In Drosophila, sex is primarily determined by the $\mathrm{X}: \mathrm{A}$ ratio, or the ratio of the number of $\mathrm{X}$ chromosomes to the number of sets of autosomes not because of the presence of a Y chromosome as in human sex determination. Although the $\mathrm{Y}$ chromosome is entirely heterochromatic, it contains at least 16 genes, many of which are thought to have male-related functions.
Related Theory
In butterflies, sex is determined by chromosome differences. But unlike in humans with the familiar $X$ and $Y$, in butterflies, it is the femates that determine the sex of offspring. Males are $\mathrm{ZZ}$, while females are $\mathrm{ZW}$