Problem Set 3 Key

Problem Set 3 Key - BICD100 FA2011 Problem Set#3 KEY 1 A...

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BICD100 FA2011 Problem Set #3 KEY 1.) A particular species of birds is normally true-breeding for black feather color. However, in a large colony of these birds living around a nuclear power plant, you find several gray birds and one white one. You perform the following crosses and observe indicated F1 progeny: black X black -> 24 black white X white -> 34 white gray X gray -> 10 black, 12 white, 21 gray black X white -> 29 gray a. Are these results consistent with the hypothesis that gray and white coloration are caused by different, recessive mutations in a single feather pigment gene, a null allele responsible for white and a leaky allele responsible for gray? If not, why? If yes, what sort of alteration at the DNA or chromosomal level would likely be responsible? No. This hypothesis predicts that gray X gray should give all gray progeny, and that black X white should give all black progeny. b. Are these results consistent with the hypothesis that gray and white coloration are both caused by the same mutation, which is a null mutation in a haploinsufficient gene needed for black pigment? If not, why? If yes, what sort of alteration at the DNA or chromosomal level would likely be responsible? Yes. If we call the wild type allele B and the mutant allele b, then black birds are BB, gray are Bb and white are bb. Thus, b would be semi-dominant, which is what we expect for a null mutation in a haploinsufficient gene. The null mutation would likely be caused by a deletion, premature stop codon (nonsense) or frameshift mutation. c. Are these results consistent with the hypothesis that gray and white coloration are both caused by the same mutation, a dominant negative mutation in a gene needed for black pigment? If not, why? If yes, what sort of alteration at the DNA or chromosomal level would likely be responsible? Yes, because dominant negative mutations are also usually semi-dominant to wild type. Genotypes as above (BB black, Bb gray, bb white). A dominant negative allele encodes a protein that is non-functional and inhibits the function of wild type protein – could be the result of a missense or nonsense (premature stop) mutation. Could also be due to deletion of part of the gene (giving a truncated protein), but not the whole gene (this would give no protein).
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