a. There are dominant mutations of the Sxl gene, called SxlM alleles, that transform XY animals into females but do not affect XX animals. Reversions of these SxlM alleles can be readily induced with mutagen treatment. These reversions, called Sxlf alleles, yield normal individuals in XY animals, but, in homozygotes, they transform XX animals into phenotypic males. Provide a possible explanation for these observations, keeping in mind that Sxl is ordinarily dispensable for male development.

b. XYY humans are fertile males. XXX humans are fertile females. What do these observations reveal about the mechanisms of sex determination and dosage compensation?

c. You have cloned a piece of DNA that contains four genes. How could you use the spatial-expression pattern of their mRNA in a wild-type embryo to identify which represents a candidate gene for the mutation in a pair-rule gene?