*11.For each of the following characteristics, indicate whether it would be considered a discontinuous characteristic or a quantitative characteristic. Briefly justify your answer.

*12.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 that each allele represented by a lowercase letter contributes 1 g to weight.

*13.Assume that three loci, each with two alleles (A and a, B and b, C and c), determine the difference in height between two homozygous strains of a plant. These genes are additive and equal in their effects on plant height. One strain (aa bb cc) is 10 cm in height. The other strain (AA BB CC) is 22 cm in height. The two strains are crossed, and the resulting F₁ are interbred to produce F₂ progeny. Give the phenotypes and the expected proportions of the F₂ progeny.

14.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. She 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.

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

16.The narrow-sense heritability of ear length in Reno rabbits is 0.4. The phenotypic variance (V_P) is 0.8, and the environmental variance (V_E) is 0.2. What is the additive genetic variance (V_A) for ear length in these rabbits?

17.A characteristic has a narrow-sense heritability of 0.6.

18.What conclusion can you draw from Figure 17.12 about the proportion of phenotypic variation in shell breadth that is due to genetic differences? Explain your reasoning.

19.Many researchers have estimated the heritability of human traits by comparing the correlation coefficients of identical and nonidentical twins. An assumption in using this method is that two identical twins experience environments that are no more similar to each other than those experienced by two nonidentical twins. How might this assumption be violated? Give some specific examples of how the environments of two identical twins might be more similar than the environments of two nonidentical twins.

*20.A genetics researcher determines that the broad-sense heritability of height among Southwestern University undergraduate students is 0.90. Which of the following conclusions would be reasonable? Explain your answer.

21. Drosophila buzzati is a fruit fly that feeds on the rotting fruits of cacti in Australia. Timothy Prout and Stuart Barker calculated the heritabilities of body size, as measured by thorax length, for a natural population of D. buzzati raised in the wild and for a population of D. buzzati collected in the wild but raised in the laboratory (T. Prout and J. S. F. Barker. 1989. Genetics 123:803–813). They found the following heritabilities.

Why do you think that the heritability measured in the laboratory-reared population is higher than that measured in the natural population raised in the wild?

22.Mr. Jones is a pig farmer. For many years, he has fed his pigs the food left over from the local university cafeteria, which is known to be low in protein, deficient in vitamins, and downright untasty. However, the food is free, and his pigs don’t complain. One day, a salesman from a feed company visits Mr. Jones. The salesman claims that his company sells a new, high-protein, vitamin-enriched feed that enhances weight gain in pigs. Although the feed is expensive, the salesman claims that the increased weight gain of the pigs will more than pay for the cost of the feed, increasing Mr. Jones’s profit. Mr. Jones responds that he took a genetics class at the university and that he has conducted some genetic experiments on his pigs; specifically, he has calculated the narrow-sense heritability of weight gain for his pigs and found it to be 0.98. Mr. Jones says that this heritability value indicates that 98% of the variance in weight gain among his pigs is determined by genetic differences, and therefore the new pig feed can have little effect on the growth of his pigs. He concludes that the feed would be a waste of his money. The salesman doesn’t dispute Mr. Jones’s heritability estimate, but he still claims that the new feed can significantly increase weight gain in Mr. Jones’s pigs. Who is correct and why?

*23.Joe is breeding cockroaches in his dorm room. He finds that the average wing length in his population of cockroaches is 4 cm. He chooses six cockroaches that have the largest wings; the average wing length among these selected cockroaches is 10 cm. Joe interbreeds these selected cockroaches. From earlier studies, he knows that the narrow-sense heritability for wing length in his population of cockroaches is 0.6.

24.Three characteristics in beef cattle—body weight, fat content, and tenderness—are measured, and the following variance components are estimated:

In this population, which characteristic would respond best to selection? Explain your reasoning.

*25.A rancher determines that the average amount of wool produced by a sheep in her flock is 22 kg per year. In an attempt to increase the wool production of her flock, the rancher picks five male and five female sheep with the greatest wool production; the average amount of wool produced per sheep by those selected is 30 kg. She interbreeds these selected sheep and finds that the average wool production among the progeny of the selected sheep is 28 kg. What is the narrow-sense heritability for wool production among the sheep in the rancher’s flock?

26.A strawberry farmer determines that the average weight of individual strawberries produced by plants in his garden is 2 g. He selects the 10 plants that produce the largest strawberries; the average weight of strawberries among these selected plants is 6 g. He interbreeds these selected plants. The progeny of these selected plants produce strawberries that weigh 5 g. If the farmer were to select plants that produce an average strawberry weight of 4 g, what would be the predicted weight of strawberries produced by the progeny of these selected plants?

27.Pigs have been domesticated from wild boars. Would you expect to find higher heritability for weight among domestic pigs or wild boars? Explain your answer.

28.Has the response to selection leveled off in the strain of corn selected for high oil content shown in Figure 17.14? What does this observation suggest about genetic variation in the strain selected for high oil content?