Name KEY SID MCB140 Practice Exam 1 Under harsh environmental conditions C elegans can enter an alternate third larval stage known as the dauer stage Animals can live in this stage for months until conditions improve allowing them exit the dauer stage and complete their life cycle Two types of mutants that are defective in dauer development exist Dauer constitutive mutants enter the dauer stage even when environmental conditions are good and dauer defective mutants are unable to enter the dauer stage The conserved insulin regulatory pathway is required for dauer development Loss of function mutations in genes of the insulin pathway cause either dauer constitutive or dauer defective phenotypes a PI3 kinase is a signaling molecule in the insulin pathway and one of the PI3 kinase subunits is encoded by the age 1 gene In a cross between a homozygous age 1 hermaphrodite and a wild type male individual hermaphrodite F1 cross progeny that are heterozygous for the loss of function age 1 mutation generate all normal not dauer F2 self progeny When individual F2 hermaphrodite self progeny are transferred to separate petri plates 3 4 have F3 self progeny that are normal and 1 4 of them have F3 self progeny that are all dauer The growth conditions used will not induce dauer formation in wild type animals Define the genotypes of the F2 progeny of the heterozygous hermaphrodite and whether they will have normal or dauer progeny Also explain why none of them are dauer 1 Name KEY SID b The FOXO transcription factor is encoded by the daf 16 gene Signaling though the insulin receptor which is encoded by the daf 2 gene activates PI3 kinase which eventually leads to the phosphorylation and inhibition of DAF 16 FOXO From what you know about age 1 and the pathway just described what will be the dauer phenotypes constitutive or defective of daf 2 and daf 16 mutants c What double mutant phenotypes would be consistent with the molecular pathway for DAF 2 AGE 1 and DAF 16 described above d You are interested in isolating suppressors of a loss of function mutation in the insulin receptor gene daf 2 You want to screen for recessive suppressors that are not maternal effect using the chemical mutagen EMS Describe what you would look for and what generation you would screen 2 Name KEY SID e In your screen you identify eight suppressors Five of the suppressor mutations suppress the daf 2 allele that you used but not other loss of function alleles of daf 2 Because interaction suppressors are rare and because you isolated several suppressors of this type in a small screen you decide that the suppressor mutations are not interaction suppressors What type of suppressors do you think that these five mutations represent Do you think that these suppressors will have effects on mutations in other genes f The remaining three suppressors named 1 3 can suppress all daf 2 alleles The three suppressors map to the same region of chromosome I Suppressor 1 is recessive and only partially suppresses the daf 2 mutant some of the double mutants still form dauers Suppressors 2 and 3 completely suppress daf 2 when homozygous Suppressor 2 is recessive but suppressor 3 is dominant Animals homozygous for daf 2 and heterozygous for 3 are partially suppressed forming dauers about 20 of the time The daf 2 homozygotes form dauers 100 of the time You suspect that all of these mutations are in the same gene How do you test this for 1 and 2 and what is the outcome if they are in the same gene Use only genetics 3 Name KEY SID g Assuming that all three suppressor mutations 1 3 are in the same gene how do you think that they might change the function of the mutated gene You test your hypothesis How do you do this and what are the expected results Use only genetics h Suppressors 1 3 are mapped to a small region that contains 20 genes How might you define which gene of the 20 is mutated No sequencing allowed 4 Name KEY SID 2 In a screen you identify several new recessive mutations that lead to a loss of R7 You are interested in defining where the genes function and carry out a mosaic analysis using white as a cell autonomous marker The results for mosaic ommatidia are shown below for each mutant What can you say about the site of function for each gene 5 Name KEY SID 3 In the pedigree below label different individuals that have the same mtDNA For example if three individuals have the same mtDNA label them all with an A Individuals with a different mtDNA with a B etc Label all individuals Because of heteroplasmy a maternally inherited trait can skip generations and show up later Is it possible that the disease trait in the pedigree is maternally inherited 6 Name KEY SID 4 You are given a haploid yeast strain that is a leucine and arginine auxotroph Leu Arg You cross the strain to wild type haploid yeast of the opposite mating type and sporulate the diploids You analyze 100 tetrads and get the following results 90 tetrads have two wild type spores and two Leu Arg spores nine tetrads have one wild type spore one Leu spore one Arg spore and one Leu Arg spore one tetrad has two Leu spores and two Arg spores What can you say about the LEU and ARG genes Don t give a quantitative answer How did the tetrad with two Leu and two Arg spores arise You are also given a Leu Arg haploid strain derived from the strain above after being mutagenized with X rays The strains is suspected of containing a rearrangement You cross the strain with a wild type strain of the opposite mating type and analyze 50 tetrads after sporulating the diploids 47 tetrads have two wild type spores and two Leu Arg spores Three tetrads have one wild type spore one Leu Arg spore and two dead spores What type of rearrangement does the Leu Arg auxotroph contain How did the tetrad containing the two dead spores arise 7 Name KEY SID 5 Most human females have roughly equal numbers of cells that express either genes from the paternal X or the maternal X Rare human females have highly biased ratios of X linked genes For example in one woman all of her cells express maternally inherited X lined genes Assume that the cause of this woman s X linke gene expression bias is a mutation in the Xist gene How do you think that the Xist gene is altered From which parent would the altered gene be inherited 8
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