The Drosophila gene Sex lethal (Sxl) is deserving of itsname. Certain alleles have no effect on XY animals butcause 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 allelesare lethal to females, while others are lethal to males.a. Would you expect a null mutation in Sxl to causelethality in males or in females? b. Why do Sxl alleles of either type cause lethality ina specific sex?The gene transformer (tra) gets its name from sexualtransformation, as some tra alleles can change XXanimals into morphological males, while other traalleles can change XY animals into morphologicalfemales.c. Which of these sex transformations would becaused by null alleles of tra and which would becaused by constitutively active alleles of tra?d. In contrast with Sxl, null tra mutations do notcause lethality either in XX or in XY animals.However, the Sxl protein regulates the productionof the Tra protein. Why then do all tra mutant animals survive?e. Predict the consequences of null mutations in tra-2on XX and XY animals. (Recall that tra-2 encodesa protein, expressed in both sexes, that is requiredfor Tra function.)f. XY males carrying loss-of-function mutations in thefruitless (fru) gene display aberrant courtship behavior. Would you predict that either XX or XY animalswith wild-type alleles of fru but loss-of-function mutations of tra would also court abnormally?

Human Anatomy & Physiology (11th Edition)
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Author:Elaine N. Marieb, Katja N. Hoehn
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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? 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?

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