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MIT 7 013 - Problem Set 2

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Name:____________________________ 1 2007 7.013 Problem Set 2 Due before 5 PM on FRIDAY, March 2, 2007. Turn answers in to the box outside of 68-120. PLEASE WRITE YOUR ANSWERS ON THIS PRINTOUT. 1. Wild-type flies are brown in color. You have discovered a gene that controls body color in flies called gene A. You have two true-breeding mutant strains, both of which have black bodies. Strain One (a1/a1) is homozygous for a mutation in gene A that causes a dominant mutant phenotype. Strain Two (a2/a2) is homozygous for a mutation in gene A that causes a recessive mutant phenotype. Group Three are flies that result from mating Strain One to Strain Two. Group Four are flies that result from mating Strain One to wild-type (A/A). Group Five are flies that result from mating Strain Two to wild-type (A/A). Predict the genotypic ratio and the phenotypic ratio of the offspring resulting from the following crosses. Make sure to label each class in each of your ratios. (a) Group Three and Group Five Genotypic ratio: Phenotypic ratio: (b) Strain Two and Group Three Genotypic ratio: Phenotypic ratio: (c) Group Four and Strain Two Genotypic ratio: Phenotypic ratio: (d) Group Four and Group Five Genotypic ratio: Phenotypic ratio:Name:____________________________ 2 2. Consider a fictitious pathway that controls coat color in the mouse. The mouse genome contains gene A, which encodes enzyme A, an enzyme that converts the precursor (a white compound) to the intermediate (a tan compound). The mouse genome contains gene B, which encodes enzyme B, an enzyme that converts the intermediate to Compound #2 (a brown compound). The mouse genome contains gene C, which encodes enzyme C, an enzyme that converts the intermediate to Compound #1 (a gray compound). Mice that produce both compound #1 and compound #2 are brownish-gray. Genes A, B, and C are all autosomal genes that lie on different mouse chromosomes. Loss-of-function mutations in both homologous versions (maternal and paternal) of any one gene (A, B, or C) causes a recessive phenotype. antigen 1antigen 2intermediateenzyme Aenzyme Cenzyme B (a) Which color coat would each following mouse have? Some are filled in for you. AABBCC = aaBBCC = AaBbcc = brownish-tan AabbCc = grayish-tan Aabbcc = tan aaBBcc = aabbCc = aabbcc = precursor Compound #2 (brown) Compound #1 (gray) (tan) (white)Name:____________________________ 3 (b) Two different true-breeding strains of mice have been isolated; neither strain can produce compound #1 or compound #2. When an individual from one strain is crossed with an individual from the other strain, all of the F1 mice produce both compounds. Write out the genotypes for both P generation strains, and the genotype for the F1 generation. (Use “A,” “B,” and “C” to designate the wild-type alleles and “a,” “b,” and “c” to designate the defective alleles of the three genes.) Also write out the coat color of each group of animals. Genotype Coat Color P generation, one strain P generation, other strain F1 generation (c) Two of the F1 mice are crossed to one another. The possible phenotypes for the F2 progeny are shown below. What fraction of the F2 generation will be represented by each phenotype on average? (Note: some fractions in the table may be zero.) Coat Color Fraction of F2 that is this coat color white tan brown brownish-gray brownish-tan grayish-tanName:____________________________ 4 3. You are studying two recessive traits in the fruit fly Drosophila melanogaster. The “h” allele causes flies to have the recessive phenotype of hairy backs (wild-type flies have hairless backs). The “t” allele causes flies to have the recessive phenotype of thick legs (wild-type flies have thin legs). You mate females from a true-breeding strain with hairy backs and normal legs to males from a true-breeding strain with normal backs and thick legs. F1 females are then mated to males that have hairy backs and thick legs to produce F2 progeny. If you analyzed 1000 MALE progeny in the F2 generation, how many flies of each possible phenotypic class would you expect, given that: (a) The two traits are determined by two unlinked autosomal genes hairy thick: hairy thin: hairless thick: hairless thin: (b) The two traits are determined by two completely linked autosomal genes hairy thick: hairy thin: hairless thick: hairless thin: (c) The two traits are determined by two autosomal genes that are 10 cM apart hairy thick: hairy thin: hairless thick: hairless thin:Name:____________________________ 5 (d) The two traits are determined by two completely linked genes on the X chromosome hairy thick: hairy thin: hairless thick: hairless thin: (e) The two traits are determined by two genes that are 20 cM apart on the X chromosome hairy thick: hairy thin: hairless thick: hairless thin: 4. You have just been hired as a genetic counselor for a royal family that still engages in a significant amount of inbreeding. As your first assignment, you are presented with the following pedigree where the filled symbol represents a male in the royal family who has a rare disease.Name:____________________________ 6 Your job is to calculate the probability that the child indicated by ? will have the disease. To do this, assume that no new mutations arise within the pedigree and that no unrelated individual is a carrier (because this is a very rare disease). Also assume complete penetrance. (a) If the disease is autosomal recessive, what is the probability that the child indicated by the ? will have the disease? (b) If the disease is X-linked recessive, what is the probability that a child will be born with the disease if that child is born male? (c) If the disease is X-linked recessive, what is the probability that a child will be born with the disease if that child is born female? 5. This problem involves two families, each of which contains individuals that express the same rare trait due to having the same rare mutation. The families are diagramed below — individuals are numbered, and those expressing the rare trait are represented by the filled symbols. For each possible mode of inheritance listed below, state what the probability is that the child will express the rare trait. Assume that no new mutations arise within the pedigree, and assume complete


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MIT 7 013 - Problem Set 2

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