Chapter 17, Problem 41P

The Drosophila gene Sex lethal (Sxl) is deserving of its name. Certain alleles have no effect on XY animals but cause XX animals to die early in development. Other alleles have no effect on XX animals but cause XY animals to die early in development. Thus, some Sxl alleles are lethal to females, while others are lethal to males.

a.	Would you expect a null mutation in Sxl to cause lethality in males or in females? What about a constitutively active Sxl mutation?
b.	Why do Sxl alleles of either type cause lethality in a specific sex? The gene transformer (tra) gets its name from sexual transformation, as some tra alleles can change XX animals into morphological males, while other tra alleles can change XY animals into morphological females.
c.	Which of these sex transformations would be caused by null alleles of tra and which would be caused by constitutively active alleles of tra?
d.	In contrast with Sxl, null tra mutations do not cause lethality either in XX or in XY animals. However, the Sxl protein regulates the production of the Tra protein. Why then do all tra mutant animals survive?
e.	Predict the consequences of null mutations in tra-2 on XX and XY animals. (Recall that tra-2 encodes a protein, expressed in both sexes, that is required for Tra function.)
f.	XY males carrying loss-of-function mutations in the fruitless (fru) gene display aberrant courtship behavior. Would you predict that either XX or XY animals with wild-type alleles of fru but loss-of-function mutations of tra would also court abnormally?