Patterns of Inheritance

Patterns of Inheritance
Chapter 9 Patterns of Inheritance

Introduction Dogs are one of man’s longest genetic experiments.
Dogs are one of man’s longest genetic experiments. Over thousands of years, humans have chosen and mated dogs with specific traits. The result has been an incredibly diverse array of dogs with distinct body types and behavioral traits. © 2012 Pearson Education, Inc. 2

Variations on Mendel’s Laws
Figure 9.0_1 Chapter 9: Big Ideas Mendel’s Laws Variations on Mendel’s Laws Figure 9.0_1 Chapter 9: Big Ideas The Chromosomal Basis of Inheritance Sex Chromosomes and Sex-Linked Genes 3

Figure 9.0_2 Figure 9.0_2 Labrador retriever puppy 4

9.1 The science of genetics has ancient roots
Pangenesis, proposed around 400 BCE by Hippocrates, was an early explanation for inheritance that suggested that particles called pangenes came from all parts of the organism to be incorporated into eggs or sperm and characteristics acquired during the parents’ lifetime could be transferred to the offspring. Aristotle rejected pangenesis and argued that instead of particles, the potential to produce the traits was inherited. Teaching Tips 1. As you begin your lectures on genetics, consider challenging your students to explain why the theories of pangenesis and blending are incorrect. Perhaps just pick one of the two. You might even ask for short responses from everyone at the start of class or as an assignment before the first lectures. In addition to arousing interest in the answers, the responses should reveal the diverse backgrounds of your students entering this discussion and reveal any preexisting confusion on the subject of genetics. 2. The concept of pangenesis is analogous to the structure of United States representation in Congress. Each congressional district sends a congressman or congresswoman (pangene) to the U.S. House of Representatives (gamete). There, all parts of the United States (body) are represented. 3. In this or future lectures addressing evolution, you may mention that pangenesis was a mechanism consistent with Lamarckian evolution. © 2012 Pearson Education, Inc. 6

Figure 9.1 Figure 9.1 Aristotle 7

9.1 The science of genetics has ancient roots
The idea that hereditary materials mix in forming offspring, called the blending hypothesis, was suggested in the 19th century by scientists studying plants but later rejected because it did not explain how traits that disappear in one generation can reappear in later generations. Teaching Tips 1. As you begin your lectures on genetics, consider challenging your students to explain why the theories of pangenesis and blending are incorrect. Perhaps just pick one of the two. You might even ask for short responses from everyone at the start of class or as an assignment before the first lectures. In addition to arousing interest in the answers, the responses should reveal the diverse backgrounds of your students entering this discussion and reveal any preexisting confusion on the subject of genetics. 2. The concept of pangenesis is analogous to the structure of United States representation in Congress. Each congressional district sends a congressman or congresswoman (pangene) to the U.S. House of Representatives (gamete). There, all parts of the United States (body) are represented. 3. In this or future lectures addressing evolution, you may mention that pangenesis was a mechanism consistent with Lamarckian evolution. © 2012 Pearson Education, Inc. 8

9.2 Experimental genetics began in an abbey garden
Heredity is the transmission of traits from one generation to the next. Genetics is the scientific study of heredity. Gregor Mendel began the field of genetics in the 1860s, deduced the principles of genetics by breeding garden peas, and relied upon a background of mathematics, physics, and chemistry. Student Misconceptions and Concerns 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 supported 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, rather than the possibility for such gradual inheritance. Teaching Tips 1. In Module 9.2, the authors make the analogy between genes and playing cards, noting that each are shuffled but retain their original identity. This analogy may form a very useful reference point for your students and can be used later, as new principles of genetics are discussed. 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. © 2012 Pearson Education, Inc. 9

In 1866, Mendel correctly argued that parents pass on to their offspring discrete “heritable factors” and stressed that the heritable factors (today called genes), retain their individuality generation after generation. A heritable feature that varies among individuals, such as flower color, is called a character. Each variant for a character, such as purple or white flowers, is a trait. Student Misconceptions and Concerns 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 supported 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, rather than the possibility for such gradual inheritance. Teaching Tips 1. In Module 9.2, the authors make the analogy between genes and playing cards, noting that each are shuffled but retain their original identity. This analogy may form a very useful reference point for your students and can be used later, as new principles of genetics are discussed. 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. © 2012 Pearson Education, Inc. 10

Figure 9.2A Figure 9.2A Gregor Mendel 11

True-breeding varieties result when self-fertilization produces offspring all identical to the parent. The offspring of two different varieties are hybrids. The cross-fertilization is a hybridization, or genetic cross. True-breeding parental plants are the P generation. Hybrid offspring are the F1 generation. A cross of F1 plants produces an F2 generation. Student Misconceptions and Concerns 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 supported 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, rather than the possibility for such gradual inheritance. Teaching Tips 1. In Module 9.2, the authors make the analogy between genes and playing cards, noting that each are shuffled but retain their original identity. This analogy may form a very useful reference point for your students and can be used later, as new principles of genetics are discussed. 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. © 2012 Pearson Education, Inc. 12

Petal Carpel Stamen Figure 9.2B
Figure 9.2B The anatomy of a garden pea flower (with one petal removed to improve visibility) Stamen 13

White Removal of stamens Stamens Carpel Transfer of pollen Parents (P)
Figure 9.2C_s1 White 1 Removal of stamens Stamens Carpel 2 Transfer of pollen Parents (P) Purple Figure 9.2C_s1 Mendel’s technique for cross-fertilization of pea plants (step 1) 14

Carpel matures into pea pod
Figure 9.2C_s2 White 1 Removal of stamens Stamens Carpel 2 Transfer of pollen Parents (P) Purple 3 Carpel matures into pea pod Figure 9.2C_s2 Mendel’s technique for cross-fertilization of pea plants (step 2) 15

Parents (P) Offspring (F1)
Figure 9.2C_s3 White 1 Removal of stamens Stamens Carpel 2 Transfer of pollen Parents (P) Purple 3 Carpel matures into pea pod 4 Seeds from pod planted Figure 9.2C_s3 Mendel’s technique for cross-fertilization of pea plants (step 3) Offspring (F1) 16

Figure 9.2D Character Traits Dominant Recessive Flower color Purple White Flower position Axial Terminal Seed color Yellow Green Seed shape Round Wrinkled Pod shape Inflated Constricted Figure 9.2D The seven pea characters studied by Mendel Pod color Green Yellow Stem length Tall Dwarf 17

Character Traits Dominant Recessive Flower color Purple White
Figure 9.2D_1 Character Traits Dominant Recessive Flower color Purple White Flower position Axial Terminal Figure 9.2D_1 The seven pea characters studied by Mendel (part 1) Seed color Yellow Green Seed shape Round Wrinkled 18

Character Traits Dominant Recessive Pod shape Inflated Constricted
Figure 9.2D_2 Character Traits Dominant Recessive Pod shape Inflated Constricted Pod color Green Yellow Figure 9.2D_2 The seven pea characters studied by Mendel (part 2) Stem length Tall Dwarf 19

9.3 Mendel’s law of segregation describes the inheritance of a single character
A cross between two individuals differing in a single character is a monohybrid cross. Mendel performed a monohybrid cross between a plant with purple flowers and a plant with white flowers. The F1 generation produced all plants with purple flowers. A cross of F1 plants with each other produced an F2 generation with ¾ purple and ¼ white flowers. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips 1. 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. 2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP  pp and (b) Pp  pp. 3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous). 4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”). © 2012 Pearson Education, Inc. 20

Purple flowers White flowers
Figure 9.3A_s1 The Experiment P generation (true-breeding parents)  Purple flowers White flowers Figure 9.3A_s1 Crosses tracking one character (flower color) (step 1) 21

Purple flowers White flowers
Figure 9.3A_s2 The Experiment P generation (true-breeding parents)  Purple flowers White flowers F1 generation All plants have purple flowers Figure 9.3A_s2 Crosses tracking one character (flower color) (step 2) 22

of plants have purple flowers of plants have white flowers
Figure 9.3A_s3 The Experiment P generation (true-breeding parents)  Purple flowers White flowers F1 generation All plants have purple flowers Fertilization among F1 plants (F1  F1) Figure 9.3A_s3 Crosses tracking one character (flower color) (step 3) F2 generation of plants have purple flowers 3 4 of plants have white flowers 1 4 23

The all-purple F1 generation did not produce light purple flowers, as predicted by the blending hypothesis. Mendel needed to explain why white color seemed to disappear in the F1 generation and white color reappeared in one quarter of the F2 offspring. Mendel observed the same patterns of inheritance for six other pea plant characters. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips 1. 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. 2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP  pp and (b) Pp  pp. 3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous). 4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”). © 2012 Pearson Education, Inc. 24

Mendel developed four hypotheses, described below using modern terminology. 1. Alleles are alternative versions of genes that account for variations in inherited characters. 2. For each characteristic, an organism inherits two alleles, one from each parent. The alleles can be the same or different. A homozygous genotype has identical alleles. A heterozygous genotype has two different alleles. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips 1. 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. 2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP  pp and (b) Pp  pp. 3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous). 4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”). © 2012 Pearson Education, Inc. 25

If the alleles of an inherited pair differ, then one determines the organism’s appearance and is called the dominant allele. The other has no noticeable effect on the organism’s appearance and is called the recessive allele. The phenotype is the appearance or expression of a trait. The genotype is the genetic makeup of a trait. The same phenotype may be determined by more than one genotype. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips 1. 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. 2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP  pp and (b) Pp  pp. 3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous). 4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”). © 2012 Pearson Education, Inc. 26

A sperm or egg carries only one allele for each inherited character because allele pairs separate (segregate) from each other during the production of gametes. This statement is called the law of segregation. Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation. The F1 hybrids all have a Pp genotype. A Punnett square shows the four possible combinations of alleles that could occur when these gametes combine. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips 1. 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. 2. Many students benefit from a little quick practice with a Punnett square. Have them try these crosses for practice: (a) PP  pp and (b) Pp  pp. 3. For students struggling with basic terminology, an analogy between a genetic trait and a pair of shoes might be helpful. A person might wear a pair of shoes in which both shoes match (homozygous), or less likely, a person might wear shoes that do not match (heterozygous). 4. Another analogy that might help struggling students is a pair of people trying to make a decision about where to eat tonight. One person wants to eat at a restaurant, the other wants to eat a meal at home. This (heterozygous) couple eats at home (the dominant allele “wins”). © 2012 Pearson Education, Inc. 27

Genetic makeup (alleles)
Figure 9.3B_s1 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P All p Figure 9.3B_s1 An explanation of the crosses in Figure 9.3A (step 1) 28

Figure 9.3B_s2 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P All p F1 generation (hybrids) All Pp Gametes 2 1 P 2 1 p Figure 9.3B_s2 An explanation of the crosses in Figure 9.3A (step 2) 29

Figure 9.3B_s3 The Explanation P generation Genetic makeup (alleles) Purple flowers White flowers PP pp Gametes All P All p F1 generation (hybrids) All Pp Alleles segregate Gametes 2 1 P 2 1 p Fertilization Figure 9.3B_s3 An explanation of the crosses in Figure 9.3A (step 3) F2 generation Sperm from F1 plant P p Phenotypic ratio 3 purple : 1 white P PP Pp Eggs from F1 plant Genotypic ratio 1 PP : 2 Pp : 1 pp p Pp pp 30

Phenotypic ratio 3 purple : 1 white P PP Pp Eggs from F1 plant
Figure 9.3B_4 F2 generation Sperm from F1 plant P p Phenotypic ratio 3 purple : 1 white P PP Pp Eggs from F1 plant Genotypic ratio 1 PP : 2 Pp : 1 pp p Pp pp Figure 9.3B_4 An explanation of the crosses in Figure 9.3A (F2 generation) 31

9.4 Homologous chromosomes bear the alleles for each character
A locus (plural, loci) is the specific location of a gene along a chromosome. For a pair of homologous chromosomes, alleles of a gene reside at the same locus. Homozygous individuals have the same allele on both homologues. Heterozygous individuals have a different allele on each homologue. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips Figure 9.4 can be of great benefit when introducing genetic terminology. For students struggling to think abstractly, such a visual aid may be essential when describing these features in lecture. © 2012 Pearson Education, Inc. 32

Homologous chromosomes
Figure 9.4 Gene loci Dominant allele P a B Homologous chromosomes P a b Recessive allele Genotype: PP aa Bb Figure 9.4 Three gene loci on homologous chromosomes Homozygous for the dominant allele Homozygous for the recessive allele Heterozygous, with one dominant and one recessive allele 33

9.5 The law of independent assortment is revealed by tracking two characters at once
A dihybrid cross is a mating of parental varieties that differ in two characters. Mendel performed the following dihybrid cross with the following results: P generation: round yellow seeds  wrinkled green seeds F1 generation: all plants with round yellow seeds F2 generation: 9/16 had round yellow seeds 3/16 had wrinkled yellow seeds 3/16 had round green seeds 1/16 had wrinkled green seeds Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips 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 occurring simultaneously. © 2012 Pearson Education, Inc. 34

Figure 9.5A Two hypotheses for segregation in a dihybrid cross
P generation RRYY rryy Gametes RY  ry Sperm F1 generation RrYy 4 1 RY 4 1 rY 4 1 Ry 4 1 ry Sperm 2 1 RY 2 1 ry 4 1 RY RRYY RrYY RRYy RrYy 2 1 RY 4 1 rY 16 9 Yellow round F2 generation Eggs RrYY rrYY RrYy rrYy Eggs 2 1 ry 4 1 16 3 Green round Ry Figure 9.5A Two hypotheses for segregation in a dihybrid cross RRYy RrYy RRyy Rryy 16 3 Yellow wrinkled 4 1 ry 16 1 Green wrinkled RrYy rrYy Rryy rryy The hypothesis of dependent assortment Data did not support; hypothesis refuted The hypothesis of independent assortment Actual results; hypothesis supported 35

P generation RRYY rryy Gametes RY  ry F1 generation RrYy
Figure 9.5A_1 P generation RRYY rryy Gametes RY  ry F1 generation RrYy Figure 9.5A_1 Two hypotheses for segregation in a dihybrid cross (part 1) 36

F1 generation RrYy Sperm RY ry RY F2 generation Eggs ry
Figure 9.5A_2 F1 generation RrYy Sperm 2 1 RY 2 1 ry 2 1 RY F2 generation Eggs 2 1 ry Figure 9.5A_2 Two hypotheses for segregation in a dihybrid cross (part 2) The hypothesis of dependent assortment Data did not support; hypothesis refuted 37

F1 generation RrYy Sperm RY rY Ry ry RY RRYY RrYY RRYy RrYy rY
Figure 9.5A_3 F1 generation RrYy Sperm 4 1 RY 4 1 4 1 4 1 rY Ry ry 4 1 RY RRYY RrYY RRYy RrYy 4 1 rY 16 9 Yellow round RrYY rrYY RrYy rrYy Eggs 16 3 4 1 Green round Ry Figure 9.5A_3 Two hypotheses for segregation in a dihybrid cross (part 3) RRYy RrYy RRyy Rryy 16 3 Yellow wrinkled 4 1 ry 16 1 Green wrinkled RrYy rrYy Rryy rryy The hypothesis of independent assortment Actual results; hypothesis supported 38

Figure 9.5B Blind Blind Phenotypes Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn Genotypes Mating of double heterozygotes (black coat, normal vision) BbNn  BbNn Blind Blind Figure 9.5B Independent assortment of two genes in the Labrador retriever Phenotypic ratio of the offspring 9 Black coat, normal vision 3 Black coat, blind (PRA) 3 Chocolate coat, normal vision 1 Chocolate coat, blind (PRA) 39

Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn
Figure 9.5B_1 Blind Phenotypes Black coat, normal vision B_N_ Black coat, blind (PRA) B_nn Genotypes Blind Figure 9.5B_1 Independent assortment of two genes in the Labrador retriever (part 1) Phenotypes Chocolate coat, normal vision bbN_ Chocolate coat, blind (PRA) bbnn Genotypes 40

Mating of double heterozygotes (black coat, normal vision) BbNn  BbNn
Figure 9.5B_2 Mating of double heterozygotes (black coat, normal vision) BbNn  BbNn Blind Blind Phenotypic ratio of the offspring 9 Black coat, normal vision 3 Black coat, blind (PRA) 3 Chocolate coat, normal vision 1 Chocolate coat, blind (PRA) Figure 9.5B_2 Independent assortment of two genes in the Labrador retriever (part 2) 41

Mendel needed to explain why the F2 offspring had new nonparental combinations of traits and a 9:3:3:1 phenotypic ratio. Mendel suggested that the inheritance of one character has no effect on the inheritance of another, suggested that the dihybrid cross is the equivalent to two monohybrid crosses, and called this the law of independent assortment. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips 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 occurring simultaneously. © 2012 Pearson Education, Inc. 42

The following figure demonstrates the law of independent assortment as it applies to two characters in Labrador retrievers: black versus chocolate color, normal vision versus progressive retinal atrophy. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips 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 occurring simultaneously. © 2012 Pearson Education, Inc. 43

9.6 Geneticists can use the testcross to determine unknown genotypes
A testcross is the mating between an individual of unknown genotype and a homozygous recessive individual. A testcross can show whether the unknown genotype includes a recessive allele. Mendel used testcrosses to verify that he had true-breeding genotypes. The following figure demonstrates how a testcross can be performed to determine the genotype of a Lab with normal eyes. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips Consider challenging your students to explain why a testcross of two black Labs of unknown genotypes might not reveal the genotype of each dog. (If both dogs are heterozygous, or homozygous, the results would reveal the genotypes because the offspring would either be three dark and one brown or all dark. But if one black Lab was homozygous and the other heterozygous, we could not determine which Lab has which genotype.) © 2012 Pearson Education, Inc. 44

What is the genotype of the black dog?
Figure 9.6 What is the genotype of the black dog? Testcross  Genotypes B_? bb Two possibilities for the black dog: BB or Bb Figure 9.6 Using a testcross to determine genotype Gametes B B b b Bb b Bb bb Offspring All black 1 black : 1 chocolate 45

9.7 Mendel’s laws reflect the rules of probability
Using his strong background in mathematics, Mendel knew that the rules of mathematical probability affected the segregation of allele pairs during gamete formation and the re-forming of pairs at fertilization. The probability scale ranges from 0 to 1. An event that is certain has a probability of 1 and certain not to occur has a probability of 0. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. © 2012 Pearson Education, Inc. 46

9.7 Mendel’s laws reflect the rules of probability
The probability of a specific event is the number of ways that event can occur out of the total possible outcomes. Determining the probability of two independent events uses the rule of multiplication, in which the probability is the product of the probabilities for each event. The probability that an event can occur in two or more alternative ways is the sum of the separate probabilities, called the rule of addition. Student Misconceptions and Concerns Students using Punnett squares need to be reminded that the calculations are expected statistical probabilities and not absolutes. We would expect that any six playing cards dealt might be half black and half red, but 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 will reflect expected ratios. Teaching Tips Many students have trouble with the basic statistics that are necessary for many of these calculations. Give your students some practice. Consider having them work in pairs, each with a pair of dice (for large class sizes, this can be done in laboratories). Let them calculate the odds of rolling three sixes in a row and other possibilities. © 2012 Pearson Education, Inc. 47

F1 genotypes Bb female Bb male Formation of eggs Formation of sperm B
Figure 9.7 F1 genotypes Bb female Bb male Formation of eggs Formation of sperm 2 1 2 1 B b Sperm 2 1 2 1 ( )  2 1 B B B b B Figure 9.7 Segregation and fertilization as chance events 4 1 4 1 F2 genotypes Eggs b B b b 2 1 b 4 1 4 1 48

9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
In a simple dominant-recessive inheritance of dominant allele A and recessive allele a, a recessive phenotype always results from a homozygous recessive genotype (aa) but a dominant phenotype can result from either the homozygous dominant genotype (AA) or a heterozygous genotype (Aa). Wild-type traits, those prevailing in nature, are not necessarily specified by dominant alleles. Student Misconceptions and Concerns 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. Teaching Tips Students seem to learn much from Figure 9.8b 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, perhaps during lecture. © 2012 Pearson Education, Inc. 49

Dominant Traits Recessive Traits Freckles No freckles Widow’s peak
Figure 9.8A Dominant Traits Recessive Traits Freckles No freckles Figure 9.8A Examples of single-gene inherited traits in humans Widow’s peak Straight hairline Free earlobe Attached earlobe 50

Figure 9.8A_1 Freckles Figure 9.8A_1 Examples of single-gene inherited traits in humans (part 1) 51

Figure 9.8A_2 No freckles Figure 9.8A_2 Examples of single-gene inherited traits in humans (part 2) 52

Figure 9.8A_3 Widow’s peak Figure 9.8A_3 Examples of single-gene inherited traits in humans (part 3) 53

Straight hairline Figure 9.8A_4
Figure 9.8A_4 Examples of single-gene inherited traits in humans (part 4) 54

Figure 9.8A_5 Free earlobe Figure 9.8A_5 Examples of single-gene inherited traits in humans (part 5) 55

Attached earlobe Figure 9.8A_6
Figure 9.8A_6 Examples of single-gene inherited traits in humans (part 6) 56

9.8 CONNECTION: Genetic traits in humans can be tracked through family pedigrees
The inheritance of human traits follows Mendel’s laws. A pedigree shows the inheritance of a trait in a family through multiple generations, demonstrates dominant or recessive inheritance, and can also be used to deduce genotypes of family members. Student Misconceptions and Concerns 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. Teaching Tips Students seem to learn much from Figure 9.8b 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, perhaps during lecture. © 2012 Pearson Education, Inc. 57

First generation (grandparents) Ff Ff ff Ff
Figure 9.8B First generation (grandparents) Ff Ff ff Ff Second generation (parents, aunts, and uncles) FF or Ff ff ff Ff Ff ff Third generation (two sisters) ff FF or Ff Figure 9.8B A pedigree showing the inheritance of attached versus free earlobes in a hypothetical family Female Male Attached Free 58

9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
Inherited human disorders show either recessive inheritance in which two recessive alleles are needed to show disease, heterozygous parents are carriers of the disease-causing allele, and the probability of inheritance increases with inbreeding, mating between close relatives. dominant inheritance in which one dominant allele is needed to show disease and dominant lethal alleles are usually eliminated from the population. Teaching Tips 1. The 2/3 fraction noted in the discussion of carriers of a recessive disorder for deafness often catches students off guard as they are expecting odds of 1/4, 1/2, or 3/4. However, when we eliminate the dd (deaf) possibility, as it would not be a carrier, we have three possible genotypes. Thus, the odds are based out of the remaining three genotypes Dd, dD, and DD. Consider adding this point of clarification to your lecture. 2. As a simple test of comprehension, ask students to explain why lethal alleles are not eliminated from a population. Several possibilities exist: a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. Ask your class a) what the odds are of a person developing Huntington’s disease if a parent has this disease (50%) and b) whether they would want this genetic test if they were a person at risk. 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. © 2012 Pearson Education, Inc. 59

Normal Dd Normal Dd Parents  D Sperm d Dd Normal (carrier) DD Normal
Figure 9.9A Normal Dd Normal Dd Parents  D Sperm d Dd Normal (carrier) DD Normal D Figure 9.9A Offspring produced by parents who are both carriers for a recessive disorder, a type of deafness Eggs Offspring Dd Normal (carrier) dd Deaf d 60

9.9 CONNECTION: Many inherited disorders in humans are controlled by a single gene
The most common fatal genetic disease in the United States is cystic fibrosis (CF), resulting in excessive thick mucus secretions. The CF allele is recessive and carried by about 1 in 31 Americans. Dominant human disorders include achondroplasia, resulting in dwarfism, and Huntington’s disease, a degenerative disorder of the nervous system. Teaching Tips 1. The 2/3 fraction noted in the discussion of carriers of a recessive disorder for deafness often catches students off guard as they are expecting odds of 1/4, 1/2, or 3/4. However, when we eliminate the dd (deaf) possibility, as it would not be a carrier, we have three possible genotypes. Thus, the odds are based out of the remaining three genotypes Dd, dD, and DD. Consider adding this point of clarification to your lecture. 2. As a simple test of comprehension, ask students to explain why lethal alleles are not eliminated from a population. Several possibilities exist: a) The lethal allele might be recessive, persisting in the population due to the survival of carriers, or b) the lethal allele might be dominant, but is not expressed until after the age of reproduction. 3. Ask your class a) what the odds are of a person developing Huntington’s disease if a parent has this disease (50%) and b) whether they would want this genetic test if they were a person at risk. 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. © 2012 Pearson Education, Inc. 61

Table 9.9 Table 9.9 Some Autosomal Disorders in Humans 62

Figure 9.9B Figure 9.9B Dr. Michael C. Ain, a specialist in the repair of bone defects caused by achondroplasia and related disorders 63

9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy
New technologies offer ways to obtain genetic information before conception, during pregnancy, and after birth. Genetic testing can identify potential parents who are heterozygous carriers for certain diseases. Teaching Tips 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 health 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? © 2012 Pearson Education, Inc. 64

Video: Ultrasound of Human Fetus
9.10 CONNECTION: New technologies can provide insight into one’s genetic legacy Several technologies can be used for detecting genetic conditions in a fetus. Amniocentesis extracts samples of amniotic fluid containing fetal cells and permits karyotyping and biochemical tests on cultured fetal cells to detect other conditions, such as Tay-Sachs disease. Chorionic villus sampling removes a sample of chorionic villus tissue from the placenta and permits similar karyotyping and biochemical tests. Teaching Tips 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 health 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? Video: Ultrasound of Human Fetus © 2012 Pearson Education, Inc. 65

Chorionic Villus Sampling (CVS)
Figure 9.10A Amniocentesis Chorionic Villus Sampling (CVS) Amniotic fluid extracted Tissue extracted from the chorionic villi Ultrasound transducer Ultrasound transducer Fetus Fetus Placenta Placenta Uterus Chorionic villi Cervix Cervix Uterus Centrifugation Amniotic fluid Biochemical and genetics tests Fetal cells Fetal cells Several hours Several hours Figure 9.10A Testing a fetus for genetic disorders Cultured cells Several weeks Several weeks Several hours Karyotyping 66

Blood tests on the mother at 14–20 weeks of pregnancy can help identify fetuses at risk for certain birth defects. Fetal imaging enables a physician to examine a fetus directly for anatomical deformities. The most common procedure is ultrasound imaging, using sound waves to produce a picture of the fetus. Newborn screening can detect diseases that can be prevented by special care and precautions. Teaching Tips 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 health 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? © 2012 Pearson Education, Inc. 67

Figure 9.10B Figure 9.10B Ultrasound scanning of a fetus 68

Figure 9.10B_1 Figure 9.10B_1 Ultrasound scanning of a fetus (part 1) 69

Figure 9.10B_2 Figure 9.10B_2 Ultrasound scanning of a fetus (part 2) 70

New technologies raise ethical considerations that include the confidentiality and potential use of results of genetic testing, time and financial costs, and determining what, if anything, should be done as a result of the testing. Teaching Tips 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 health 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? © 2012 Pearson Education, Inc. 71

9.11 Incomplete dominance results in intermediate phenotypes
Mendel’s pea crosses always looked like one of the parental varieties, called complete dominance. For some characters, the appearance of F1 hybrids falls between the phenotypes of the two parental varieties. This is called incomplete dominance, in which neither allele is dominant over the other and expression of both alleles occurs. Student Misconceptions and Concerns 1. After reading the preceding modules, students might expect all traits to be governed by a single gene with two alleles, one dominant over the other. Modules 9.11–9.15 describe deviations from simplistic models of inheritance. 2. As these variations of Mendel’s laws are introduced, students are likely to get confused and become uncertain about the prior definitions. Consider keeping a clear definition of these different patterns of inheritance available for the class to refer to as new patterns are discussed (perhaps as a handout for student reference). 3. As your class size increases, the chances increase that at least one student will have a family member with one of the genetic disorders discussed. Some students may find this embarrassing, but others might have a special interest in learning more about these topics, and may even be willing to share some of their family’s experiences with the class. Teaching Tips 1. Incomplete dominance is analogous to a compromise, or a gray shade. The key concept is that both “sides” have input. Complete dominance is analogous to an authoritarian style, overruling others and insisting on things being a certain way. Although these analogies might seem obvious to instructors, 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 can catch. Heterozygotes for hypercholesterolemia have fewer “fishing poles” for cholesterol. Thus, fewer “fish” are caught and more “fish” remain in the water. © 2012 Pearson Education, Inc. 73

Red RR White rr Pink hybrid Rr
Figure 9.11A P generation  Red RR White rr Gametes R r F1 generation Pink hybrid Rr 2 1 2 1 Gametes R r Figure 9.11A Incomplete dominance in snapdragon flower color F2 generation Sperm 2 1 2 1 R r 2 1 R RR rR Eggs 2 1 Rr rr r 74

P generation  Red RR White rr Gametes R r Figure 9.11A_1
Figure 9.11A_1 Incomplete dominance in snapdragon flower color (part 1) Gametes R r 75

Patterns of Inheritance

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