Eye pigment in a particular strain of fly is determined by two genes. An autosomal gene that controls the color of the pigments in the eye has two alleles: a dominant allele (\(R\)) that results in red eyes and a recessive allele (\(r\)) that results in sepia eyes. A sex-linked gene that controls the expression of the colored pigments also has two alleles: a dominant allele (\(T\)) that allows for expression of the colored pigments and a recessive allele (\(t\)) that does not allow for expression of the colored pigments. Individuals without a \(T\) allele have white eyes regardless of the alleles of other eye-color genes.

Which of the following represents a cross between a white-eyed female and a red-eyed male?

A while-eyed female must be XX. It must also not have a \(T\) allele since it is white. The \(T\) and \(t\) alleles are on the \(\text{X}\) chromosome. Only the last answer choice works. It is irrelevant what the color of the pigments are since the female will have white-eyes regardless.

The male is red eyed, so it must be XY with at least one \(R\) allele. Since the red pigment is expressed, the X chromosome must also contain the \(T\) allele.