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

A rancher determines that the average amount of wool produced by a sheep in his flock is 22 kg per year. In an attempt to increase the wool production of his 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. He 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? The narrow-sense heritability of wing length in a population of Drosophila melanogaster is .8. The narrow-sense heritability of head width in the same population is .9. The genetic correlation between wing length and head width is —.86. If a geneticist selects for increased wing length in these flies, what will happen to head width?

CHALLENGE QUESTIONS]_

42. We have explored some of the difficulties in separating genetic and environmental components of human behavioral characteristics. Considering these difficulties and what you know about calculating heritability, propose an experimental design for accurately measuring the heritability of musical ability.

43. A student who has just learned about quantitative genetics says, "Heritability estimates are worthless! They don't tell you anything about the genes that affect a characteristic. They don't provide any information about the types of offspring to expect from a cross. Heritability estimates measured in one population can't be used for other populations; so they don't even give you any general information about how much of a characteristic is genetically determined. I can't see that heritabilities do anything other than make undergraduate students sweat during tests." How would you respond to this statement? Is the student correct? What good are heritabilities, and why do geneticists bother to calculate them?

44. A geneticist selects for increased size in a population of fruit flies that she is raising in her laboratory. She starts with the two largest males and the two largest females and uses them as the parents for the next generation. From the progeny produced by these selected parents, she selects the two largest males and the two largest females and mates them. She repeats this procedure each generation. The average weight of flies in the initial population was 1.1 mg. The flies respond to selection, and their body size steadily increases. After 20 generations of selection, the average weight is 2.3 mg. However, after about 20 generations, the response to selection in subsequent generations levels off, and the average size of the flies no longer increases. At this point, the geneticist takes a long vacation; while she is gone, the fruit flies in her population interbreed randomly. When she returns from vacation, she finds that the average size of the flies in the population has decreased to 2.0 mg.

(a) Provide an explanation for why the response to selection leveled off after 20 generations.

(b) Why did the average size of the fruit flies decrease when selection was no longer applied during the geneticist's vacation?

45. Manic-depressive illness is a psychiatric disorder that has a strong hereditary basis, but the exact mode of inheritance is not known. Previous research has shown that siblings of patients with manic-depressive illness are more likely also to develop the disorder than are siblings of unaffected persons. A recent study demonstrated that the ratio of manic-depressive brothers to manic-depressive sisters is higher when the patient is male than when the patient is female. In other words, relatively more brothers of manic-depressive patients also have the disease when the patient is male than when the patient is female. What does this new observation suggest about the inheritance of manic-depressive illness?

SUGGESTED READINGS)_

Barton, N. H. 1989. Evolutionary quantitative genetics: how little do we know? Annual Review of Genetics 23:337-3370. A review of how quantitative genetics is used to study the process of evolution.

Cunningham, P. 1991. The genetics of thoroughbred horses. Scientific American 264(5):92-98.

An interesting account of how quantitative genetics is being applied to the breeding of thoroughbred horses. Dudley, J. W. 1977. 76 generations of selection for oil and protein percentage in maize. In E. Pollak, O. Kempthorne, and T. B. Bailey, Jr., Eds. Proceedings of the International Conference on Quantitative Genetics, pp. 459 - 473. Ames, IA: Iowa State University Press.

A report on the progress of one of the longest running selection experiments. East, E. M. 1910. A Mendelian interpretation of variation that is apparently continuous. American Naturalist 44:65-82. East's interpretation of how individual genes acting collectively produce continuous variation, including a discussion of Nilsson-Ehle's research on kernel color in wheat.

East, E. M. 1916. Studies on size inheritance in Nicotiana. Genetics 1:164 -176.

East's study of flower length in Nicotiana. Falconer, D. S., and T. F. C. MacKay (Contributor). 1996. Introduction to Quantitative Genetics, 4th ed. New York: Addison-Wesley.

An excellent basic text of quantitative genetics. Frary, A., T. C. Nesbitt, A. Frary, S. Grandillo, E. van der Knaap, et al. 2000. A quantitative trait locus key to the evolution of tomato fruit size. Science 289:85-88. A report of the discovery and cloning of one QTL that is responsible for the quantitative difference in fruit size between wild tomatoes and cultivated varieties.

Gillham, N. W. 2001. Sir Francis Galton and the birth of eugenics. Annual Review of Genetics 2001:83 -101. A history of Galton's contributions to the eugenics movement. Mackay, T. F. C. 2001. The genetic architecture of quantitative traits. Annual Review of Genetics 35:303 -339. A review of techniques for QLT mapping and results from current studies on QLTs.

Martienssen, R. 1997. The origin of maize branches out. Nature 386:443 -445.

Discusses the identification of QTLs that contributed to the domestication of corn. Moore, K. J., and D. L. Nagle. 2000. Complex trait analysis in the mouse: the strengths, the limitations, and the promise yet to come. Annual Review of Genetics 43:653 -686. A review of the genetic analysis of complex characteristics in mice, particularly emphasizing those that are medically important.

Paterson, A. H., E. S. Lander, J. D. Hewitt, S. Peterson, S. E. Lincoln, and S. D. Tanksley. 1988. Resolution of quantitative traits into Mendelian factors by using a complete linkage map of restriction fragment length polymorphisms. Nature 335:721 - 726.

A study identifying QTLs that control fruit mass, pH, and other important characteristics in tomatoes.

Plomin, R. 1999. Genetics and general cognitive ability. Nature 402:C25 - C29.

A good discussion of the genetics of general intelligence and the search for QTLs that influence it. Tanksley, S. D. 1993. Mapping polygenes. Annual Review of Genetics 27:205 -233.

This review article summarizes some of the efforts to map QTLs. It discusses the methodology and some of the findings that are emerging from this research.

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