Ch. 19 - Genetic Analysis of Quantitative Traits
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Problem 1
Which of the following traits would you expect to be inherited as quantitative traits?
fruit weight in tomatoesProblem 1
Which of the following traits would you expect to be inherited as quantitative traits?
milk production in cattleProblem 1
Which of the following traits would you expect to be inherited as quantitative traits?
growth rate in sheepProblem 1
Which of the following traits would you expect to be inherited as quantitative traits?
body weight in chickensProblem 11
In selective breeding experiments, it is frequently observed that the strains respond to artificial selection for many generations, with the selected phenotype changing in the desired direction. Often, however, the response to artificial selection reaches a plateau after many generations, and the phenotype no longer changes as it did in past generations. Once a plateau has been reached, is the heritability of the trait very high or is it very low? Explain.Problem 12
Two inbred lines of sunflowers (P₁and P₂) produce different total weights of seeds per flower head. The mean weight of seeds (grams) and the variance of seed weights in different generations are as follows. Generation Mean Weight/Head (g) Variance P₁ 105 3.0 P₂ 135 3.8 F₁ 122 3.5 F₂ 125 7.4 Use the information above to determine VG, VE, and VP for this trait.
Problem 13
What is a quantitative trait locus (QTL)? Suppose you wanted to search for QTLs influencing fruit size in tomatoes. Describe the general structure of a QTL experiment, including the kind of tomato strains you would use, how molecular markers should be distributed in the genome, how the genetic marker alleles should differ between the two strains, and how you would use the F₁ progeny in a subsequent cross to obtain information about the possible location(s) of QTLs of interest.Problem 14
In Nicotiana, two inbred strains produce long (PL) and short (PS) corollas. These lines are crossed to produce F₁, and the F₁ are crossed to produce F₂ plants in which corolla length and variance are measured. The following table summarizes mean and variance of corolla length in each generation. Calculate H² for corolla length in Nicotiana. Generation Mean Corolla Length (mm) Variance PL 85.75 4.21 PS 43.15 2.89 F₁ 62.26 3.62 F₂ 67.37 38.10Problem 15
Suppose the length of maize ears has narrow sense heritability (h²) of 0.70. A population produces ears that have an average length of 28 cm, and from this population a breeder selects a plant producing 34-cm ears to cross by self-fertilization. Predict the selection differential (S) and the response to selection (R) for this cross.Problem 16
In a line of cherry tomatoes, the average fruit weight is 16 g. A plant producing tomatoes with an average weight of 12 g is used in one self-fertilization cross to produce a line of smaller tomatoes, and a plant producing tomatoes of 24 g is used in a second cross to produce larger tomatoes. What is the selection differential (S) for fruit weight in each cross?Problem 16
In a line of cherry tomatoes, the average fruit weight is 16 g. A plant producing tomatoes with an average weight of 12 g is used in one self-fertilization cross to produce a line of smaller tomatoes, and a plant producing tomatoes of 24 g is used in a second cross to produce larger tomatoes. If narrow sense heritability (h²) for this trait is 0.80, what are the expected responses to selection (R) for fruit weight in the crosses?Problem 17
Two pure-breeding wheat strains, one producing dark red kernels and the other producing white kernels, are crossed to produce F₁ with pink kernel color. When an F₁ plant is self-fertilized and its seed collected and planted, the resulting F₂ consist of 160 plants with kernel colors as shown in the following table. Kernel Color Number White 9 Dark red 12 Red 39 Light pink 41 Pink 59 Based on the F₂ progeny, how many genes are involved in kernel color determination?Problem 17
Two pure-breeding wheat strains, one producing dark red kernels and the other producing white kernels, are crossed to produce F₁ with pink kernel color. When an F₁ plant is self-fertilized and its seed collected and planted, the resulting F₂ consist of 160 plants with kernel colors as shown in the following table. Kernel Color Number White 9 Dark red 12 Red 39 Light pink 41 Pink 59 How many additive alleles are required to explain the five phenotypes seen in the F₂?Problem 17
Two pure-breeding wheat strains, one producing dark red kernels and the other producing white kernels, are crossed to produce F₁ with pink kernel color. When an F₁ plant is self-fertilized and its seed collected and planted, the resulting F₂ consist of 160 plants with kernel colors as shown in the following table. Kernel Color Number White 9 Dark red 12 Red 39 Light pink 41 Pink 59 If an F₁ plant is crossed to a dark red plant, what are the expected progeny phenotypes and what is the expected proportion of each phenotype?Problem 17
Two pure-breeding wheat strains, one producing dark red kernels and the other producing white kernels, are crossed to produce F₁ with pink kernel color. When an F₁ plant is self-fertilized and its seed collected and planted, the resulting F₂ consist of 160 plants with kernel colors as shown in the following table. Kernel Color Number White 9 Dark red 12 Red 39 Light pink 41 Pink 59 Using clearly defined allele symbols of your choice, give genotypes for the parental strains and the F₁. Describe the genotypes that produce the different phenotypes in the F₂.Problem 18
In studies of human MZ and DZ twin pairs of the same sex who are reared together, the following concordance values are identified for various traits. Based on the values shown, describe the relative importance of genes versus the influence of environmental factors for each trait. Trait Concordance MZ DZ _ Blood type 100 65 Chicken pox 89 87 Bipolar disorder 67 13 Schizophrenia 72 12 Diabetes 62 15 Cleft lip 51 6 Club foot 40 4Problem 19
During a visit, your grandparents comment on how tall you are compared with them. You tell them that in your genetics class, you learned that height in humans has high heritability, although environmental factors also influence adult height. You correctly explain the meaning of heritability, and your grandfather asks, 'How can height be highly heritable and still be influenced by the environment?' What explanation do you give your grandfather?Problem 20
An association of racehorse owners is seeking a new genetic strategy to improve the running speed of their horses. Traditional breeding of fast male and female horses has proven expensive and time-consuming, and the breeders are interested in an approach using quantitative trait loci as a basis for selecting breeding pairs of horses. Write a brief synopsis (∼50 words) of QTL mapping to explain how genes influencing running speed might be identified in horses.Problem 22
Suppose a polygenic system for producing color in kernels of a grain is controlled by three additive genes, G, M, and T. There are two alleles of each gene, G₁ and G₂, M₁, and M₂, and T₁ and T₂. The phenotypic effects of the three genotypes of the G gene are G₁G₁ = 6 units of color, G₁G₂ = 3 units of color, and G₂G₂ = 1 unit of color. The phenotypic effects for genes M and T are similar, giving the phenotype of a plant with the genotype G₁G₁M₁M₁T₁T₁ a total of 18 units of color and a plant with the genotype G₂G₂M₂M₂T₂T₂ a total of 3 units of color. Assuming the threshold model applies to this kernel-color system, what proportion of the progeny of the cross G₁G₂M₁M₂T₂T₂xG₁G₂M₁M₂T₁T₂ do you expect to display colored kernels?Problem 22
Suppose a polygenic system for producing color in kernels of a grain is controlled by three additive genes, G, M, and T. There are two alleles of each gene, G₁ and G₂, M₁, and M₂, and T₁ and T₂. The phenotypic effects of the three genotypes of the G gene are G₁G₁ = 6 units of color, G₁G₂ = 3 units of color, and G₂G₂ = 1 unit of color. The phenotypic effects for genes M and T are similar, giving the phenotype of a plant with the genotype G₁G₁M₁M₁T₁T₁ a total of 18 units of color and a plant with the genotype G₂G₂M₂M₂T₂T₂ a total of 3 units of color. Two trihybrid plants are mated. What is the expected proportion of progeny plants displaying 9 units of color? Explain your answer.Problem 22
Suppose a polygenic system for producing color in kernels of a grain is controlled by three additive genes, G, M, and T. There are two alleles of each gene, G₁ and G₂, M₁, and M₂, and T₁ and T₂. The phenotypic effects of the three genotypes of the G gene are G₁G₁ = 6 units of color, G₁G₂ = 3 units of color, and G₂G₂ = 1 unit of color. The phenotypic effects for genes M and T are similar, giving the phenotype of a plant with the genotype G₁G₁M₁M₁T₁T₁ a total of 18 units of color and a plant with the genotype G₂G₂M₂M₂T₂T₂ a total of 3 units of color. Suppose that instead of an additive genetic system, kernel-color determination in this organism is a threshold system. The appearance of color in kernels requires nine or more units of color; otherwise, kernels have no color and appear white. In other words, plants whose phenotypes contain eight or fewer units of color are white. Based on the threshold model, what proportion of the F₂ progeny produced by the trihybrid cross in part (b) will be white? Explain your answer.Problem 22
Suppose a polygenic system for producing color in kernels of a grain is controlled by three additive genes, G, M, and T. There are two alleles of each gene, G₁ and G₂, M₁, and M₂, and T₁ and T₂. The phenotypic effects of the three genotypes of the G gene are G₁G₁ = 6 units of color, G₁G₂ = 3 units of color, and G₂G₂ = 1 unit of color. The phenotypic effects for genes M and T are similar, giving the phenotype of a plant with the genotype G₁G₁M₁M₁T₁T₁ a total of 18 units of color and a plant with the genotype G₂G₂M₂M₂T₂T₂ a total of 3 units of color. How many units of color are found in trihybrid plants?Problem 23
New Zealand lamb breeders measure the following variance values for their herd. Trait VP VG VA Body mass (kg) 42.4 20.5 7.4 Body fat (%) 38.9 16.2 5.7 Body length (cm) 51.6 26.4 8.1 Calculate the broad sense heritability (H²) and the narrow sense heritability (h²) for each trait in this lamb herd.Problem 23
New Zealand lamb breeders measure the following variance values for their herd. Trait VP VG VA Body mass (kg) 42.4 20.5 7.4 Body fat (%) 38.9 16.2 5.7 Body length (cm) 51.6 26.4 8.1 How would you characterize the potential response to selection (R) for each trait?Problem 24
Cattle breeders would like to improve the protein content and butterfat content of milk produced by a herd of cows. Narrow sense heritability values are 0.60 for protein content and 0.80 for butterfat content. The average percentages of these traits in the herd and the percentages of the traits in cows selected for breeding are as follows. Trait Herd Average Selected Cows Protein content 20.2% 22.7% Butterfat content 6.5% 7.4% Determine the selection differential (S) for each trait in this herd.Problem 24
Cattle breeders would like to improve the protein content and butterfat content of milk produced by a herd of cows. Narrow sense heritability values are 0.60 for protein content and 0.80 for butterfat content. The average percentages of these traits in the herd and the percentages of the traits in cows selected for breeding are as follows. Trait Herd Average Selected Cows Protein content 20.2% 22.7% Butterfat content 6.5% 7.4% Which trait is likely to be the most responsive to artificial selection applied by the cattle breeders through selection of cows for mating?Problem 25
In human gestational development, abnormalities of the closure of the lower part of the mid-face can result in cleft lip, if the lip alone is affected by the closure defect, or in cleft lip and palate (the roof of the mouth), if the closure defect is more extensive. Cleft lip and cleft lip with cleft palate are multifactorial disorders that are threshold traits. A family with a history of either condition has a significantly increased chance of a recurrence of mid-face cleft disorder in comparison with families without such a history. However, the recurrence risk of a mid-face cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone. Suppose a friend of yours who has not taken genetics asks you to explain these observations. Construct a genetic explanation for the increased recurrence risk of mid-face clefting in families that have a history of cleft disorders versus families without a history of such disorders.Problem 25
In human gestational development, abnormalities of the closure of the lower part of the mid-face can result in cleft lip, if the lip alone is affected by the closure defect, or in cleft lip and palate (the roof of the mouth), if the closure defect is more extensive. Cleft lip and cleft lip with cleft palate are multifactorial disorders that are threshold traits. A family with a history of either condition has a significantly increased chance of a recurrence of mid-face cleft disorder in comparison with families without such a history. However, the recurrence risk of a mid-face cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone. Construct a similar explanation of why the recurrence risk of a cleft disorder is higher in families with a history of cleft lip with cleft palate than in families with a history of cleft lip alone.Problem 26
The children of couples in which one partner has blood type O (genotype ii) and the other partner has blood type AB (genotype IᴬIᴮ) are studied. What is the expected concordance rate for blood type of MZ twins in this study? Explain your answer.Problem 26
The children of couples in which one partner has blood type O (genotype ii) and the other partner has blood type AB (genotype IᴬIᴮ) are studied. What is the expected concordance rate for blood type of DZ twins in this study? Explain why this answer is different from the answer to part (a).
