1 7 03 Problem Set 1 Due before 5 PM on Thursday September 20 2001 Hand in answers in recitation section or in the box outside the class 1 You have isolated 10 new mutant yeast strains that are defective in synthesis of leucine an amino acid These Leu mutants do not grow on minimal medium but they do grow on minimal medium supplemented with leucine Your Leu mutants numbered 1 through 10 were all isolated in a strain of mating type a MAT a As it turns out your high school classmate now at Harvard has independently isolated 10 new Leu mutants numbered 11 through 20 all in a strain of mating type a MAT a You and your high school classmate decide to combine your resources and determine how many different genes are represented by your 20 mutant strains You cross each of the MAT a strains to each of the MAT a strains and you include crosses to the appropriate wild type strains Your experimental observations are shown in the table below where indicates diploids that did not grow on minimal medium and indicates diploids that did grow on minimal medium strains of mating type a strains of mating type a wild type 1 2 3 4 5 6 7 8 9 10 wild type 11 12 13 14 15 16 17 18 19 20 a What property do mutants 2 and 16 share b Which mutations do you know to be in the same gene 2 c Could mutations 3 and 16 be in the same gene d Based on these experiments what is the minimum number of genes required for leucine synthesis e What is the maximum number of genes that these 20 mutants could represent 2 Being a well rounded geneticist you also maintain a colony of mice a One day you spot a mouse in your colony with a novel and interesting phenotype a kinked tail You breed the kinked tail mouse a male with several wild type females and observe that about half of the offspring both males and females have kinked tails and half have normal tails Is the kinked tail mutation dominant or recessive to wild type b When two of the kinked tail offspring from part a are crossed what fraction of the resulting mice would you expect to have kinked tails c When you cross kinked tail offspring from part a you find that one third of the resulting kinked tail males produce no sperm and thus are sterile The other two thirds of the resulting kinked tail males and all of the normal tail males and all of the females are fertile Propose a model to account for these findings d Not to be outdone your high school classmate informs you that he has isolated a purebreeding mouse strain in which males produce no sperm but have normal tails and in which females are phenotypically normal fertile normal tails You explain to your high school classmate that this is impossible Why e Eventually your classmate convinces you that the male sterile mutant that he has discovered displays autosomal recessive inheritance You decide to test whether your male sterile mutant and your classmate s male sterile mutant are in the same gene Diagram the series of crosses required to conduct an appropriate complementation experiment including the expected ratios in the final generation of infertile to fertile males for two competing models 1 two mutations in the same gene and 2 mutations in two different genes on different chromosomes 3 f Breeding experiments involving the two mutant strains are simplified when your classmate discovers that his mutant with recessive male sterility also displays a dominant phenotype in both sexes short tail Through extensive breeding your classmate identifies a series of short tail females that when crossed with wild type produce exclusively short tail progeny You cross these short tail females with fertile kinked tail males and observe the following offspring 35 short kinked tail females 32 short nonkinked tail females 28 short kinked tail males 36 short nonkinked tail males all offspring fertile Propose a model to account for these ratios g To test your model from part f you select from among the offspring from part f short kinked tail females and short kinked tail males and you cross them If your model is correct what phenotypic classes do you expect to observe and in what ratios 3 Each of the families below exhibits a different extremely rare genetic disorder Individuals expressing the trait the disorder are indicated by solid symbols Assume that no new mutations have arisen in any of the individuals shown Consider the following possible modes of inheritance i X linked recessive with complete penetrance ii autosomal recessive with complete penetrance iii autosomal recessive with 70 penetrance iv autosomal dominant with complete penetrance v autosomal dominant with 70 penetrance For each pedigree state which if any of these five modes of inheritance are not possible For the modes of inheritance that are possible calculate the probability that the individual indicated by a is affected 4 a b c d 7 03 Problem Set 1 Answer Key Fall 2001 1a Mutants 2 and 16 have dominant Leu alleles Remember dominant or recessive alleles are tested by crossing a mutant to wildtype and observing the phenotype of the heterozygote 1b The following mutants are in the same genes 1 19 3 8 18 4 13 5 6 11 15 and 10 14 Mutant alleles in these strains fail to complement each other in the heterozygote Nothing can be said definitively about strain 2 and 16 because complementation tests cannot be done with dominant alleles 1c Yes Different alleles of the same gene can be both dominant and recessive Again complementation tests cannot be done with dominant alleles 1d There is a minimum of seven genes Five groups defined by recessive alleles that noncomplement each other One group minimally defined by mutants 7 and 9 and a second group defined by mutants 12 17 and 20 7 and 9 complement 11 20 excluding 16 and thus are not a part of any previously defined complementation group They may however be in the same gene because we do not have data about complementation between mutants 7 and 9 Similar logic follows that 12 17 and 20 represent at least one new gene These groups represent at least 2 new genes because they do not complement each other 1e There is a maximum of twelve genes Five complementation groups in addition to mutants 7 9 12 17 20 which don t belong to any defined complementation groups and mutants 2 and 16 which contain alleles that have a dominant phenotype and may represent new genes 2a The kinked tail allele is dominant Parental are male tKt x t t female where tK allele with dominant kinked tail phenotype t allele with normal tail phenotype wild type
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