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PS4_F11Key.CA

# PS4_F11Key.CA - Bio D137 Human and Eukaryotic Genetics...

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Bio D137 Human and Eukaryotic Genetics J.L.Marsh F'11 ANSWER KEY Prob Set 4 due Oct, 18 1] Black body color, scarlet eye color and hooked bristles are three autosomal recessivemutations in Drosophila governed by the genes b , st , hk respectively. The results of a trihybrid testcross are given below: 245 black 239 black, scarlet 13 black, hook 12 black, hook, scarlet 228 hook 233 hook, scarlet 16 scarlet 14 wild type A]What would the genotype of the tester be? Why?B]From the data deduce the linkage relationships of these genes. Be sure to figure out the map distances between linked genes. C]Can you deduce the parental linkage relationships of the trihybrid (triple heterozygote) parent(ie what alleles were contributed by the father and what alleles were contributed by the mother? -ie coupling and repulsion). Please indicate the parental genotype. Hint : Solve the problem by first examining pairwise segregation in order to determine theparental linkage relationships (eg ask who is independent of whom?). Answer : A] The tester should be the triple homozygote recessive b st hk/ b st hk so that you can score the genotype of all the offspring (1 point) . B] The failure to approximate a 1:1:1:1:1:1:1:1 ratio indicates that the three loci are not assorting independently. Therefore two or all three of these loci may be linked. To determine thelinkage relationships, make three comparisons between two genes, ignoring the third. (1) Black vs Scarlet (1 point) 228 + 14 = 242 neither black nor scarlet 245 + 13 = 258 black, but not scarlet 233 + 16 = 249 scarlet but not black 239 + 12 = 251 black and scarlet (2) Black vs Hook (1 point) 16 + 14 = 30 neither black nor hook 239 + 245 = 484 black, but not hook 233 + 228 = 461 hook but not black 12 + 13 = 25 black and hook (3) Hook vs Scarlet (1 point) 245 + 14 = 259 neither hook nor scarlet 228 + 13 = 241 hook, but not scarlet 239 + 16 = 255 scarlet but not hook 233 + 12 = 245 hook and scarlet Notice that in comparisons (1) &(3) a 1:1:1:1 ratio is approximated by the data, indicating that black and scarlet are assorting independently, and so are hook and scarlet. Comparison (2) indicated a gross deviation from a 1:1:1:1 ratio indicating that black and hook are on the same chromosome. From comparison (2) the parental types black (484) and hook (461) indicate that

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Bio D137 Human and Eukaryotic Genetics J.L.Marsh F'11 Prob Set 4 due Oct, 18 the trihybrid parent was b +/ + hk; +/st . The crossover types (neither black nor hook {30};black plus hook {25}) indicate that (30 + 25)/1000 = 0.055 or 5.5 map units between the black and hook loci (1 point).
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PS4_F11Key.CA - Bio D137 Human and Eukaryotic Genetics...

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