Name_____KEY_________________________ SID_____________________1MCB140: Practice Exam1. Under harsh environmental conditions, C. elegans can enter an alternate third larvalstage known as the dauer stage. Animals can live in this stage for months untilconditions improve, allowing them exit the dauer stage and complete their life cycle.Two types of mutants that are defective in dauer development exist. Dauerconstitutive mutants enter the dauer stage even when environmental conditions aregood, and dauer defective mutants are unable to enter the dauer stage. The conservedinsulin regulatory pathway is required for dauer development. Loss of functionmutations in genes of the insulin pathway cause either dauer constitutive or dauerdefective phenotypes.a. PI3 kinase is a signaling molecule in the insulin pathway, and one of the PI3 kinasesubunits is encoded by the age-1 gene. In a cross between a homozygous age-1hermaphrodite and a wild-type male, individual hermaphrodite F1 cross progeny thatare heterozygous for the loss-of-function age-1 mutation generate all normal (notdauer) F2 self progeny. When individual F2 hermaphrodite self progeny aretransferred to separate petri plates, 3/4 have F3 self progeny that are normal, and 1/4of them have F3 self progeny that are all dauer. (The growth conditions used will notinduce dauer formation in wild-type animals.) Define the genotypes of the F2 progenyof the heterozygous hermaphrodite and whether they will have normal or dauerprogeny. Also explain why none of them are dauer.Name_____KEY_________________________ SID_____________________2b. The FOXO transcription factor is encoded by the daf-16 gene. Signaling though theinsulin receptor, which is encoded by the daf-2 gene, activates PI3 kinase, whicheventually leads to the phosphorylation and inhibition of DAF-16/FOXO. From whatyou know about age-1 and the pathway just described, what will be the dauerphenotypes (constitutive or defective) of daf-2 and daf-16 mutants.c. What double mutant phenotypes would be consistent with the molecular pathwayfor DAF-2, AGE-1 and DAF-16 described above.d. You are interested in isolating suppressors of a loss-of-function mutation in theinsulin receptor gene daf-2. You want to screen for recessive suppressors that are notmaternal effect using the chemical mutagen EMS. Describe what you would look forand what generation you would screen.Name_____KEY_________________________ SID_____________________3e. In your screen, you identify eight suppressors. Five of the suppressor mutationssuppress 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 suppressorsof this type in a small screen, you decide that the suppressor mutations are notinteraction suppressors. What type of suppressors do you think that these fivemutations represent? Do you think that these suppressors will have effects onmutations in other genes?f. The remaining three suppressors (named #1-#3) can suppress all daf-2 alleles. Thethree suppressors map to the same region of chromosome I. Suppressor #1 is recessiveand only partially suppresses the daf-2 mutant—some of the double mutants still formdauers. Suppressors #2 and #3 completely suppress daf-2 when homozygous.Suppressor #2 is recessive, but suppressor #3 is dominant. Animals homozygous fordaf-2 and heterozygous for #3 are partially suppressed, forming dauers about 20% ofthe time. (The daf-2 homozygotes form dauers 100% of the time.) You suspect that allof these mutations are in the same gene? How do you test this for #1 and #2 and whatis the outcome if they are in the same gene? Use only genetics.Name_____KEY_________________________ SID_____________________4g. Assuming that all three suppressor mutations #1-3 are in the same gene, how doyou think that they might change the function of the mutated gene? You test yourhypothesis. 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 mightyou define which gene of the 20 is mutated? No sequencing allowed.Name_____KEY_________________________ SID_____________________52. 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 mosaicanalysis using white as a cell autonomous marker. The results for mosaic ommatidiaare shown below for each mutant. What can you say about the site of function foreach gene?Name_____KEY_________________________ SID_____________________63. In the pedigree below label different individuals that have the same mtDNA. Forexample, 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 showup later. Is it possible that the disease trait in the pedigree is maternally inherited?Name_____KEY_________________________ SID_____________________74. 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 thediploids. You analyze 100 tetrads and get the following results: 90 tetrads have two wild-typespores and two Leu-, Arg- spores; nine tetrads have one wild-type spore, one Leu- spore, oneArg- 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 beingmutagenized with X rays). The strains is suspected of containing a rearrangement. You crossthe strain with a wild-type strain of the opposite mating type and analyze 50 tetrads aftersporulating 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?Name_____KEY_________________________ SID_____________________85. Most human females have roughly equal numbers of cells that express either genes fromthe paternal X or the maternal X. Rare human females have highly
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