20. Quantitative Genetics

Heritability

Open Question
The following variances were calculated for two traits in a herd of hogs.

Trait Vₚ VG VA _
Back fat 30.6 12.2 8.44
Body length 52.4 26.4 11.70

Which of the two traits will respond best to selection by a breeder? Why?
Open Question
The mean and variance of plant height of two highly inbred strains (P₁ and P₂) and their progeny (F₁ and F₂) are shown here.

Strain Mean (cm) Variance _
P₁ 34.2 4.2
P₂ 55.3 3.8
F₁ 44.2 5.6
F₂ 46.3 10.3

Calculate the broad-sense heritability (H²) of plant height in this species.
Open Question
A hypothetical study investigated the vitamin A content and the cholesterol content of eggs from a large population of chickens. The following variances (V) were calculated.

Which trait, if either, is likely to respond to selection? <>
Open Question
A hypothetical study investigated the vitamin A content and the cholesterol content of eggs from a large population of chickens. The following variances (V) were calculated.

Calculate the narrow-sense heritability (h²) for both traits. <>
Open Question
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) VariancePL 85.75 4.21PS 43.15 2.89F₁ 62.26 3.62F₂ 67.37 38.10
Open Question
In a herd of dairy cows the narrow-sense heritability for milk protein content is 0.76, and for milk butterfat it is 0.82. The correlation coefficient between milk protein content and butterfat is 0.91. If the farmer selects for cows producing more butterfat in their milk, what will be the most likely effect on milk protein content in the next generation?
Open Question
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.
Open Question
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?
Open Question
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?
Open Question
Suppose you want to develop a population of Drosophila that would rapidly learn to avoid certain substances the flies could detect by smell. Based on the heritability estimate you obtained in Problem 16, do you think it would be worth doing this by artificial selection? Why or why not?
Open Question
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 ConcordanceMZ DZ _Blood type 100 65Chicken pox 89 87Bipolar disorder 67 13Schizophrenia 72 12Diabetes 62 15Cleft lip 51 6Club foot 40 4
Open Question
In a population of tomato plants, mean fruit weight is 60 g and h² is 0.3. Predict the mean weight of the progeny if tomato plants whose fruit averaged 80 g were selected from the original population and interbred.
Open Question
In a population of 100 inbred, genotypically identical rice plants, variance for grain yield is 4.67. What is the heritability for yield? Would you advise a rice breeder to improve yield in this strain of rice plants by selection?
Open Question
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?
Open Question
In a cross between a strain of large guinea pigs and a strain of small guinea pigs, the F₁ are phenotypically uniform, with an average size about intermediate between that of the two parental strains. Among 1014 F₂ individuals, 3 are about the same size as the small parental strain and 5 are about the same size as the large parental strain. How many gene pairs are involved in the inheritance of size in these strains of guinea pigs?