29.Bipolar illness is a psychiatric disorder with a strong hereditary basis, but the exact mode of inheritance is not known. Research has shown that siblings of patients with bipolar illness are more likely to develop the disorder than are siblings of unaffected persons. Findings from one study demonstrated that the ratio of bipolar brothers to bipolar sisters is higher when the patient is male than when the patient is female. In other words, relatively more brothers of patients with bipolar illness also have the disease when the patient is male than when the patient is female. What does this observation suggest about the inheritance of bipolar illness?

30.We have explored some of the difficulties in separating the 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.

31.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. Heritabilities do nothing but 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?

32. Eugene Eisen selected for increased 12-day litter weight (total weight of a litter of offspring 12 days after birth) in a population of mice (E. J. Eisen. 1972. Genetics 72:129–142). The 12-day litter weight of the population steadily increased, but then leveled off after about 17 generations. At generation 17, Eisen took one family of mice from the selected population and reversed the selection procedure: in this group, he selected for decreased 12-day litter weight. This group immediately responded to the reversed selection: the 12-day litter weight dropped 4.8 g within 1 generation and dropped 7.3 g after 5 generations. On the basis of the results of the reverse selection, what is the most likely explanation for the leveling off of 12-day litter weight in the original population?