Patterns of Inheritance

Patterns of Inheritance
Chapter 9 Patterns of Inheritance

Biology And Society: A Matter of Breeding
Genetics is the scientific study of heredity. Genetics explains why the offspring of purebred dogs are like their parents. Inbreeding of dogs makes some genetic disorders common. A dog’s behavior is determined by its Genes Environment

HERITABLE VARIATION AND PATTERNS OF INHERITANCE
Heredity is the transmission of traits from one generation to the next. Gregor Mendel Worked in the 1860s Was the first person to analyze patterns of inheritance Deduced the fundamental principles of genetics 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

In an Abbey Garden Mendel studied garden peas because they
Are easy to grow Come in many readily distinguishable varieties Are easily manipulated Can self-fertilize 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Petal Stamen (makes sperm- producing pollen) Carpel (produces eggs)
Figure 9.2 Figure 9.2 The structure of a pea flower.

A character is a heritable feature that varies among individuals.
A trait is a variant of a character. Each of the characters Mendel studied occurred in two distinct forms. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Hybrids are the offspring of two different true-breeding varieties.
Mendel Created true-breeding varieties of plants Crossed two different true-breeding varieties Hybrids are the offspring of two different true-breeding varieties. The parental plants are the P generation. Their hybrid offspring are the F1 generation. A cross of the F1 plants forms the F2 generation. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

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

Mendel’s Law of Segregation
Mendel performed many experiments. He tracked the inheritance of characters that occur as two alternative traits. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

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.

Figure 9.4 Figure 9.4 Photo of pea plants

Blast Animation: Single-Trait Crosses
Monohybrid Crosses A monohybrid cross is a cross between parent plants that differ in only one character. Blast Animation: Single-Trait Crosses 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

P Generation (true-breading parents) Purple flowers White flowers
F1 Generation All plants have purple flowers Fertilization among F1 plants (F1  F1) F2 Generation 3 4 1 4 of plants have purple flowers of plants have white flowers Figure 9.5-3 Figure 9.5 Mendel's cross tracking one character (flower color). (Step 3)

Mendel developed four hypotheses from the monohybrid cross:
1. There are alternative versions of genes, called alleles. 2. For each character, an organism inherits two alleles, one from each parent. An organism is homozygous for that gene if both alleles are identical. An organism is heterozygous for that gene if the alleles are different. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

3. If two alleles of an inherited pair differ
The allele that determines the organism’s appearance is the dominant allele The other allele, which has no noticeable effect on the appearance, is the recessive allele 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

4. Gametes carry only one allele for each inherited character.
The two members of an allele pair segregate (separate) from each other during the production of gametes. This statement is the law of segregation. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Blast Animation: Genetic Variation: Fusion of Gametes
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. Blast Animation: Genetic Variation: Fusion of Gametes 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Figure 9.6-3 P Generation Genetic makeup (alleles) Purple flowers
White flowers PP pp Alleles carried by parents Gametes All P All p F1 Generation (hybrids) Purple flowers Alleles segregate All Pp 1 2 1 2 Gametes P p F2 Generation (hybrids) Sperm from F1 plant 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 9.6-3 Figure 9.6 The law of segregation. (Step 3)

Geneticists distinguish between an organism’s physical traits and its genetic makeup.
An organism’s physical traits are its phenotype. An organism’s genetic makeup is its genotype. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Genetic Alleles and Homologous Chromosomes
Homologous chromosomes have Genes at specific loci Alleles of a gene at the same locus 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Gene loci Dominant allele P a B Homologous chromosomes P a b Recessive
Genotype: PP aa Bb Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous Figure 9.7 Figure 9.7 The relationship between alleles and homologous chromosomes.

Mendel’s Law of Independent Assortment
A dihybrid cross is the crossing of parental varieties differing in two characters. What would result from a dihybrid cross? Two hypotheses are possible: 1. Dependent assortment 2. Independent assortment Blast Animation: Two-Trait Crosses 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

(a) Hypothesis: Dependent assortment
(b) Hypothesis: Independent assortment P Generation RRYY rryy RRYY rryy Gametes RY ry Gametes RY ry F1 Generation RrYy RrYy F2 Generation Sperm 1 4 1 4 1 4 1 4 RY rY Ry ry Sperm 1 2 1 2 RY ry 1 4 RY RRYY RrYY RRYy RrYy 1 2 RY 1 4 rY 16 9 Yellow round Eggs RrYY rrYY RrYy rrYy Eggs 1 2 ry 1 4 Ry 16 3 Green round RRYy RrYy RRyy Rryy 16 3 Yellow wrinkled 1 4 ry RrYy rrYy Rryy rryy 1 16 Green wrinkled Predicted results (not actually seen) Actual results (support hypothesis) Figure 9.8 Figure 9.8 Testing alternative hypotheses for gene assortment in a dihybrid cross.

Figure 9.8 Figure 9.8 Photo of peas

This is the law of independent assortment.
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 the law of independent assortment. Independent assortment is also seen in two hereditary characters in Labrador retrievers. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Mating of double heterozygotes (black coat, normal vision)
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 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 Figure 9.9 Independent assortment of genes in Labrador retrievers.

Using a Testcross to Determine an Unknown Genotype
A testcross is a mating between An individual of dominant phenotype (but unknown genotype) A homozygous recessive individual © 2010 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Two possible genotypes for the black dog:
Testcross Genotypes B_ bb Two possible genotypes for the black dog: BB or Bb Gametes B B b b Bb b Bb bb Offspring All black 1 black : 1 chocolate Figure 9.10 Figure 9.10 A Labrador retriever testcross

The Rules of Probability
Mendel’s strong background in mathematics helped him understand patterns of inheritance. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

F1 Genotypes Bb female Bb male Formation of eggs Formation of sperm
Male gametes 1 2 1 2 B b B B B 1 2 b 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.

Family Pedigrees Mendel’s principles apply to the inheritance of many human traits. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

DOMINANT TRAITS RECESSIVE TRAITS Freckles Widow’s peak Free earlobe
No freckles Straight hairline Attached earlobe Figure 9.12 Figure 9.12 Examples of inherited traits thought to be controlled by a single gene.

Dominant traits are not necessarily
Normal or More common Wild-type traits are Those seen most often in nature Not necessarily specified by dominant 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

A family pedigree Shows the history of a trait in a family
Allows geneticists to analyze human traits 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

First generation (grandparents) Ff Ff ff Ff Second generation
(parents, aunts, and uncles) FF ff ff Ff Ff ff or Ff Third generation (brother and sister) ff FF or Ff Female Male Attached Free Figure 9.13 Figure 9.13 A family pedigree showing inheritance of free versus attached earlobes.

Human Disorders Controlled by a Single Gene
Many human traits Show simple inheritance patterns Are controlled by single genes on autosomes 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Table 9.1 Table 9.1 Some Autosomal Disorders in Humans

Recessive Disorders Most human genetic disorders are recessive.
Individuals who have the recessive allele but appear normal are carriers of the disorder. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Parents Hearing Hearing Dd Dd Offspring D d Sperm Dd DD D Hearing
(carrier) Hearing Eggs Dd dd d Hearing (carrier) Deaf Figure 9.14 Figure 9.14 Predicted offspring when both parents are carriers for a recessive disorder.

Cystic fibrosis Is the most common lethal genetic disease in the United States Is caused by a recessive allele carried by about one in 25 people of European ancestry Prolonged geographic isolation of certain populations can lead to inbreeding, the mating of close relatives. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Dominant Disorders Some human genetic disorders are dominant.
Huntington’s disease, which leads to degeneration of the nervous system, does not begin until middle age. Achondroplasia is a form of dwarfism. The homozygous dominant genotype causes death of the embryo. Thus, only heterozygotes have this disorder. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Normal (no achondroplasia) Dwarf (achondroplasia)
Parents Normal (no achondroplasia) Dwarf (achondroplasia) dd Dd d d Sperm Dd Dd D Dwarf Dwarf Eggs dd dd d Normal Normal Molly Jo Matt Amy Zachary Jake Jeremy Figure 9.16 Figure 9.16 A family with and without achondroplasia.

The Process of Science: What Is the Genetic Basis of Hairless Dogs?
Observation: Dogs come in a wide variety of physical types. Question: What is the genetic basis for the hairless phenotype? Hypothesis: A comparison of genes of coated and hairless dogs would identify the gene or genes responsible. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Prediction: A mutation in a single gene accounts for the hairless appearance.
Experiment: Compared DNA sequences of 140 hairless dogs from 3 breeds with 87 coated dogs from 22 breeds. Results: Every hairless dog, but no coated dogs, had a single change in a single 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

Figure 9.17 Figure 9.17 Hairless versus coated Chinese crested dogs.

Genetic Testing Today many tests can detect the presence of disease-causing alleles. Most genetic testing is performed during pregnancy. Amniocentesis collects cells from amniotic fluid. Chorionic villus sampling removes cells from placental tissue. Genetic counseling helps patients understand the results and implications of genetic testing. 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 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. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) a PP x pp and (b) Pp x pp. 4. 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. 5. 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. 6. Students also seem to learn much from Figure 9.13 by analyzing the possible genotypes for the people whose complete genotype is not known. Consider challenging your students to suggest the possible genotypes for these people. 7. The 2⁄3 fraction noted in the discussion of carriers of recessive disorder (and in Figure 9.14) often catches students off guard as they are 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 they have the Huntington’s allele. The test is especially important to the children of a parent with Huntington’s disease. Consider asking your class: (1) what are the odds of developing Huntington’s disease if a parent has this disease (50%) and (2) 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. 9. As a simply 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.

VARIATIONS ON MENDEL’S LAWS
Some patterns of genetic inheritance are not explained by Mendel’s laws. 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. 2. In larger classes, the chances increase that at least one student has a family member with one of the genetic disorders discussed. Some students may find this embarrassing while others might have a special interest in learning more about these potentially personal topics. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. 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 and Blue 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 is the AB blood type. No socks, as already noted, represent 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 Associations’ website address ( 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.

Incomplete Dominance in Plants and People
In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents. 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. 2. In larger classes, the chances increase that at least one student has a family member with one of the genetic disorders discussed. Some students may find this embarrassing while others might have a special interest in learning more about these potentially personal topics. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. 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 and Blue 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 is the AB blood type. No socks, as already noted, represent 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 Associations’ website address ( 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.

P Generation Red White RR rr Gametes R r F1 Generation Pink Rr 1 2 1 2
Sperm 1 2 1 2 R r 1 2 R RR Rr Eggs 1 2 r Rr rr Figure Figure 9.18 Incomplete dominance in snapdragons. (Step 3)

Hypercholesterolemia
Is characterized by dangerously high levels of cholesterol in the blood. Is a human trait that is incompletely dominant. Heterozygotes have blood cholesterol levels about twice normal. Homozygotes have blood cholesterol levels about five times normal. 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. 2. In larger classes, the chances increase that at least one student has a family member with one of the genetic disorders discussed. Some students may find this embarrassing while others might have a special interest in learning more about these potentially personal topics. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. 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 and Blue 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 is the AB blood type. No socks, as already noted, represent 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 Associations’ website address ( 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.

HH Hh hh Homozygous for ability to make LDL receptors Heterozygous
for inability to make LDL receptors GENOTYPE LDL LDL receptor PHENOTYPE Cell Normal Mild disease Severe disease Figure 9.19 Figure 9.19 Incomplete dominance in human hypercholesterolemia.

ABO Blood Groups: An Example of Multiple Alleles and Codominance
The ABO blood groups in humans are an example of multiple alleles. © 2010 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. 2. In larger classes, the chances increase that at least one student has a family member with one of the genetic disorders discussed. Some students may find this embarrassing while others might have a special interest in learning more about these potentially personal topics. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. 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 and Blue 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 is the AB blood type. No socks, as already noted, represent 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 Associations’ website address ( 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.

Figure 9.20 Blood Group (Phenotype) Antibodies Present in Blood
Reactions When Blood from Groups Below Is Mixed with Antibodies from Groups at Left Genotypes Red Blood Cells O A B AB Carbohydrate A IAIA or IAi A Anti-B Carbohydrate B IBIB or IBi B Anti-A AB IAIB — Anti-A Anti-B O ii Figure 9.20 Figure 9.20 Multiple alleles for the ABO blood groups.

Blood Group (Phenotype) Genotypes Red Blood Cells Carbohydrate A IAIA
or IAi A Carbohydrate B IBIB or IBi B AB IAIB O ii Figure 9.20a Figure 9.20a Multiple alleles for the ABO blood groups.

Reactions When Blood from Groups Below Is
Antibodies Present in Blood Reactions When Blood from Groups Below Is Mixed with Antibodies from Groups at Left O A B AB Anti-B Anti-A — Anti-A Anti-B Figure 9.20b Figure 9.20b Multiple alleles for the ABO blood groups.

Figure 9.20 Figure 9.20 A laboratory test to determine blood type

The clumping reaction is the basis of a blood-typing lab test.
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. 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. 2. In larger classes, the chances increase that at least one student has a family member with one of the genetic disorders discussed. Some students may find this embarrassing while others might have a special interest in learning more about these potentially personal topics. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. 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 and Blue 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 is the AB blood type. No socks, as already noted, represent 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 Associations’ website address ( 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.

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.

Figure 9.21 Figure 9.21 Sickle-cell disease, multiple effects of a single human gene.

The human blood type alleles IA and IB exhibit codominance: Both alleles are expressed in the phenotype. 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. 2. In larger classes, the chances increase that at least one student has a family member with one of the genetic disorders discussed. Some students may find this embarrassing while others might have a special interest in learning more about these potentially personal topics. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. 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 and Blue 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 is the AB blood type. No socks, as already noted, represent 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 Associations’ website address ( 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.

Pleiotropy and Sickle-Cell Disease
Pleiotropy is the impact of a single gene on more than one character. Sickle-cell disease Exhibits pleiotropy Results in abnormal hemoglobin production Causes disk-shaped red blood cells to deform into a sickle shape with jagged edges 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. 2. In larger classes, the chances increase that at least one student has a family member with one of the genetic disorders discussed. Some students may find this embarrassing while others might have a special interest in learning more about these potentially personal topics. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. 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 and Blue 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 is the AB blood type. No socks, as already noted, represent 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 Associations’ website address ( 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.

Figure 9.22 P Generation aabbcc (very light) AABBCC (very dark)
F1 Generation AaBbCc AaBbCc F2 Generation Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 64 20 1 8 1 8 64 15 Eggs 1 8 Fraction of population 1 8 64 6 1 8 1 8 1 64 Skin pigmentation 1 64 64 6 64 15 64 20 64 15 64 6 1 64 Figure 9.22 Figure 9.22 A model for polygenic inheritance of skin color.

Figure 9.22a P Generation aabbcc (very light) AABBCC (very dark)
F1 Generation AaBbCc AaBbCc F2 Generation Sperm 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 1 8 Eggs 1 8 1 8 1 8 1 8 1 64 64 6 64 15 64 20 64 15 64 6 1 64 Figure 9.22a Figure 9.22a A model for polygenic inheritance of skin color.

Fraction of population
64 20 64 15 Fraction of population 64 6 64 1 Skin pigmentation Figure 9.22b Figure 9.22b A model for polygenic inheritance of skin color.

Polygenic Inheritance
Polygenic inheritance is the additive effects of two or more genes on a single phenotype. 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. 2. In larger classes, the chances increase that at least one student has a family member with one of the genetic disorders discussed. Some students may find this embarrassing while others might have a special interest in learning more about these potentially personal topics. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. 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 and Blue 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 is the AB blood type. No socks, as already noted, represent 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 Associations’ website address ( 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.

Figure 9.23 Figure 9.23 As a result of environmental influences, even identical twins can look different.

The Role of Environment
Many human characters result from a combination of heredity and environment. Only genetic influences are inherited. 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. 2. In larger classes, the chances increase that at least one student has a family member with one of the genetic disorders discussed. Some students may find this embarrassing while others might have a special interest in learning more about these potentially personal topics. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. 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 and Blue 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 is the AB blood type. No socks, as already noted, represent 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 Associations’ website address ( 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.

All round-yellow seeds
P Generation Round-yellow seeds (RRYY) Wrinkled-green seeds (rryy) Y r y R Y R r y MEIOSIS FERTILIZATION Gametes R Y y r F1 Generation All round-yellow seeds (RrYy) R r y Law of Segregation: Follow the long chromosomes (carrying R and r) taking either the left or right branch. Law of Independent Assortment: Follow both the long and the short chromosomes. Y MEIOSIS R r Metaphase I (alternative arrangements) r R They are arranged in either of two equally likely ways at metaphase I. The R and r alleles segregate in anaphase I of meiosis. Y y Y y Metaphase II R r 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 4 1 4 1 4 1 4 1 RY ry rY Ry Figure Figure 9.24 The chromosomal basis of Mendel's laws. (Step 3)

All round-yellow seeds
P Generation Round-yellow seeds (RRYY) Wrinkled-green seeds (rryy) Y r y R Y R r y MEIOSIS FERTILIZATION Gametes R Y y r F1 Generation All round-yellow seeds (RrYy) R r y Law of Segregation: Follow the long chromosomes (carrying R and r) taking either the left or right branch. Law of Independent Assortment: Follow both the long and the short chromosomes. Y MEIOSIS R r Metaphase I (alternative arrangements) r R They are arranged in either of two equally likely ways at metaphase I. The R and r alleles segregate in anaphase I of meiosis. Y y Y y Metaphase II R r 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 4 1 4 1 4 1 4 1 RY ry rY 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 Figure 9.24 The chromosomal basis of Mendel's laws. (Step 4)

THE CHROMOSOMAL BASIS OF INHERITANCE
The chromosome theory of inheritance states that Genes are located at specific positions on chromosomes The behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns It is chromosomes that undergo segregation and independent assortment during meiosis and thus account for Mendel’s laws. Student Misconceptions and Concerns 1. This section of the chapter relies upon 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 upon our natural expectations that items that are closely 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. 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 Gone With the Wind. Clearly, the closer two frames of film are together, the more likely they are to move or remain together.

Gray body, long wings (wild-type) Black body, short wings (mutant)
Dihybrid testcross Gray body, long wings (wild-type) Black body, short wings (mutant) GgLl ggll Female Male Figure Figure 9.25 Thomas Morgan's experiment and results.

Recombinant phenotypes 17%
Dihybrid testcross Gray body, long wings (wild-type) Black body, short wings (mutant) GgLl ggll Female Male Results Offspring Gray-long GgLl Black-short ggll Gray-short Ggll Black-long ggLl 965 944 206 185 Parental phenotypes 83% Recombinant phenotypes 17% Figure Figure 9.25 Thomas Morgan's experiment and results.

Linked Genes Linked genes
Are located close together on a chromosome May be inherited together Using the fruit fly Drosophila melanogaster, Thomas Hunt Morgan determined that some genes were linked based on the inheritance patterns of their traits. Student Misconceptions and Concerns 1. This section of the chapter relies upon 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 upon our natural expectations that items that are closely 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. 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 Gone With the Wind. Clearly, the closer two frames of film are together, the more likely they are to move or remain together.

Pair of homologous chromosomes
B a b A B Parental gametes a b A b a B Pair of homologous chromosomes Crossing over Recombinant gametes Figure 9.26 Figure 9.26 Review: Crossing over can produce recombinant gametes.

Genetic Recombination: Crossing Over
Crossing over can Separate linked alleles Produce gametes with recombinant chromosomes Produce offspring with recombinant phenotypes Student Misconceptions and Concerns 1. This section of the chapter relies upon 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 upon our natural expectations that items that are closely 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. 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 Gone With the Wind. Clearly, the closer two frames of film are together, the more likely they are to move or remain together.

FERTILIZATION G L g l GgLl (female) ggll (male) g l g l Crossing over
Sperm Parental gametes Recombinant gametes Eggs FERTILIZATION Offspring G L g l G l g L g l g l g l g l Parental Recombinant Figure 9.27 Figure 9.27 Explaining the unexpected results from the dihybrid testcross in Figure 9.25.

ggll (male) GgLl (female)
Crossing over G L g l G l g L g l Sperm Parental gametes Recombinant gametes Eggs Figure 9.27a Figure 9.27a Explaining the unexpected results from the dihybrid testcross in Figure 9.25.

G L g l G l g L g l Sperm Parental gametes Recombinant gametes Eggs
FERTILIZATION Offspring G L g l G l g L g l g l g l g l Parental Recombinant Figure 9.27b Figure 9.27b Explaining the unexpected results from the dihybrid testcross in Figure 9.25.

The percentage of recombinant offspring among the total is called the recombination frequency.
Student Misconceptions and Concerns 1. This section of the chapter relies upon 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 upon our natural expectations that items that are closely 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. 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 Gone With the Wind. Clearly, the closer two frames of film are together, the more likely they are to move or remain together.

Recombination frequencies
Chromosome g c l 17% 9% 9.5% Recombination frequencies Figure 9.28 Figure 9.28 Using crossover data to map genes.

Linkage Maps Early studies of crossing over were performed using the fruit fly Drosophila melanogaster. Alfred H. Sturtevant, a student of Morgan, developed a method for mapping gene loci, which resulted in the creation of linkage maps. A diagram of relative gene locations on a chromosome is a linkage map. Student Misconceptions and Concerns 1. This section of the chapter relies upon 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 upon our natural expectations that items that are closely 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. 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 Gone With the Wind. Clearly, the closer two frames of film are together, the more likely they are to move or remain together.

44  XY 44  XX Somatic cells 22  X 22  Y 22  X 44  XX 44  XY
Male Female 44  XY 44  XX Somatic cells 22  X 22  Y 22  X Sperm Egg 44  XX 44  XY Offspring Female Male Figure 9.29 Figure 9.29 The chromosomal basis of sex determination.

Y X Colorized SEM Figure 9.29
Figure 9.29 The chromosomal basis of sex determination.

SEX CHROMOSOMES AND SEX-LINKED GENES
Sex chromosomes influence the inheritance of certain traits. Student Misconceptions and Concerns 1. The prior 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. 2. The likelihood that at least some students are color-blind 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. 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. 2. For additional information about hemophilia, consider visiting the Website of the National Hemophilia Foundation at

Sex Determination in Humans
Nearly all mammals have a pair of sex chromosomes designated X and Y. Males have an X and Y. Females have XX. Student Misconceptions and Concerns 1. The prior 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. 2. The likelihood that at least some students are color-blind 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. 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. 2. For additional information about hemophilia, consider visiting the Website of the National Hemophilia Foundation at

Sex-Linked Genes Any gene located on a sex chromosome is called a sex-linked gene. Most sex-linked genes are found on the X chromosome. Red-green color blindness is a common human sex-linked disorder. Student Misconceptions and Concerns 1. The prior 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. 2. The likelihood that at least some students are color-blind 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. 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. 2. For additional information about hemophilia, consider visiting the Website of the National Hemophilia Foundation at

Figure 9.30 Figure 9.30 A test for red-green colorblindness.

XNXN XnY XNXn XNY XNXn XnY Xn Y XN Y Xn Y XNXN XNY XN XNXn XNY XN XN
Sperm Sperm Sperm Xn Y XN Y Xn Y XNXN XNY XN XNXn XNY XN XN XNXn XNY Eggs Eggs Eggs XN XNXn XNY Xn XNXn XnY Xn XnXn XnY (a) (b) (c) Normal female  colorblind male Carrier female  normal male Carrier female  colorblind male Key Unaffected individual Carrier Colorblind individual Figure 9.31 Figure 9.31 Inheritance of colorblindness, a sex-linked recessive trait.

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 Student Misconceptions and Concerns 1. The prior 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. 2. The likelihood that at least some students are color-blind 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. 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. 2. For additional information about hemophilia, consider visiting the Website of the National Hemophilia Foundation at

Queen Victoria Czar Nicholas II of Russia
Albert Alice Louis Alexandra Czar Nicholas II of Russia Alexis Figure 9.32 Figure 9.32 Hemophilia in the royal family of Russia.

Evolution Connection: Barking Up the Evolutionary Tree
About 15,000 years ago in East Asia, humans began to cohabit with ancestral canines that were predecessors of modern wolves and dogs. As people settled into geographically distinct populations, different canines became separated and inbred. © 2010 Pearson Education, Inc. Student Misconceptions and Concerns 1. The prior 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. 2. The likelihood that at least some students are color-blind 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. 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. 2. For additional information about hemophilia, consider visiting the Website of the National Hemophilia Foundation at

In 2005 researchers sequenced the complete genome of a dog.
An evolutionary tree of dog breeds was created. Student Misconceptions and Concerns 1. The prior 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. 2. The likelihood that at least some students are color-blind 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. 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. 2. For additional information about hemophilia, consider visiting the Website of the National Hemophilia Foundation at

Ancestral canine Alaskan malamute
Wolf Chinese shar-pei Ancestral canine Siberian husky Akita Alaskan malamute Basenji Afghan hound Saluki Rottweiler Sheepdog Retriever Figure 9.33 Figure 9.33 An evolutionary tree of dog breeds.

(allele pairs separate)
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 separate) Figure 9.UN1 Figure 9.UN1 Summary: Law of Segregation

Phenotype Dominant Recessive Genotype pp P ? or Phenotype All dominant
Conclusion Unknown parent is PP Unknown parent is Pp Figure 9.UN2 Figure 9.UN2 Summary: Testcross

Intermediate phenotype (incomplete dominance)
Dominant phenotype (RR) Recessive phenotype (rr) Intermediate phenotype (incomplete dominance) (Rr) Figure 9.UN3 Figure 9.UN3 Summary: Incomplete Dominance

Pleiotropy Single gene Multiple traits (e.g., sickle-cell disease)
Figure 9.UN4 Figure 9.UN4 Summary: Pleiotropy

Polygenic inheritance Single trait (e.g., skin color) Multiple genes
Figure 9.UN5 Figure 9.UN5 Summary:Polygenic Inheritance

Male Female 44  XY 44  XX Somatic cells Figure 9.UN6
Figure 9.UN6 Summary: Sex Determination in Humans

Figure 9.UN7 Figure 9.UN7 Summary: Sex Linked Traits

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