21. 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 females? What about constitutively active Sxl mutation? b. Why do Sxl alleles of eitehr 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 others change XY animals into morphological females. c. Which of these sex transformation 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 in either XX or XY animals. However, the Sxl protein regulates the production of 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 fruitlessi (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?.

1. 21. 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 females? What about
constitutively active Sxl mutation?
b. Why do Sxl alleles of eitehr 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 others change XY animals into morphological females.
c. Which of these sex transformation 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 in either XX or XY animals.
However, the Sxl protein regulates the production of 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 fruitlessi (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?