Final answer:
In Drosophila, sex is determined by the X:A ratio where X refers to the number of X chromosomes and A refers to the number of haploid sets of autosomes. Two X chromosomes to two sets of autosomes results in a female, whereas one X chromosome to two sets of autosomes results in a male.
Step-by-step explanation:
In the fruit fly Drosophila melanogaster, sex determination is based on the ratio of the number of X chromosomes to the number of haploid sets of autosomes (X:A ratio). Females typically have two X chromosomes (XX) and males have one X and no Y chromosome (XO). This system is different from humans, where females are XX and males are XY, and the presence of a Y chromosome determines maleness.
In Drosophila, the balance between the autosomes and the number of X chromosomes results in the development of sexual characteristics. If the ratio is 1.0 (two X chromosomes to two sets of autosomes, or XX:AA), the fly develops as female. If the ratio is 0.5 (one X chromosome to two sets of autosomes, or X:AA), the fly develops as male. This chromosomal mechanism of sex determination is unique to Drosophila and some other organisms such as C.