Chapter 19, Problem 24P

Recall from Chapter 17 that in Drosophila, sex determination depends on the number of X chromosomes: XX flies are female, and XY flies are male. The elaboration of female morphology depends on a cascade of gene regulation initiated by the expression of the Sxl gene in XX embryos (Fig. 17.35 and Fig. 17.36). Sxl causes splicing of the tra gene transcript in a manner that allows translation of Tra protein. Tra protein in turn causes female-specific splicing of the dsx mRNA and expression of Dsx-F protein, a transcription factor. Dsx-F activates the transcription of female morphology genes and represses male morphology gene expression. The ix gene product, Ix protein, is required for Dsx-F to repress the expression of male morphology genes.

a.	Drosophila sex determination is a switch/regulation pathway. Define the signal and the switch in this pathway. In what cells is the switch ON? OFF?
b.	Describe the tra- and ix- mutant phenotypes in XX flies and XY flies
c.	Predict the mutant phenotype of tra- ix- null double mutants. Which mutation would you expect to be epistatic?
d.	In the two examples discussed previously concerning epistasis analysis in the study of eye development-sev- and RasG12V described in the text, and sev- and yan- in Problem 23-the epistatic mutation defined the downstream gene in the pathway. Can you explain why that is not the case in this example for the sex determination pathway?