PS2B KEY - BICD100 Genetics W08 Problem Set 2B ! Chisholm...

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Problem Set 2B Tetrad Analysis 1. From a cross of a trp1 x ! trp + you sporulate and dissect 300 tetrads. You get two classes as below. Determine the genetic map. I II a trp1 a trp+ a trp1 a trp+ " trp+ " trp1 " trp+ " trp1 157 153 Type I is PD and type II is NPD. PD = NPD (approximately) so trp and the mating type locus ( mat ) are unlinked. [As there are no TT, they both must be close to their centromeres] 2. The yeast cross a leu2 x ! leu+ produces the following tetrads. Determine the genetic map. I II a leu2 a leu2 a leu2 a leu+ ! leu+ ! leu2 ! leu+ ! leu+ 127 15 Type 1 is a PD. Type II is TT. There are no NPDs. PD>>NPD so the genes are linked. Use map distance = (0.5 TT)/Total = 7.5/142 = 0.053. Multiply by 100 to get 5.3 map units or centiMorgans. 3. You cross two haploid yeast strains, the triple mutant his2 leu1 cdc5 x his+ leu+ cdc+ (i.e. wild type) ( erroneously written as trp+ in PS) to obtain a triply heterozygous diploid which you then sporulate to obtain tetrads. The his2 gene is tightly linked to the centromere of chromosome VII. The leu1 and cdc5 genes are both located on the same arm of chromosome III, with leu1 closer to the centromere than cdc5 . For each of the following types of tetrads, diagram the meiosis at the 4-chromatid stage. The diagram should show the location of crossover(s) if any, relevant centromeres, and clearly indicate which sister chromatids segregate together to explain the genotypes of the 4 spores. For full credit you must give the simplest answer (i.e. the minimum number of crossovers that can explain the tetrad). If there are >1 equally simple answers, give only one example. (a) Tetratype for his2 and cdc5 As his2 and cdc5 are unlinked, TT can only arise from recombination between one of the genes and its centromere. Either SCO between his2 and centromere VII or SCO between cdc5 and centromere III (either side of leu1--we are not considering leu1 here) would be an acceptable answer. As we are told his2 is close to its centromere, a his2 SCO would be much less likely. Drawing the chromatids as they would be after
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This note was uploaded on 06/07/2008 for the course BICD 100 taught by Professor Nehring during the Winter '08 term at UCSD.

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PS2B KEY - BICD100 Genetics W08 Problem Set 2B ! Chisholm...

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