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*26. Assume that plant weight is determined by a pair of alleles at each of two independently assorting loci (A and a, B and b) that are additive in their effects. Further, assume that each allele represented by an uppercase letter contributes 4 g to weight and each allele represented by a lowercase letter contributes 1 g to weight.

(a) If a plant with genotype AABB is crossed with a plant with genotype aabb, what weights are expected in the

F1 progeny?

(b) What is the distribution of weight expected in the F2 progeny?

27. Assume that three loci, each with two alleles (A and a, B and b, C and c), determine the differences in height between two homozygous strains of a plant. These genes are additive and equal in their effects on plant height. One strain

(aabbcc) is 10 cm in height. The other strain (AABBCC) is 22 cm in height. The two strains are crossed, and the resulting Fj are interbred to produce F2 progeny. Give the phenotypes and the expected proportions of the F2 progeny.

*28. A farmer has two homozygous varieties of tomatoes. One variety, called Little Pete, has fruits that average only 2 cm in diameter. The other variety, Big Boy, has fruits that average a whopping 14 cm in diameter. The farmer crosses Little Pete and Big Boy; he then intercrosses the F1 to produce F2 progeny. He grows 2000 F2 tomato plants and doesn't find any F2 offspring that produce fruits as small as Little Pete or as large as Big Boy. If we assume that the differences in fruit size of these varieties are produced by genes with equal and additive effects, what conclusion can we make about the minimum number of loci with pairs of alleles determining the differences in fruit size of the two varieties?

29. Seed size in a plant is a polygenic characteristic. A grower crosses two pure-breeding varieties of the plant and measures seed size in the F1 progeny. He then backcrosses the F1 plants to one of the parental varieties and measures seed size in the backcross progeny. The grower finds that seed size in the backcross progeny has a higher variance than does seed size in the F1 progeny. Explain why the backcross progeny are more variable.

*30. Phenotypic variation in tail length of mice has the following components:

Additive genetic variance (VA) = .5

Dominance genetic variance (VD) = .3

Genic interaction variance (Vj) = .1

Environmental variance (VE) = .4

Genetic - environmental interaction variance (VGE) = .0

(a) What is the narrow-sense heritability of tail length?

(b) What is the broad-sense heritability of tail length?

31. The narrow-sense heritability of ear length in Reno rabbits is .4. The phenotypic variance (VP) is .8 and the environmental variance (VE) is .2. What is the additive genetic variance (VA) for ear length in these rabbits?

*32. Assume that human ear length is influenced by multiple genetic and environmental factors. Suppose you measured ear length on three groups of people, in which group A consists of five unrelated persons, group B consists of five siblings, and group C consists of five first cousins.

(a) Assuming that the environment for each group is similar, which group should have the highest phenotypic variance? Explain why.

(b) Is it realistic to assume that the environmental variance for each group is similar? Explain your answer.

33. A characteristic has a narrow-sense heritability of .6.

(a) If the dominance variance (VD) increases and all other variance components remain the same, what will happen to the narrow-sense heritability? Will it increase, decrease, or remain the same? Explain.

(b) What will happen to the broad-sense heritability? Explain.

(c) If the environmental variance (VE) increases and all other variance components remain the same, what will happen to the narrow-sense heritability? Explain.

(d) What will happen to the broad-sense heritability? Explain.

34. Flower color in the pea plants that Mendel studied is controlled by alleles at a single locus. A group of peas homozygous for purple flowers is grown in a garden. Careful study of the plants reveals that all their flowers are purple, but there is some variability in the intensity of the purple color. If heritability were estimated for this variation in flower color, what would it be. Explain your answer.

*35. A graduate student is studying a population of bluebonnets along a roadside. The plants in this population are genetically variable. She counts the seeds produced by 100 plants and measures the mean and variance of seed number. The variance is 20. Selecting one plant, the graduate student takes cuttings from it, and cultivates these cuttings in the greenhouse, eventually producing many genetically identical clones of the same plant. She then transplants these clones into the roadside population, allows them to grow for 1 year, and then counts the number of seeds produced by each of the cloned plants. The graduate student finds that the variance in seed number among these cloned plants is 5. From the phenotypic variance of the genetically variable and genetically identical plants, she calculates the broad-sense heritability.

(a) What is the broad-sense heritability of seed number for the roadside population of bluebonnets?

(b) What might cause this estimate of heritability to be inaccurate?

*36. The length of the middle joint of the right index finger was measured on 10 sets of parents and their adult offspring. The mean parental lengths and the mean offspring lengths for each family are listed in the following table. Calculate the regression coefficient for regression of mean offspring length against mean parental length and estimate the narrow-sense heritability for this characteristic.

Mean parental length (mm)

0 0

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