A geneticist selects for increased body weight in a population of fruit flies that she is raising in her laboratory. She measures body weight in her population and selects the five heaviest males and the five heaviest females and uses them as the parents for the next generation. From the progeny produced by these parents, she selects the five heaviest males and five heaviest females and mates them. She repeats this procedure each generation. The average body weight of flies in the original population was $1.1$ mg. The flies respond to selection, and their body weight steadily increases. After $20$ generations of selection, the average body weight is $2.3$ mg. However, after about $$ generations, the response to selection in subsequent generations levels off, and the average body weight of the flies no longer increases. At this point, the geneticist takes a long vacation; while she is gone, the fruit flies in her population interbreed randomly. When she returns from vacation, she finds that the average body weight of the flies in the population has decreased to $2.0$ mg.\ Provide an explanation for why the response to selection leveled off after $20$ generations. Why did the average size of the fruit flies decrease when selection was no longer applied during the geneticist’s vacation? $$

The geneticist carries out the same experiment described in Problem 32 , but this time she mixes phages with genotypes $c^{+} \mathrm{m}^{-}$and $c^{-} \mathrm{m}^{+}$. What results are expected from this cross?

How is the standard deviation related to the variance?

Do you think genetics influences a person’s knowledge of the game of football? Explain your answer.

What information do the mean and variance provide about a distribution?

Eugene Eisen was 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 was 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?

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.

How are epialleles different from genetic alleles, such as those that encode differences in the shape of peas or blood types?

A drug causes the degradation of Cactus protein. What would be the effect of administering this drug to developing Drosophila embryos?

Bloom syndrome is an autosomal recessive disease that exhibits haploinsufficiency. A recent survey showed that people heterozygous for mutations at the $B L M$ locus are at increased risk of colon cancer. Suppose you are a genetic counselor. A young woman is referred to you whose mother has Bloom syndrome; the young woman's father has no family history of Bloom syndrome. The young woman asks whether she is likely to experience any other health problems associated with her family history of Bloom syndrome. What advice would you give her?

If a gene had similar rates of nonsynonymous and synonymous substitutions, what might that suggest about the evolution of that gene?

Bipolar disorder is a psychiatric illness with a strong hereditary basis, but the exact mode of its inheritance is not known. Research has shown that siblings of patients with bipolar disorder are more likely to develop the disorder than siblings of unaffected people. 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 disorder 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 disorder?

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

The narrow-sense heritability of wing length in a population of Drosophila melanogaster is 0.8. The narrow-sense heritability of head width in the same population is 0.9. The genetic correlation between wing length and head width is −0.86. If a geneticist selects for increased wing length in these flies, what will happen to head width?