Final answer:
In Drosophila, fruit flies with XXY sex chromosomes are phenotypically female, as the presence of two X chromosomes, despite also having a Y chromosome, leads to the development of female characteristics.
Step-by-step explanation:
Fruit flies (Drosophila melanogaster) with XXY sex chromosomes exhibit the sexual phenotype of a female. This is because, in Drosophila, the sex is determined by the presence or absence of the Y chromosome. If only X chromosomes are present (as in XX), or if there is an extra X chromosome alongside the standard XY set (as in XXY), the fly will develop female characteristics. This occurs despite the presence of a Y chromosome because the impact of extra X chromosome(s) on the phenotype is generally more substantial than that of a single Y chromosome.
Nondisjunction during meiosis can produce sex chromosome abnormalities such as the XXY combination. Unlike in mammals, where XXY individuals are male but with Klinefelter syndrome, in Drosophila, the presence of a Y chromosome does not always override the effects of an extra X chromosome, and the presence of two X chromosomes leads to a female phenotype. This is an example of how chromosomal differences can influence sexual phenotype differently across species.