Drosophila melanogaster (D. melanogaster) is a species of fruit fly frequently used by researchers in genetic studies. Members of this species have two of each of four different chromosomes: the sex chromosome (flies have X and Y) and three autosomes (chromosomes 2, 3, and 4). Researchers studying D. melanogaster conducted genetic crosses to investigate a particular X-linked recessive trait encoded by a single gene (Table 1). Affected flies have the trait. (a) Identify the genotypes of the male and female flies used in cross 2.

(b) Identify the cross in which the female parent was most likely heterozygous.

(c) The researchers hypothesize that crossing any unaffected female and an affected male will result in a 0% chance of producing an affected male offspring. Evaluate the validity of the hypothesis.

(d) Explain how the results exclude the possibility that the trait is encoded by a mitochondrial gene.