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2005_pset3_ans - 7.06 Problem Set#3 Spring 2005 1 The...

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1 7.06 Problem Set #3, Spring 2005 1. The Drosophila compound eye is composed of about 800 units called ommatidia. Each ommatidium contains eight photoreceptor neurons (R1 through R8), which develop in a fixed order. Mutations have been isolated that perturb the differentiation of the R7 cells. Two such mutations are in genes called Sev ( Sevenless ) and Boss ( Bride Of Sevenless ). R7 progenitor cells in fly eyes that do not express either (or both) of these genes differentiate into non-neural cone cells (instead of R7 cells). Too many R7 cells develop in fly eyes that are overexpressing either BOSS or SEV (or both). a. Overexpression of SEV in fly eyes that do not express functional Boss leads to ommatidia with too many R7 cells. Overexpression of BOSS in fly eyes that do not express functional Sev generates ommatidia with no R7 cells. What do these results imply about the relationship between Sev and Boss ? This epistasis analysis allows one to order genes into a genetic pathway. Epistasis is inferred from the phenotype of double mutant cells or organisms. One analyzes the phenotype of the double mutant organism, and compares it to the two single mutant phenotypes. Whichever phenotype is displayed by the double mutant is the phenotype of the mutant that is further downstream in the pathway. Overexpression of SEV alone causes too many R7 cells to develop, and a loss of function in Boss alone leads to no R7 cells. The double mutant leads to too many R7 cells developing. Thus one would place the Sev gene downstream of the Boss gene in the pathway for the development of the R7 cell. (Downstream means closer to the end of the pathway, or the output. Upstream means closer to the signal for the pathway, or the input.) b. You have isolated a gain of function mutation in Sos ; you call this allele Sos *. You find that flies that are expressing Sos* in their eyes have high numbers of R7 cells. What two strains would you make (given that you have gain of function and loss of function alleles in SEV and BOSS available to you) if you didn’t already know the order of Sos in the fly eye developmental pathway with Sev and Boss , and what phenotype would each strain have? Sos is downstream of Sev and Boss in the fly eye developmental pathway. (BOSS is the ligand for SEV (the RTK receptor), and SOS is a downstream effector in this RTK-Ras signaling pathway. To determine the order ( Boss Sev Sos an R7 cell), you could have made: Flies with eyes expressing Sos* that do not express functional Boss ; these would have extra R7 cells. AND
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2 Flies with eyes expressing Sos* that do not express functional Sev ; these would have extra R7 cells. Note that you have to use double mutants that contain two single mutations that yield opposite phenotypes. That is, you couldn’t use a fly that was expressing Sos* and overexpressing SEV in its eyes, for instance, because both single mutant phenotypes = too many R7 cells.
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