Final answer:
The Sex lethal (Sxl) gene is a splicing factor that regulates downstream targets by alternative splicing, which leads to female development in fruit flies by promoting the production of female-specific proteins and characteristics.
Step-by-step explanation:
The Sex lethal (Sxl) gene plays a critical role in female development in Drosophila melanogaster, commonly known as the fruit fly. When the X:A ratio is 1:1, which occurs in females with two X chromosomes, Sxl is active and functions as a splicing factor.
Sxl specifically binds to the pre-mRNA of the transformer (tra) gene and facilitates its splicing into a version that supports female development. This resultingly allows the Tra protein to impact the splicing of other genes, such as doublesex (dsx) and fruitless (fru), which leads to the production of female-specific proteins and the expression of female sexual characteristics. Therefore, the correct answer to the student's question is that Sxl regulates downstream targets by alternative splicing, thereby promoting female development in fruit flies.