371B_exam3_yellow.html

Genetics 371B, Autumn 2000

Exam 3 YELLOW VERSION: 11 December

1. (20 pts total)
The short-tailed voles of the Canadian north have tails that measure 7 cm. In contrast, voles of the same species in Siberia have tails that are 11 cm long. A cross between a Canadian vole and a Siberian vole always gives F₁ progeny with 9 cm tails. F₂ progeny from crosses between these F₁ animals show a range of tail lengths -- most have tails that are 9 cm long, while the shortest tails are 5 cm long and the longest tails are 13 cm long. Of 1061 F₂ progeny, four had 5 cm tails and four had 13 cm tails.

(a) Assuming that tail length in these voles is a polygenic trait where all additive alleles make equal contributions, how many genes do you think are involved in determination of tail length? How much (in inches) does each additive allele contribute to tail length? Show your work. (6 pts)

(b) Using A/a, B/b, etc., for the tail-length genes, give one set of genotypes that could explain the phenotypes of the parents and the F1 progeny. Your answer must be consistent with the observation that all the F1 animals have 1.8-inch tails. (8 pts)

(c) Give two reasons to be cautious about your conclusion about the number of tail-length genes (in part a). (6 pts)
Reason 1

Reason 2

2.	(16 pts total) You are studying seizures using a mouse model. You have a true-breeding strain of mice that has frequent, spontaneous seizures. You cross these to another true-breeding strain that does not have seizures. The F₁ all have seizures, and a single mating of two F₁ animals produces 2 normal mice and 7 mice with seizures.
	(a)	Suppose this is a single gene trait. Assuming that there was no observational error and that there was no environmental influence on seizure vs. non-seizure, what is the simplest explanation for the F₁ and F₂ phenotype ratios? Provide genotypes for all individuals in the cross. State what phenotype ratio you expect to get and what approximation you are making. Use A/a for the allele names. (4 pts)
	(b)	Now suppose this trait is controlled by two genes, and that the F₁ animals are heterozygous for both genes. Give the simplest explanation for the F₂ phenotype ratios -- but no need for Punnett squares! Use A/a and B/b for the gene names. (8 pts)
	(c)	What is the simplest experimental strategy that would allow you to distinguish between these two possibilities? (ONE sentence!) (4 pts)

3.	(20 pts total) The Great Northwestern Dragonfly develops in a manner similar to Drosophila , with the following difference: while Drosophila has one set of wings and one set of halteres (flight balancers) in the segment immediately posterior to the wing segment, the dragonfly has two sets of wings. The dragonfly larva has seven segments, shown here with their future adult fates.
	(a)	Suppose that the common ancestor of Drosophila and the dragonfly had wings and halteres (as Drosophila does). What type of mutation may have occurred in the dragonfly lineage to produce extra wings? Specify the class of developmental gene, and whether the mutation was loss-of-function or gain-of-function. (6 pts)
	(b)	A French group working on dragonfly development has discovered three maternal genes expressed in the developing egg: fromage, pamplemousse (ppm), and chien. The expression pattern of these three genes in wild type and homozygous null mutant embryos is shown. Based on these data, suggest how these protein gradients are set up in wild type embryos, specifying the function of fromage and chien. Your model should explain the mutant phenotypes shown. (8 pts)
	(c)	The expression of the zygotic gene bateau requires 1 unit of chien protein and is blocked by 1 unit of ppm protein. What segments/adult structure(s) might be missing in a mutant homozygous for a null allele of bateau? What class of developmental genes does bateau belong to? (6 pts)

4.	(22 pts total) An improbable pair of operons in the mythical bacterium Confusus inhabitabilis is outlined below: E, F, G, H, J, and K are genes coding for proteins; P and O are the promoter and operator for each operon. G and K are known to be short-lived proteins. The sugar mannose induces transcription of the mannose operon, while melibiose induces transcription of the melibiose operon.
	(a)	A mutation in G (allele g) causes constitutively low levels of melibiose operon transcription, while a mutation in K (allele k) gives constitutive inactivation of the mannose operon. Explain the mutant phenotypes (specifying gain-of-function vs. loss-of-function) if: G is an activator of the melibiose operon and K is a repressor of the mannose operon (4 pts) G is a repressor of the melibiose operon and K is an activator of the mannose operon(4 pts)
	(b)	State what you think mannose and melibiose do according to Model (i). (What are their targets, and what do they do to those proteins?) (6 pts)
	(c)	Addition of mannose to wild type cells causes only a brief induction of mannose operon transcription, while addition of melibiose causes sustained induction of melibiose operon transcription (until the melibiose is used up). Which one of the two models do you think is MORE CONSISTENT with this observation? Explain BRIEFLY. (8 pts)

5.	(22 pts total) In a certain remote island nation (Island #1), 91% of the inhabitants are normal (phenotype T), while the remainder show an autosomal recessive trait (t).
	(a)	Assuming Hardy-Weinberg conditions, what fraction of the population are carriers of the trait? Show your work! (6 pts)
	(b)	It turns out the the population is not in Hardy-Weinberg conditions after all, as the fitness of the two phenotypes is not the same: w_{T_} = 1, and w_tt = 1/9. Starting with the frequencies given above, predict the frequencies of alleles T and t in the next generation. Show your work! (6 pts)
	(c)	On a different island (Island #2), where the frequency of allele t = 1/1000, a radical and ill-informed politician argues that modern health care is causing a "degradation in the quality of the gene pool" because recessive traits, instead of being selected against, are allowed to propagate. He is proposing to "clean up the gene pool" by making it illegal for people showing the recessive trait t to have children. Explain why this policy will have little influence on the frequency of allele t in that population. (6 pts)
	(d)	Which island do you think will show higher heritability (in the broad sense) for the trait? Explain in 1-2 sentences. (4 pts)

1.	(20 pts total) The short-tailed voles of the Canadian north have tails that measure 7 cm. In contrast, voles of the same species in Siberia have tails that are 11 cm long. A cross between a Canadian vole and a Siberian vole always gives F₁ progeny with 9 cm tails. F₂ progeny from crosses between these F₁ animals show a range of tail lengths -- most have tails that are 9 cm long, while the shortest tails are 5 cm long and the longest tails are 13 cm long. Of 1061 F₂ progeny, four had 5 cm tails and four had 13 cm tails.
	(a)	Assuming that tail length in these voles is a polygenic trait where all additive alleles make equal contributions, how many genes do you think are involved in determination of tail length? How much (in inches) does each additive allele contribute to tail length? Show your work. (6 pts)
	(b)	Using A/a, B/b, etc., for the tail-length genes, give one set of genotypes that could explain the phenotypes of the parents and the F1 progeny. Your answer must be consistent with the observation that all the F1 animals have 1.8-inch tails. (8 pts)
	(c)	Give two reasons to be cautious about your conclusion about the number of tail-length genes (in part a). (6 pts) Reason 1 Reason 2