2006problemset5key - Problem Set#5 KEY BICD 100 W 06(L...

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1 Problem Set #5 KEY BICD 100 W’ 06 (L. Smith) 1.) In Drosophila , a recessive mutation ca causes eyes to be claret in color when homozygous, and a recessive mutation ro causes the eye to have a rough surface when homozygous. Another recessive mutation, e, causes body color to be ebony when homozygous. A fly from a truebreeding claret eye line is crossed to one from a true-breeding rough eye, ebony body line. F 1 progeny are crossed to claret eye, rough eye, ebony body testers to generate the following F 2 progeny: wild type 49 ebony body 1 claret eye 395 rough eye 119 ebony body, rough eye 370 ebony body, claret eye 89 ebony body, rough eye, claret eye 66 From this data determine the linkage relationships between these 3 genes. Draw a map showing the gene order and the distances between genes in cM. (To answer these questions you will first have to consider why there are only 7 progeny classes. If you’d like to try a more straightforward 3 point cross problem to start with, try one from the book: 5-17 - the second one about foxgloves). One class of double recombinants has an ebony body. The other class of double recombinants would have rough, claret eyes, but as it happens there are none (not too surprising since there is only 1 ebony body fly – there must be a lot of interference here suppressing double recombination events). E--------19 cM -------Ro----11 cM----Ca 2.) A geneticist wanting to identify genes required for biosynthesis of arginine in Saccharomyces cerevisiae (baker’s yeast) isolates two arg - mutants that arose independently from one another. To determine whether these two mutations affect the same gene or different genes, a haploid strain with mutation 1 is crossed to a haploid strain with mutation 2. The diploids are sporulated, and the resulting tetrads are analyzed to determine spore genotypes (arg - requires addition of arginine to the medium for growth and arg + does not). 40% of the resulting tetrads had two arg + and two arg - spores, 40% had all arg - spores, and 20% had 3 arg - spores and 1 arg + spore. Do these mutations affect the same gene, two linked genes, or two unlinked genes? (Hint: figure out what you would expect to happen in each case and see which one fits the data). The two mutations must affect different genes, or else you would never see any arg + progeny. So the cross was: arg-1 + x + arg-2. PD tetrads would have four arg - spores (two arg-1 + and two + arg-2); TT tetrads would have three arg - spores (arg-1 + , + arg-2, and arg-1 arg- 2) and 1 arg + spore (+ + ); NPD tetrads would have two arg - spores (arg-1 arg-2) and two arg + spores ( + + ). If two genes are unlinked, we expect % PD = % NPD, which is what we have here. So the two genes are unlinked.
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2 3.) An Irish strain of Neurospora makes big, green colonies due to mutations b and g, whereas wild type Neurospora makes small, orange colonies (has + alleles for these genes). The Irish strain was mated with a wild type strain, and the resulting asci were analyzed to determine the genotypes of the spores and their
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2006problemset5key - Problem Set#5 KEY BICD 100 W 06(L...

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