2008 B00 final exam key for 2010

2008 B00 final exam key for 2010 - BICD100 Final Exam...

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BICD100 Final Exam 3/21/08 (B00) KEY (MEAN 139/200 points) 1.) A yeast geneticist studying genetic control of sugar metabolism isolated two different haploid lac - mutants in Saccharomyces cerevisiae (these mutants cannot utilize lactose as a carbon source so can only grow if a different sugar is added to the medium). These mutants were crossed together and the resulting diploids were sporulated. Analysis of the tetrads gave the following results: tetrad type number 4 lac - spores 123 PD 3 lac - spores and 1 lac + spore 42 TT 2 lac - spores and 2 lac + spores 11 NPD a. Do the two lac - mutations affect the same gene or different genes? If they affect different genes, what is the linkage relationship between them including map distance in cM if applicable (12 pts)? Different genes (if they were the same gene, there would be no tetrads with lac + spores) map distance in cM = [(NPD + 1/2TT)/total tetrads] X 100 = (11 + ½ *42)/176 = 0.18 ->> the genes are 18 cM apart b. Based on your answer to a, what phenotype would you expect to find (lac - or lac + ) in the diploids produced by crossing the type lac- haploids together (5 pts)? Since the two lac - mutations affect different genes, we expect to see complementation in the diploid – it should be lac + 2.) Much of the variation in skin, eye, hair and fur pigmentation in mammals and other vertebrates is due to mutations affecting the melanin biosynthetic pathway. The melanocortin 1 receptor (MC1R or sometimes written Mc1R) is a key regulator of this pathway. When melanocyte stimulating hormone ( α -MSH) binds to MC1R on the surface of melanocytes, melanin synthesis is stimulated. Thus, loss of function mutations in MC1R cause pigmentation to be reduced or absent (they are recessive). Mice completely lacking MC1R function are blonde. The Agouti protein is a negative regulator of this pathway. Like α -MSH, Agouti binds to MC1R but inhibits the activation of the receptor rather than stimulating it. In mice, dominant, gain of function Agouti mutations reduce melanin synthesis causing a pale coat color (“Agouti”) phenotype. a. Based on this information, what phenotype do you expect to find in mice that are homozygous for gain of function Agouti and loss of function MC1R mutations (5 pts)? blonde (if there is no functional MC1R receptor, then there is nothing for the excess/hyperactive Agouti protein to do! b. What genetic phenomenon explains your answers in part a (5 pts)? epistasis (blonde phenotype masks Agouti phenotype in double mutant) Hopi Hoekstra (a professor at UCSD until last year when she was lured away to Harvard) and her colleagues have investigated the roles of Agouti and MC1R in body color differences between two subspecies of “oldfield mice”: Peromyscus subgriseus , which are darkly colored and are widely distributed throughout the southeastern U.S., and Peromyscus leucocephalus , which are a pale color and are found only on the barrier islands off the coast of Florida. The pale color of
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2008 B00 final exam key for 2010 - BICD100 Final Exam...

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