APPLICATION QUESTIONS AND PROBLEMS

Section 17.1

Question 11

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

  1. Kernel color in a strain of wheat, in which two codominant alleles segregating at a single locus determine the color. Thus, there are three phenotypes present in this strain: white, light red, and medium red.

  2. Body weight in a family of Labrador retrievers. An autosomal recessive allele that causes dwarfism is present in this family. Two phenotypes are recognized: dwarf (less than 13 kg) and normal (greater than 23 kg).

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  3. Presence or absence of leprosy. Susceptibility to leprosy is determined by multiple genes and numerous environmental factors.

  4. Number of toes in guinea pigs, which is influenced by genes at many loci.

  5. Number of fingers in humans. Extra (more than five) fingers are caused by the presence of an autosomal dominant allele.

Question 12

*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.

  1. If a plant with genotype AA BB is crossed with a plant with genotype aa bb, what weights are expected in the F1 progeny?

  2. What is the distribution of weight expected in the F2 progeny?

Question 13

*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.

Question 14

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.

Section 17.3

Question 15

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

Additive genetic variance (VA) = 0.5
Dominance genetic variance (VD) = 0.3
Gene interaction variance (VI) = 0.1
Environmental variance (VE) = 0.4
Genetic–environmental interaction variance (VGE) = 0.0
  1. What is the narrow-sense heritability of tail length?

  2. What is the broad-sense heritability of tail length?

Question 16

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

Question 17

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

  1. 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.

  2. What will happen to the broad-sense heritability? Explain.

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

  4. What will happen to the broad-sense heritability? Explain.

Question 18

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.

Question 19

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.

Question 20

*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.

  1. Since Sally is a Southwestern University undergraduate student, 10% of her height is determined by nongenetic factors.

  2. Ninety percent of variation in height among all undergraduate students in the United States is due to genetic differences.

  3. Ninety percent of the height of Southwestern University undergraduate students is determined by genes.

  4. Ten percent of the variation in height among Southwestern University undergraduate students is determined by variation in nongenetic factors.

  5. Because the heritability of height among Southwestern University students is so high, any change in the students’ environment will have minimal effect on their height.

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Question 21

21.image 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.

Population Heritability of body size (± standard error)
Wild population 0.0595 ± 0.0123
Laboratory-reared population 0.3770 ± 0.0203

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?

Question 22

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?

Section 17.4

Question 23

*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.

  1. Calculate the selection differential and expected response to selection for wing length in these cockroaches.

  2. What should be the average wing length of the progeny of the selected cockroaches?

Question 24

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

Body weight Fat content Tenderness
VA 22 45 12
VD 10 25 5
VI 3 8 2
VE 42 64 8
VGE 0 0 1

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

Question 25

*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?

Question 26

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?

Question 27

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.

Question 28

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?