabilities of the independent events. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 33

Segregation of alleles and fertilization as chance events
F1 Genotypes Bb female × Bb male Formation of eggs Formation of sperm F2 Genotypes Formation of gametes is like flipping a coin (Head or tail) Chance of having a head or a tail is ½ The rule of multiplication Multiply from the outside of the table to the inside BB= 25%; bb= 25%; Bb=50% The rule of addition for heterozygote (Bb) Inside the table you add probabilities 1/4 + 1/4= 1/2 Male gametes 1 2 1 2 B b B B B b 1 2 B 1 4 1 4 ( 1 2 1 2 ) Female gametes × 1 2 b b b B b 1 4 1 4 Figure 9.11 Figure 9.11 Segregation of alleles and fertilization as chance events

Genotypes Homozygous = same alleles = PP, pp
Heterozygous = different alleles = Pp homozygous dominant homozygous recessive

Phenotype vs. genotype PP purple Pp purple
2 organisms can have the same phenotype but have different genotypes PP homozygous dominant purple Pp heterozygous purple

Mendel’s Law of Independent Assortment
A dihybrid cross is the mating of parental varieties differing in two characters. Mendel studied inheritance of pattern of two characters one at a time as in a monohybrid ratio seed color (yellow is dominant to green) seed shape (round shape is dominant to wrinkled seed) What would result from a dihybrid cross? Two hypotheses are possible Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 37

If the genes for the two characters were inherited together, then the F1 hybrids would produce only the same two kinds of gametes that they received from their parents, and the F2 generation would show a 3:1 phenotypic ratio. Independent assortment: If, however, the two seed characters sorted independently, then the F1 generation would produce four gamete genotypes (RY, rY, Ry, and ry) in equal quantities, and the F2 generation would have nine different genotypes producing four different phenotypes in a ratio of 9:3:3:1. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 38

Testing alternative hypotheses for gene assortment in a dihybrid cross
(a) Hypothesis: Dependent assortment (b) Hypothesis: Independent assortment P Generation RRYY rryy RRYY rryy Gametes RY ry Gametes RY ry F1 Generation RrYy RrYy Were the two characters transmitted to offspring as a package (capital letter staying together), or was each character inherited independently of each other (consider all possible combinations )? Figure 9.8-1 Figure Testing alternative hypotheses for gene assortment in a dihybrid cross (part 1: hypothesis)

Predicted results Contradict hypothesis (not actually seen)
(a) Hypothesis: Dependent assortment (b) Hypothesis: Independent assortment P Generation RRYY rryy RRYY rryy Gametes Gametes RY ry RY ry F1 Generation RrYy RrYy F2 Generation Sperm 1 4 RY 1 4 rY 1 4 Ry 1 4 ry Sperm 1 2 RY 1 2 ry 1 4 RY RRYY RrYY RRYy RrYy 1 2 RY 1 4 rY Yellow round Eggs 9 16 RrYY rrYY RrYy rrYy Eggs 1 2 ry 1 4 Ry 3 16 Green round RRYy RrYy RRyy Rryy 1 4 Yellow wrinkled ry 3 16 RrYy rrYy Rryy rryy Predicted results Contradict hypothesis (not actually seen) Actual results (support hypothesis) Green wrinkled 1 16 Figure 9.8 Figure 9.8 Testing alternative hypotheses for gene assortment in a dihybrid cross a) Dependent assortment: leads to prediction that F2 plants will have seeds that match the parents, either RY or GW b) Independent assortment: leads to prediction that F2 plants will have 4 different seed phenotypes

Figure 9.8-4 Figure Testing alternative hypotheses for gene assortment in a dihybrid cross (part 4: peas photo)

Mendel’s dihybrid cross supported the hypothesis that each pair of alleles segregates independently of the other pairs during gamete formation. Thus, the inheritance of one character has no effect on the inheritance of another. This is called Mendel’s law of independent assortment. Independent assortment is seen in the inheritance of two characters in Labrador retrievers. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 42

Practice In a certain plant, the alleles A, B, and C are dominant to the alleles a, b, and c. A plant with the genotype AABbcc will have the same phenotype as the plant with the genotype _____. ( Heritable Variation and Patterns of Inheritance) AAbbcc aabbcc AaBBcc AABBCc none of the above

Practice Assume tall (T) is dominant to dwarf (t). If a homozygous dominant individual is crossed with a homozygous dwarf, the offspring will _____. ( Heritable Variation and Patterns of Inheritance) all be intermediate in height all be tall 1/2 tall and 1/2 dwarf be 3/4 tall and 1/4 dwarf all be short

Independent assortment of genes in Labrador retrievers
Blind dog Blind dog Phenotypes Black coat, normal vision Black coat, blind (PRA) Chocolate coat, normal vision Chocolate coat, blind (PRA) Genotypes B_N_ B_nn bbN_ bbnn (a) Possible phenotypes of Labrador retrievers Mating of double heterozygotes (black coat, normal vision) BbNn BbNn Blind Blind Phenotypic ratio of offspring 9 black coat, normal vision 3 black coat, blind (PRA) 3 chocolate coat, normal vision 1 chocolate coat, blind (PRA) (b) A Labrador dihybrid cross Figure 9.9 Independent assortment of genes in Labrador retrievers

So how do you figure out the genotype?
Dominant phenotypes It is not possible to determine the genotype of an organism with a dominant phenotype by looking at it. So how do you figure out the genotype? PP? Pp?

Using a Testcross to Determine an Unknown Genotype
A testcross is a mating between an individual of dominant phenotype (but unknown genotype) and a homozygous recessive individual. x is it PP or Pp? pp Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 47

Test cross x x PP pp Pp pp p p p p Pp Pp Pp Pp P P P Pp Pp p pp pp
100% 50%:50% 1:1 P Pp Pp p pp pp

A Labrador retriever testcross
Two possible genotypes for the black dog: B_ bb Testcross Genotypes Bb B b or BB Gametes Offspring All black 1 black : 1 chocolate You can tell the genotype of Labrador with a chocolate coat is recessive. What if you have black Lab? The genotype may be either BB or Bb

Practice A testcross is applied to determine
whether the dominance of the trait is incomplete. None of these whether the parent displaying the dominant phenotype is a dominant heterozygote or dominant homozygote. the genotype of the parent that displays the recessive phenotype. whether a trait is dominant or recessive.

Practice Suppose we have a pea plant with purple flowers, determined by the dominant allele P. How might you determine whether the plant is homozygous (PP) or heterozygous (Pp)? ( Heritable Variation and Patterns of Inheritance) Cross the plant with a true-breeding purple plant. Attempt to cross the plant with a pink-flowered snapdragon. Examine the plant's chromosomes with a microscope. Perform a testcross: Cross the plant with a known heterozygote, Pp. Perform a testcross where you cross the plant with a white plant, which must be homozygous recessive, or pp.

Examples of inherited traits controlled by single gene
Family Pedigrees Mendel’s principles apply to the inheritance of many human traits. Freckles Widow’s peak Free earlobe No freckles Straight hairline Attached earlobe RECESSIVE TRAITS DOMINANT TRAITS Dominant traits are not necessarily normal or more common Wild-type traits are those seen most often in nature and not necessarily specified by dominant alleles Examples of inherited traits controlled by single gene Examples of inherited traits in humans thought to be controlled by a single gene.

Family Pedigrees How can human genetics be studied? Geneticists
analyze the results of matings that have already occurred and assemble this information into a family tree, called a pedigree. A family pedigree can deduce which trait is recessive can also deduce the genotype of most of the people in pedigree shows the history of a trait in a family and allows geneticists to analyze human traits. Mendel’s laws enable us to deduce the genotypes for most of the people in the pedigree. 53

First generation (grandparents) Aaron Ff Betty Ff Cletus ff Debbie Ff
Second generation (parents, aunts, and uncles) Evelyn FF or Ff Fred ff Gabe ff Hal Ff Ina Ff Julia ff Third generation (brother and sister) Kevin ff Lisa FF or Ff Female Male Female Male Attached Free Figure Figure A family pedigree showing inheritance of free versus attached earlobes (part 1: pedigree) Figure 9.13 A family pedigree showing inheritance of free versus attached earlobes How do you know that a particular trait is inherited? Test crosses can be performed with plants but the researcher can not control the mating of their subjects in humans. Instead, he can collect as much information possible about a family’s history for that trait and assemble this information into a family tree – pedigree. This figure shows a pedigree tracing the incidence of free vs. attached ear lobe Explain rounds and squares, F (dominant) and f By applying Mendel’s law, we can deduce attached lobe is recessive because that is the only way that one of the 3rd generation children can have attached earlobes when both the parents did not. We can also deduce the genotype of most of the people in the pedigree

Human Disorders Controlled by a Single Gene
Many human traits show simple inheritance patterns and are controlled by single genes on autosomes (not the sex chromosomes X or Y )

Recessive Disorders Most human genetic disorders are recessive.
Individuals who have the recessive allele but appear normal are carriers of the disorder. Using Mendel’s laws, we can predict the fraction of affected offspring that is likely to result from a marriage between two carriers. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 56

Parents Hearing Dd Hearing Dd Offspring D d Sperm Dd Hearing (carrier) Suppose, 2 heterozygotes carriers have a child. What is the probability that the child will be deaf? DD Hearing D Eggs Dd Hearing (carrier) dd Deaf d Figure 9.14 Figure 9.14 Predicted offspring when both parents are carriers for a recessive disorder

Recessive Disorders Cystic fibrosis is
the most common lethal genetic disease in the USA and occurs when mucus build up in many organs caused by a recessive allele carried by about 1 in 31 Americans. Prolonged geographic isolation of certain populations can lead to inbreeding, the mating of close relatives. It is relatively unlikely that two carriers of a rare allele will meet and mate. Inbreeding increases the chance of offspring that are homozygous for a harmful recessive trait. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 58

Achondroplasia, a dominant trait
Dominant Disorders A number of human disorders are caused by dominant alleles. Achondroplasia is a form of dwarfism. The homozygous dominant genotype (AA) causes death of the embryo. Only heterozygotes (Aa), individuals with a single copy of the defective allele, have this disorder. This also means that a person with achondroplasia has a 50% chance of passing the condition on to any children. Achondroplasia, a dominant trait © 2016 Pearson Education, Inc. Some dominant disorders are non-lethal : extra fingers or toes Achondroplasia: head and torso develop normally, arms and legs will be short Huntington’s disease: any child born to a parent with this allele has 50% chance of inheriting this allele and the disorder 59

Parents Normal (no achondroplasia) dd Dwarf (achondroplasia) Dd d d
Sperm Dd Dwarf Dd Dwarf D Eggs dd Normal dd Normal d Figure 9.15 Figure 9.15 A Punnett square illustrating a family with and without achondroplasia

Dominant Disorders Dominant alleles that cause lethal disorders are much less common than lethal recessive alleles. One example is the allele that causes Huntington’s disease, a degeneration of the nervous system that usually does not begin until middle age. Once the deterioration of the nervous system begins, it is irreversible and inevitably fatal. Because the allele for Huntington’s disease is dominant, any child born to a parent with the allele has a 50% chance of inheriting the allele and the disorder. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 61

Genetic Testing Today many tests can detect the presence of disease-causing alleles. Most genetic tests are performed during pregnancy if the prospective parents are aware that they have an increased risk of having a baby with a genetic disease. In amniocentesis, a physician uses a needle to extract about 2 teaspoons of the fluid that bathes the developing fetus. In chorionic villus sampling, a physician inserts a narrow, flexible tube through the mother’s vagina and into her uterus, removing some placental tissue. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 62

Genetic Testing Newer genetic screening procedures involve isolating tiny amounts of fetal cells or DNA released into the mother’s bloodstream. These newer technologies are gradually replacing more invasive screening methods because they are more accurate and can be performed earlier than other tests. Once cells are obtained, they can be screened for genetic diseases. Patients seeking genetic testing should receive counseling both before and after to explain the test and to help them cope with the results. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 63

Extending Mendelian genetics
Mendel worked with a simple system peas are genetically simple most traits are controlled by single gene each gene has only 2 version 1 completely dominant (A) 1 recessive (a) But its usually not that simple!

Variations on Mendel’s Laws
Mendel’s two laws explain inheritance in terms of genes that are passed along from generation to generation according to simple rules of probability. These laws are valid for all sexually reproducing organisms. But Mendel’s laws stop short of explaining some patterns of genetic inheritance. In fact, for most sexually reproducing organisms, the observed inheritance patterns are more complex. Additional tests have revealed some special cases of genetic inheritance : Incomplete dominance Multi-allelic dominance Single genes affecting multiple traits (pleiotropy) Polygenic inheritance Student Misconceptions and Concerns 1. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference). 2. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a shade of gray. The key concept is that both “sides” have input. Complete dominance is more analogous to an authoritarian style, overruling others and insisting on things being a certain way. Although these analogies might seem obvious to us, many students new to genetics appreciate them. 2. Another analogy for cholesterol receptors is fishing poles. The more fishing poles you use, the more fish you will likely catch. Heterozygotes for hypercholesterolemia have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more “fish” remain in the water. 3. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of amber (A) and blue (B) socks. Type A blood can have an amber sock with either another amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock represent the AB blood type. Having no socks, as already noted, represents type O. 4. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison. 5. The American Sickle Cell Anemia Association’s website ( is a good place to get additional details. 6. Polygenic inheritance makes it possible for children to inherit genes to be taller, or shorter, than either parent. Similarly, skin tones can be darker or lighter than either parent. The environment also contributes significantly to the final phenotype for both of these traits. 7. The authors note that polygenic inheritance is the converse of pleiotropy. This is worth noting in lecture as these concepts are discussed. We often remember concepts better when they are contrasted in pairs. 8. As the authors are careful to note, although genetics and the environment both contribute to the final phenotypes, only the genetic factors are inherited. This distinction is important to understanding the limitations of Lamarck’s mechanisms of evolution. If you will address principles of evolution soon after this chapter, this may be an important distinction to reinforce in lecture. References to tattoos, piercing, and circumcision may also help to distinguish between environmental influences and inheritance. Students with tattoos will not produce children born with tattoos! And for thousands of years, fathers who are circumcised have not fathered boys also circumcised! Active Lecture Tips 1. See the activity Who Wants to Be a Millionaire? Inheritance on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 65

1. Incomplete dominance in Plants and People
Snapdragons F1 hybrids have an appearance in between the phenotypes of the two parents. This is the result of incomplete dominance. Pure red and pure white snapdragons produce all- pink offspring (mixed color). Two alleles produce three phenotypes ? Differences & similarities with the Mendelian genetics? F1 Generation RR rr Gametes P Generation F2 Generation Sperm Red White R r Rr % Pink Rr Eggs 1 2 Some patterns of genetic inheritance are not explained by Mendel’s laws

Incomplete dominance P 1st 100% 1:2:1 2nd X true-breeding red flowers
white flowers 100% 100% pink flowers 1st generation (hybrids) self-pollinate 25% white 2nd generation red 1:2:1 50% pink

Incomplete dominance RR R W RW RR RW R W RW WW RW WW RW x RW 25% 25%
genotype % phenotype RR 25% 25% R W male / sperm 50% 50% RW RR RW R W female / eggs RW WW 25% 25% RW WW 1:2:1 1:2:1

Incomplete Dominance in Plants and People
Hypercholesterolemia is a human trait that is incompletely dominance is characterized by dangerously high levels of cholesterol in the blood. heterozygotes have blood cholesterol levels about twice normal, while homozygotes have about five times the normal amount of blood cholesterol and may have heart attacks as early as age 2. Student Misconceptions and Concerns 1. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference). 2. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a shade of gray. The key concept is that both “sides” have input. Complete dominance is more analogous to an authoritarian style, overruling others and insisting on things being a certain way. Although these analogies might seem obvious to us, many students new to genetics appreciate them. 2. Another analogy for cholesterol receptors is fishing poles. The more fishing poles you use, the more fish you will likely catch. Heterozygotes for hypercholesterolemia have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more “fish” remain in the water. 3. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of amber (A) and blue (B) socks. Type A blood can have an amber sock with either another amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock represent the AB blood type. Having no socks, as already noted, represents type O. 4. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison. 5. The American Sickle Cell Anemia Association’s website ( is a good place to get additional details. 6. Polygenic inheritance makes it possible for children to inherit genes to be taller, or shorter, than either parent. Similarly, skin tones can be darker or lighter than either parent. The environment also contributes significantly to the final phenotype for both of these traits. 7. The authors note that polygenic inheritance is the converse of pleiotropy. This is worth noting in lecture as these concepts are discussed. We often remember concepts better when they are contrasted in pairs. 8. As the authors are careful to note, although genetics and the environment both contribute to the final phenotypes, only the genetic factors are inherited. This distinction is important to understanding the limitations of Lamarck’s mechanisms of evolution. If you will address principles of evolution soon after this chapter, this may be an important distinction to reinforce in lecture. References to tattoos, piercing, and circumcision may also help to distinguish between environmental influences and inheritance. Students with tattoos will not produce children born with tattoos! And for thousands of years, fathers who are circumcised have not fathered boys also circumcised! Active Lecture Tips 1. See the activity Who Wants to Be a Millionaire? Inheritance on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 69

Incomplete dominance in People
HH Homozygous for ability to make LDL receptors Hh Heterozygous hh Homozygous for inability to make LDL receptors GENOTYPE LDL LDL receptor PHENOTYPE Cell Normal Mild disease Make half of the Receptors Low diet in cholesterol can alleviate Severe disease Unable to lower excess cholesterol Low diet in cholesterol + something else Figure 9.19 Incomplete dominance in human hypercholesterolemia

Practice In snapdragons, heterozygotes have pink flowers, whereas the dominant and recessive homozygotes have red and white flowers, respectively. When red-flowered plants are crossed with white-flowered plants, what percentage of the progeny will have pink flowers

Practice Andalusian chickens with the genotype CBCB are black, those with the genotype CWCW are white, and those with the genotype CBCW are gray. What is the expected phenotypic ratio of a CBCB x CBCW cross? 1 black : 1 white 3 black : 1 white 1 black : 2 gray : 1 white 3 gray : 1 white 1 black : 1 gray

ABO Blood Groups: An Example of Multiple Alleles and Codominance
The ABO blood groups in humans are an example of multiple alleles and Codomiance. The immune system produces blood proteins called antibodies that bind specifically to foreign carbohydrates. If a donor’s blood cells have a carbohydrate (A or B) that is foreign to the recipient, the blood cells may clump together, potentially killing the recipient. The clumping reaction is the basis of a blood-typing lab test. The human blood type alleles IA and IB are codominant, meaning that both alleles are expressed in heterozygous individuals who have type AB blood. both A & B alleles are dominant over i allele 73

2. Multiple allele and Codominance
There are three common alleles for ABO blood type Allele IA for the ability to make substance A Allele IB for the ability to make substance B Allele i for the ability to make neither of the substance Four blood groups were made from 3 different alleles type A : IA IA homozygote dominant for A or IAi heterozygote for A type B : IB IB homozygote dominant for B or IBi heterozygote for B type AB : IA IB heterozygote for A and B; codominance type O : ii (recessive) The human blood type alleles IA and IB exhibit codominance: Both alleles are expressed in the phenotype The genes code for different sugars on the surface of red blood cells So far we studied the inheritance pattern that involves 2 alleles per gene. But sometimes more than 2 alleles for a gene exists in the population

Multiple Alleles for the ABO blood group
(Phenotype) O Genotypes Antibodies Present in Red Blood Cells Reactions When Blood from Groups Below Is Mixed with Antibodies from Groups at Left A B AB ii IAIB IBIB or IBi IAIA IAi Carbohydrate A Carbohydrate B Anti-A Anti-B — clotting clotting clotting Universal recipient Universal donor clotting clotting clotting I = for immune factor can be either A or B; IA = allele A; IB = allele B i = absence of the immune factor IAIB is codominance; IA and IB are two codominant alleles Three alleles give four phenotypes The immune system produces blood proteins called antibodies that can bind specifically to blood cell carbohydrates. Blood cells may clump together if blood cells of a different type enter the body. The clumping reaction is the basis of a blood-typing lab test.

Practice If one parent is blood type AB and the other is type O, what fraction of their offspring will be blood type A? a. 0.5 b. 0.25 c. 0.75 d. 1.0 e. 0.0

3. Structure/Function: Pleiotropy and Sickle-Cell Disease
The genes that we have covered so far affect only one trait But most genes affect many traits Pleiotropy is when one gene influences several characters. Sickle-cell disease exhibits pleiotropy, results in abnormal hemoglobin proteins, and causes disk-shaped red blood cells to deform into a sickle shape with jagged edges. In most cases, only people who are homozygous for the sickle-cell allele have sickle-cell disease. Student Misconceptions and Concerns 1. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference). 2. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a shade of gray. The key concept is that both “sides” have input. Complete dominance is more analogous to an authoritarian style, overruling others and insisting on things being a certain way. Although these analogies might seem obvious to us, many students new to genetics appreciate them. 2. Another analogy for cholesterol receptors is fishing poles. The more fishing poles you use, the more fish you will likely catch. Heterozygotes for hypercholesterolemia have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more “fish” remain in the water. 3. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of amber (A) and blue (B) socks. Type A blood can have an amber sock with either another amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock represent the AB blood type. Having no socks, as already noted, represents type O. 4. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison. 5. The American Sickle Cell Anemia Association’s website ( is a good place to get additional details. 6. Polygenic inheritance makes it possible for children to inherit genes to be taller, or shorter, than either parent. Similarly, skin tones can be darker or lighter than either parent. The environment also contributes significantly to the final phenotype for both of these traits. 7. The authors note that polygenic inheritance is the converse of pleiotropy. This is worth noting in lecture as these concepts are discussed. We often remember concepts better when they are contrasted in pairs. 8. As the authors are careful to note, although genetics and the environment both contribute to the final phenotypes, only the genetic factors are inherited. This distinction is important to understanding the limitations of Lamarck’s mechanisms of evolution. If you will address principles of evolution soon after this chapter, this may be an important distinction to reinforce in lecture. References to tattoos, piercing, and circumcision may also help to distinguish between environmental influences and inheritance. Students with tattoos will not produce children born with tattoos! And for thousands of years, fathers who are circumcised have not fathered boys also circumcised! Active Lecture Tips 1. See the activity Who Wants to Be a Millionaire? Inheritance on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 77

Individual homozygous for sickle-cell allele
Sickle-cell (abnormal) hemoglobin Abnormal hemoglobin crystallizes into long, flexible chains, causing red blood cells to become sickle-shaped. Colorized SEM Sickled cells can lead to a cascade of symptoms, such as weakness, pain, organ damage, and paralysis. Figure 9.21 Figure 9.21 Sickle-cell disease: multiple effects of a single human gene

4. Polygenic Inheritance
Polygenic inheritance is the additive effects of two or more genes on a single phenotypic character and the logical opposite of pleiotropy, in which one gene affects several characters. There is evidence that height in people is controlled by several genes that are inherited separately. (Actually, human height is probably affected by a great number of genes, but we’ll simplify here.) © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference). 2. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a shade of gray. The key concept is that both “sides” have input. Complete dominance is more analogous to an authoritarian style, overruling others and insisting on things being a certain way. Although these analogies might seem obvious to us, many students new to genetics appreciate them. 2. Another analogy for cholesterol receptors is fishing poles. The more fishing poles you use, the more fish you will likely catch. Heterozygotes for hypercholesterolemia have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more “fish” remain in the water. 3. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of amber (A) and blue (B) socks. Type A blood can have an amber sock with either another amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock represent the AB blood type. Having no socks, as already noted, represents type O. 4. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison. 5. The American Sickle Cell Anemia Association’s website ( is a good place to get additional details. 6. Polygenic inheritance makes it possible for children to inherit genes to be taller, or shorter, than either parent. Similarly, skin tones can be darker or lighter than either parent. The environment also contributes significantly to the final phenotype for both of these traits. 7. The authors note that polygenic inheritance is the converse of pleiotropy. This is worth noting in lecture as these concepts are discussed. We often remember concepts better when they are contrasted in pairs. 8. As the authors are careful to note, although genetics and the environment both contribute to the final phenotypes, only the genetic factors are inherited. This distinction is important to understanding the limitations of Lamarck’s mechanisms of evolution. If you will address principles of evolution soon after this chapter, this may be an important distinction to reinforce in lecture. References to tattoos, piercing, and circumcision may also help to distinguish between environmental influences and inheritance. Students with tattoos will not produce children born with tattoos! And for thousands of years, fathers who are circumcised have not fathered boys also circumcised! Active Lecture Tips 1. See the activity Who Wants to Be a Millionaire? Inheritance on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 79

Fraction of population
P Generation = short allele = tall allele aabbcc (very short) AABBCC (very tall) F1 Generation AaBbCc (medium height) AaBbCc (medium height) F2 Generation Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 20 64 1 8 1 8 15 64 1 8 Eggs 1 8 Fraction of population 1 8 6 64 1 8 1 8 1 64 Adult height 1 64 6 64 15 64 20 64 15 64 6 64 1 64 Very short Very tall Figure 9.22 Figure 9.22 A model for polygenic inheritance of height

a) incomplete dominance
Summary: Variations in Mendel’s law a) incomplete dominance b) Codominance Dominant phenotype (RR) Recessive phenotype (rr) Intermediate phenotype (incomplete dominance) (Rr) Blood Group (Phenotype) Genotypes Red Blood Cells O A B AB ii IAIB IBIB or IBi IAIA IAi Carbohydrate A Carbohydrate B c) Pleiotropy Multiple traits (e.g., sickle-cell disease) Single gene d) Polygenic inheritance Multiple genes Single trait (e.g., skin color) Figure 9.UN4 Summary: Pleiotropy

The Role of Environment
Many human characters result from a combination of heredity and environment. Coat color in arctic fox influenced by heat sensitive alleles Human skin color is influenced by both genetics & environmental conditions Color of Hydrangea flowers is influenced by soil pH Another example – trees, exercise in humanFigure 9.23 As a result of environmental influences, even identical twins can look different.

Epigenetics and the Role of Environment
Whether human characters are more influenced by genes or by the environment—nature or nurture—is a very old and hotly contested issue. Spending time with identical twins will convince anyone that environment, and not just genes, affects a person’s traits. In general, only genetic influences are inherited and effects of the environment are not passed to the next generation. In recent years, however, biologists have begun to recognize the importance of epigenetic inheritance, the transmission of traits by mechanisms not directly involving DNA sequence. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference). 2. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a shade of gray. The key concept is that both “sides” have input. Complete dominance is more analogous to an authoritarian style, overruling others and insisting on things being a certain way. Although these analogies might seem obvious to us, many students new to genetics appreciate them. 2. Another analogy for cholesterol receptors is fishing poles. The more fishing poles you use, the more fish you will likely catch. Heterozygotes for hypercholesterolemia have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more “fish” remain in the water. 3. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of amber (A) and blue (B) socks. Type A blood can have an amber sock with either another amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock represent the AB blood type. Having no socks, as already noted, represents type O. 4. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison. 5. The American Sickle Cell Anemia Association’s website ( is a good place to get additional details. 6. Polygenic inheritance makes it possible for children to inherit genes to be taller, or shorter, than either parent. Similarly, skin tones can be darker or lighter than either parent. The environment also contributes significantly to the final phenotype for both of these traits. 7. The authors note that polygenic inheritance is the converse of pleiotropy. This is worth noting in lecture as these concepts are discussed. We often remember concepts better when they are contrasted in pairs. 8. As the authors are careful to note, although genetics and the environment both contribute to the final phenotypes, only the genetic factors are inherited. This distinction is important to understanding the limitations of Lamarck’s mechanisms of evolution. If you will address principles of evolution soon after this chapter, this may be an important distinction to reinforce in lecture. References to tattoos, piercing, and circumcision may also help to distinguish between environmental influences and inheritance. Students with tattoos will not produce children born with tattoos! And for thousands of years, fathers who are circumcised have not fathered boys also circumcised! Active Lecture Tips 1. See the activity Who Wants to Be a Millionaire? Inheritance on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 83

Epigenetics and the Role of Environment
For example, components of chromosomes can be chemically modified by adding or removing chemical groups on the DNA and/or protein components of chromosomes. Over a lifetime, the environment plays a role in these changes, which may explain how one identical twin can suffer from a genetically based disease whereas the other twin does not, despite their identical genomes. Epigenetic modifications—and the changes in gene activity that result—may even be carried on to the next generation. Unlike alterations to the DNA sequence, chemical changes to the chromosomes can be reversed. Student Misconceptions and Concerns 1. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference). 2. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a shade of gray. The key concept is that both “sides” have input. Complete dominance is more analogous to an authoritarian style, overruling others and insisting on things being a certain way. Although these analogies might seem obvious to us, many students new to genetics appreciate them. 2. Another analogy for cholesterol receptors is fishing poles. The more fishing poles you use, the more fish you will likely catch. Heterozygotes for hypercholesterolemia have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more “fish” remain in the water. 3. Students can think of blood types as analogous to socks on their feet. You can have socks that match, a sock on one foot but not the other, you can wear two socks that do not match, or you can even go barefoot (type O blood)! Developed further, think of amber (A) and blue (B) socks. Type A blood can have an amber sock with either another amber sock or a bare foot (or “zero” sock). Blue socks work the same way. One amber and one blue sock represent the AB blood type. Having no socks, as already noted, represents type O. 4. Consider specifically comparing the principles of codominance (expression of both alleles) and incomplete dominance (expression of one intermediate trait). Students will likely benefit from this direct comparison. 5. The American Sickle Cell Anemia Association’s website ( is a good place to get additional details. 6. Polygenic inheritance makes it possible for children to inherit genes to be taller, or shorter, than either parent. Similarly, skin tones can be darker or lighter than either parent. The environment also contributes significantly to the final phenotype for both of these traits. 7. The authors note that polygenic inheritance is the converse of pleiotropy. This is worth noting in lecture as these concepts are discussed. We often remember concepts better when they are contrasted in pairs. 8. As the authors are careful to note, although genetics and the environment both contribute to the final phenotypes, only the genetic factors are inherited. This distinction is important to understanding the limitations of Lamarck’s mechanisms of evolution. If you will address principles of evolution soon after this chapter, this may be an important distinction to reinforce in lecture. References to tattoos, piercing, and circumcision may also help to distinguish between environmental influences and inheritance. Students with tattoos will not produce children born with tattoos! And for thousands of years, fathers who are circumcised have not fathered boys also circumcised! Active Lecture Tips 1. See the activity Who Wants to Be a Millionaire? Inheritance on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 84

The Chromosomal Basis of Inheritance
The chromosome theory of inheritance states that genes are located at specific positions (loci) on chromosomes and the behavoir of chromosomes during meiosis and fertilization accounts for inheritance patterns. It is chromosomes that undergo segregation and independent assortment during meiosis and account for Mendel’s laws. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. This section of the chapter relies on a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis was not otherwise addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes. 2. The nature of linked genes builds on our natural expectations that items that are closer together are less likely to be separated. Yet students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart. You might demonstrate this simply by drawing a line down a page of text. The likelihood that the line separates any pair of words increases, as the distance between the words grows farther apart. 3. The discussion of “linked genes” addresses a different relationship than the use of the similar termed “sex-linked genes.” The nature of the linkage is quite different. Consider emphasizing this distinction for your students. 4. The likelihood that at least some students are colorblind in larger classes is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect. Teaching Tips 1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances. 2. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of a Shrek film. Clearly, the closer that two frames of film are together, the more likely they are to move or remain together. 3. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on your computer. If you only have one copy and it is damaged, you have to live with the damaged file. Having two X chromosomes in females provides a “backup copy” that can function if one of the sex-linked genes is damaged. 4. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease. 5. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at Active Lecture Tips 1. See the activity Pairs of Shoes and Pairs of Chromosomes on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 2. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 85

The chromosomal basis of Mendel's laws All round-yellow seeds
P Generation Round-yellow seeds (RRYY) Wrinkled-green Seeds (rryy) Y y r R Y R r y MEIOSIS FERTILIZATION Gametes R Y y r F1 Generation All round-yellow seeds (RrYy) MEIOSIS I R y r Law of Independent Assortment: Follow both the long and the short chromosomes. Law of Segregation: Follow the long chromosomes (carrying R and r) taking either the left or right branch. . Y MEIOSIS R r r R Metaphase I (alternative arrangements) They are arranged in either of two equally likely ways at metaphase I. The R and r alleles segregate in anaphase I. Y y Y y R r Metaphase II r R Only one long chromosome ends up in each gamete. They sort independently, giving four gamete types. Y y Y y Y y Y y Y Y y y Gametes R R r r r r R R 1 4 RY 1 4 ry 1 4 rY 1 4 Ry Fertilization results in the 9:3:3:1 phenotypic ratio in the F2 generation. Fertilization recombines the r and R alleles at random. FERTILIZATION AMONG THE F1 PLANTS F2 Generation 9 : 3 : 3 :1 Figure 9.24 Figure 9.24 The chromosomal basis of Mendel’s laws

Linked Genes Linked genes
are located near each other on the same chromosome and tend to travel together during meiosis and fertilization. Such genes are often inherited as a set and therefore often do not follow Mendel’s law of independent assortment. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. This section of the chapter relies on a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis was not otherwise addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes. 2. The nature of linked genes builds on our natural expectations that items that are closer together are less likely to be separated. Yet students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart. You might demonstrate this simply by drawing a line down a page of text. The likelihood that the line separates any pair of words increases, as the distance between the words grows farther apart. 3. The discussion of “linked genes” addresses a different relationship than the use of the similar termed “sex-linked genes.” The nature of the linkage is quite different. Consider emphasizing this distinction for your students. 4. The likelihood that at least some students are colorblind in larger classes is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect. Teaching Tips 1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances. 2. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of a Shrek film. Clearly, the closer that two frames of film are together, the more likely they are to move or remain together. 3. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on your computer. If you only have one copy and it is damaged, you have to live with the damaged file. Having two X chromosomes in females provides a “backup copy” that can function if one of the sex-linked genes is damaged. 4. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease. 5. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at Active Lecture Tips 1. See the activity Pairs of Shoes and Pairs of Chromosomes on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 2. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 87

Sex Determination in Humans
Many animals, including all mammals, have a pair of sex chromosomes, designated X and Y, that determine an individual’s sex. Individuals with one X chromosome and one Y chromosome are males. XX individuals are females. Human males and females both have 44 autosomes (chromosomes other than sex chromosomes). © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. This section of the chapter relies on a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis was not otherwise addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes. 2. The nature of linked genes builds on our natural expectations that items that are closer together are less likely to be separated. Yet students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart. You might demonstrate this simply by drawing a line down a page of text. The likelihood that the line separates any pair of words increases, as the distance between the words grows farther apart. 3. The discussion of “linked genes” addresses a different relationship than the use of the similar termed “sex-linked genes.” The nature of the linkage is quite different. Consider emphasizing this distinction for your students. 4. The likelihood that at least some students are colorblind in larger classes is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect. Teaching Tips 1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances. 2. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of a Shrek film. Clearly, the closer that two frames of film are together, the more likely they are to move or remain together. 3. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on your computer. If you only have one copy and it is damaged, you have to live with the damaged file. Having two X chromosomes in females provides a “backup copy” that can function if one of the sex-linked genes is damaged. 4. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease. 5. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at Active Lecture Tips 1. See the activity Pairs of Shoes and Pairs of Chromosomes on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 2. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 88

The chromosomal basis of sex determination in humans
Male Female 44  XY Somatic cells 44  XX Y X Colorized SEM 22  X 22  Y 22  X Sperm Egg 44  XX 44  XY Offspring Female Male Figure 9.25 Figure 9.25 The chromosomal basis of sex determination in humans

Sex-Linked Genes A gene located on a sex chromosome is called a sex-linked gene. Most sex-linked genes are found on the X chromosome. A number of human conditions, including red-green colorblindness, hemophilia, and a type of muscular dystrophy, result from sex-linked recessive alleles. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. This section of the chapter relies on a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis was not otherwise addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes. 2. The nature of linked genes builds on our natural expectations that items that are closer together are less likely to be separated. Yet students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart. You might demonstrate this simply by drawing a line down a page of text. The likelihood that the line separates any pair of words increases, as the distance between the words grows farther apart. 3. The discussion of “linked genes” addresses a different relationship than the use of the similar termed “sex-linked genes.” The nature of the linkage is quite different. Consider emphasizing this distinction for your students. 4. The likelihood that at least some students are colorblind in larger classes is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect. Teaching Tips 1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances. 2. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of a Shrek film. Clearly, the closer that two frames of film are together, the more likely they are to move or remain together. 3. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on your computer. If you only have one copy and it is damaged, you have to live with the damaged file. Having two X chromosomes in females provides a “backup copy” that can function if one of the sex-linked genes is damaged. 4. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease. 5. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at Active Lecture Tips 1. See the activity Pairs of Shoes and Pairs of Chromosomes on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 2. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 90

Sex-Linked Genes Red-green colorblindness is a common sex- linked disorder caused by a malfunction of light- sensitive cells in the eyes. Mostly males are affected, but heterozygous females have some defects, too. Figure 9.26 shows a simple test for red-green colorblindness. © 2016 Pearson Education, Inc. Figure 9.26 A test for red-green colorblindness 91

Sex-Linked Genes Because they are located on the sex chromosomes, sex-linked genes exhibit unusual inheritance patterns. Figure 9.27a illustrates what happens when a colorblind male has offspring with a homozygous female with normal color vision. Figure 9.27b illustrates what happens if a female carrier mates with a male who has normal color vision. Because the colorblindness allele is recessive, a female will be colorblind only if she receives that allele on both X chromosomes. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. This section of the chapter relies on a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis was not otherwise addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes. 2. The nature of linked genes builds on our natural expectations that items that are closer together are less likely to be separated. Yet students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart. You might demonstrate this simply by drawing a line down a page of text. The likelihood that the line separates any pair of words increases, as the distance between the words grows farther apart. 3. The discussion of “linked genes” addresses a different relationship than the use of the similar termed “sex-linked genes.” The nature of the linkage is quite different. Consider emphasizing this distinction for your students. 4. The likelihood that at least some students are colorblind in larger classes is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect. Teaching Tips 1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances. 2. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of a Shrek film. Clearly, the closer that two frames of film are together, the more likely they are to move or remain together. 3. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on your computer. If you only have one copy and it is damaged, you have to live with the damaged file. Having two X chromosomes in females provides a “backup copy” that can function if one of the sex-linked genes is damaged. 4. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease. 5. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at Active Lecture Tips 1. See the activity Pairs of Shoes and Pairs of Chromosomes on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 2. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 92

(a) Normal female  colorblind male (b) Carrier female  normal male
XNXN XnY XNXn XNY Xn Y Sperm XN Y Sperm Eggs XN XNXn XNY Eggs XN XNXN XNY XN XNXn XNY Xn XNXn XnY (a) Normal female  colorblind male (b) Carrier female  normal male XNXn XnY Key Xn Y Sperm Unaffected individual Carrier Eggs XN XNXn XNY Colorblind individual Xn XnXn XnY (c) Carrier female  colorblind male Figure 9.27 Figure 9.27 Inheritance of colorblindness, a sex-linked recessive trait

Sex-Linked Genes Hemophilia is a sex-linked recessive trait with a long, well-documented history. Hemophiliacs bleed excessively when injured because they have inherited an abnormal allele for a factor involved in blood clotting. The most seriously affected individuals may bleed to death after relatively minor bruises or cuts. The age-old practice of strengthening international alliances by marriage effectively spread hemophilia through the royal families of several nations. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. This section of the chapter relies on a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis was not otherwise addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes. 2. The nature of linked genes builds on our natural expectations that items that are closer together are less likely to be separated. Yet students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart. You might demonstrate this simply by drawing a line down a page of text. The likelihood that the line separates any pair of words increases, as the distance between the words grows farther apart. 3. The discussion of “linked genes” addresses a different relationship than the use of the similar termed “sex-linked genes.” The nature of the linkage is quite different. Consider emphasizing this distinction for your students. 4. The likelihood that at least some students are colorblind in larger classes is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect. Teaching Tips 1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances. 2. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of a Shrek film. Clearly, the closer that two frames of film are together, the more likely they are to move or remain together. 3. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on your computer. If you only have one copy and it is damaged, you have to live with the damaged file. Having two X chromosomes in females provides a “backup copy” that can function if one of the sex-linked genes is damaged. 4. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease. 5. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at Active Lecture Tips 1. See the activity Pairs of Shoes and Pairs of Chromosomes on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 2. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 94

Hemophilia is a sex-linked recessive trait defined by the absence of one or more clotting factors.
These proteins normally slow and then stop bleeding. Individuals with hemophilia have prolonged bleeding because a firm clot forms slowly. Bleeding in muscles and joints can be painful and lead to serious damage. Individuals can be treated with intravenous injections of the missing protein.

Sex-Linked Genes Hemophilia
is a sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruises has plagued royal families of Europe Albert Queen Victoria Alice Louis Alexandra Czar Nicholas II of Russia Alexis

Practice To determine the phenotype of an individual who expresses a dominant trait, you would cross that individual with an individual who ______. expresses the dominant trait is homozygous recessive for that trait has the genotype Aa is homozygous dominant for that trait is heterozygous for that trait

Practice An individual who is homozygous ______.
expresses the dominant trait carries two different alleles for a gene is a carrier of a genetic disorder carries two copies of the same allele for a gene expresses the recessive trait

Practice The observable traits of an organism are its phenotype.
genotype. characteristics that do not become dormant in succeeding generations. sociobiology. pedigree.

Practice In people with sickle-cell disease, red blood cells break down, clump, and clog the blood vessels. In addition, the red cells can accumulate in the spleen. Among other things this leads to physical weakness, heart failure, pain, and brain damage. Such a suite of symptoms can be explained by __________. ( Variations on Mendel's Laws) side effects of the drugs used to cure sickle-cell disease the disease being the result of the inheritance of two linked genes a bacterial infection interacting with the sickle-cell allele the pleiotropic effects of the sickle-cell allele the polygenic nature of sickle-cell disease

Practice The inheritance of human characteristics, such as height and weight, can best be described as __________. (Variations on Mendel's Laws) all symptoms of Huntington's disease being caused by microorganisms c. Polygenic d. simple dominant-recessive inheritance e. the results of a bad lifestyle

Challenge ?

Challenge ? In mice, the allele for black fur is dominant to the allele for brown fur. A biology student decides to mate a male homozygous black mouse with a female homozygous brown mouse. A. What percentage of the offspring do you predict will have black fur? B. What percentage of the offspring do you predict will be homozygous?

Evolution Connection: Barking Up the Evolutionary Tree
About 15,000 years ago, in East Asia, people began to cohabit with ancestral canines that were predecessors of both modern wolves and dogs. As people moved into permanent, geographically isolated settlements, populations of canines were separated from one another and eventually became inbred. A 2010 study indicated that small dogs were first bred within early agricultural settlements of the Middle East around 12,000 years ago. Continued over millennia, such genetic tinkering has resulted in a diverse array of dog body types and behaviors. Student Misconceptions and Concerns 1. This section of the chapter relies on a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis was not otherwise addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes. 2. The nature of linked genes builds on our natural expectations that items that are closer together are less likely to be separated. Yet students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart. You might demonstrate this simply by drawing a line down a page of text. The likelihood that the line separates any pair of words increases, as the distance between the words grows farther apart. 3. The discussion of “linked genes” addresses a different relationship than the use of the similar termed “sex-linked genes.” The nature of the linkage is quite different. Consider emphasizing this distinction for your students. 4. The likelihood that at least some students are colorblind in larger classes is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect. Teaching Tips 1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances. 2. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of a Shrek film. Clearly, the closer that two frames of film are together, the more likely they are to move or remain together. 3. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on your computer. If you only have one copy and it is damaged, you have to live with the damaged file. Having two X chromosomes in females provides a “backup copy” that can function if one of the sex-linked genes is damaged. 4. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease. 5. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at Active Lecture Tips 1. See the activity Pairs of Shoes and Pairs of Chromosomes on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 2. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 106

Evolution Connection: Barking Up the Evolutionary Tree
Our understanding of canine evolution took a big leap forward when researchers sequenced the complete genome of a dog. Using the genome sequence and a wealth of other data, canine geneticists produced an evolutionary tree based on a genetic analysis of 85 breeds. The formulation of an evolutionary tree for the domestic dog shows that new technologies can provide important insights into genetic and evolutionary questions about life on Earth. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. This section of the chapter relies on a good understanding of the chromosome sorting process of meiosis. If students were not assigned Chapter 8, and meiosis was not otherwise addressed, it will be difficult for students to understand the chromosomal basis of inheritance or linked genes. 2. The nature of linked genes builds on our natural expectations that items that are closer together are less likely to be separated. Yet students may find such concepts initially foreign. Whether it is parents holding the hands of children or people and their pets, we generally know that separation is more likely when things are farther apart. You might demonstrate this simply by drawing a line down a page of text. The likelihood that the line separates any pair of words increases, as the distance between the words grows farther apart. 3. The discussion of “linked genes” addresses a different relationship than the use of the similar termed “sex-linked genes.” The nature of the linkage is quite different. Consider emphasizing this distinction for your students. 4. The likelihood that at least some students are colorblind in larger classes is very high. Some of these students might find this interesting and want to discuss it further. However, others might be embarrassed by what might be perceived as a defect. Teaching Tips 1. Building on the shoe analogy developed in Chapter 8, linked genes are like a shoe and its shoelaces. The two are usually transferred together but can be moved separately under special circumstances. 2. Crossing over (from Chapter 8) is like randomly editing out a minute of film from two movies and swapping them. Perhaps the fifth minute of Bambi is swapped for the fifth minute of a Shrek film. Clearly, the closer that two frames of film are together, the more likely they are to move or remain together. 3. In some ways, sex-linked genes reflect the risk of not having a backup copy of a file on your computer. If you only have one copy and it is damaged, you have to live with the damaged file. Having two X chromosomes in females provides a “backup copy” that can function if one of the sex-linked genes is damaged. 4. Hemophilia and other genetic diseases may also result from spontaneous mutations in a family with no known history of the disease. Although rare, this possibility should always be considered when tracing the history of an inherited disease. 5. Female hemophiliacs are very rare because both X chromosomes would need to have the recessive trait. Although very unlikely, female hemophiliacs are known. Students may enjoy searching for details of these rare cases. For additional information about hemophilia, consider visiting the website of the National Hemophilia Foundation at Active Lecture Tips 1. See the activity Pairs of Shoes and Pairs of Chromosomes on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 2. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 107

An evolutionary tree of dog breeds
Wolf Chinese shar-pei Ancestral canine Siberian husky Akita Alaskan malamute Basenji Afghan hound Saluki Rottweiler An evolutionary tree of dog breeds Sheepdog Retriever Figure 9.29 Figure 9.29 An evolutionary tree of dog breeds

(allele pairs separated)
Figure 9.UN01 Fertilization Alleles Meiosis Gamete from other parent Diploid zygote (contains paired alleles) Diploid cell (contains paired alleles, alternate forms of a gene) Haploid gametes (allele pairs separated) Figure 9.UN01 Summary of key concepts: law of segregation

Dominant P ? Recessive pp
Figure 9.UN02 Phenotype Dominant P ? Recessive pp Genotype or Phenotype All dominant 1 dominant:1 recessive Conclusion Unknown parent is PP Unknown parent is Pp Figure 9.UN02 Summary of key concepts: testcross

Summary Figure 9.UN7 Summary: Sex Linked Traits

The Process of Science: What Is the Genetic Basis of Coat Variation in Dogs?
Observation: Dogs come in a wide variety of physical types. Question: What is the genetic basis for canine coats? Hypothesis: A comparison of genes of a wide variety of dogs with different coats would identify the genes responsible. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 112

Figure 9.16 Figure 9.16 Smooth versus wired fox terrier

The Process of Science: What Is the Genetic Basis of Coat Variation in Dogs?
Prediction: Mutations in just a few genes account for the coat appearance. Experiment: Researchers compared DNA sequences of 622 dogs from dozens of breeds. Results: Three genes in different combinations produced seven different coat appearances, from very short hair to full, thick, wired hair. © 2016 Pearson Education, Inc. Student Misconceptions and Concerns 1. Students might think that dominant alleles are naturally (a) more common, (b) more likely to be inherited, and (c) better for an organism. The text notes that this is not necessarily true. However, this might need to be emphasized further in lecture. 2. Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. Just as we would expect that any six playing cards dealt might be half black and half red, we frequently find that this is not true. This might be a good time to show how larger sample sizes increase the likelihood that sampling reflects expected ratios. 3. The authors note that Mendel’s work was published in 1866, seven years after Darwin published On the Origin of Species. Consider challenging your students to consider whether Mendel’s findings were supportive of Darwin’s ideas. Some scientists have noted that Darwin often discussed the evolution of traits by matters of degree. Yet Mendel’s selection of pea plant traits typically showed complete dominance. Mendel’s pea traits did not show the possibility for such gradual inheritance. Teaching Tips 1. Medical technology raises many ethical issues. Consider asking your students this practical question: How much routine fetal testing do we want our insurance companies to cover and at what cost for insurance? Ultrasound, for example, is routinely performed on pregnant women as a normal part of prenatal care. What other tests should be standard? Who should decide? Who should pay? 2. This early material introduces many definitions that are vital to understanding the later discussions in this chapter. Therefore, students need to be encouraged to master these definitions immediately. This may be a good time for a short quiz to encourage their progress. 3. Consider this analogy for dominance of a trait in the heterozygous condition, which may help struggling students. Two people are trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. If this “heterozygous” couple eats at home, the dominant allele “wins.” 4. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP × pp and (b) Pp × pp. 5. Understanding dihybrid crosses may be the most difficult concept in this chapter. Consider spending additional time to make these ideas very clear. As the text indicates, dihybrid crosses are essentially two monohybrid crosses. 6. Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. Explanation: If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three black and one chocolate or all black. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype. 7. The 2/3 fraction noted in the discussion of carriers of a recessive disorder (and in Figure 9.14) often catches students off guard, as they may be expecting odds of 1/4, 1/2, or 3/4. However, we eliminate the dd (deaf) possibility, as it would not be a carrier. So, the odds are based out of the remaining three genotypes Dd, dD, and DD. 8. Genetic tests are now available to inform a person whether he or she has the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class (a) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (b) whether or not they would want this genetic test. The Huntington Disease Society website ( offers many additional details. It is a good starting point for those who want to explore this disease in more detail. Active Lecture Tips 1. Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. 2. As a simple test of comprehension, ask students to work in pairs to explain why lethal alleles are not eliminated from a population. Several possibilities exist: (a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or (b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. See the Media Review: “Learn.Genetics” Genetic Science Learning Center from the University of Utah on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 4. See the activity Interactive Celebrity Parents Genetic Inheritance Game on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 5. See the activity Personal Genomics: Would You Give Your DNA Away? on the Instructor Exchange. Visit the Instructor Exchange in the MasteringBiology instructor resource area for a description of this activity. 114

Patterns of Inheritance

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