Final answer:
Sex determination in Drosophila involves the Sex lethal (Sxl) gene activated based on the X chromosome-to-autosome ratio, leading to female-specific splicing events, while in humans, it is based on the presence or absence of the Y chromosome, particularly the SRY gene which determines male characteristics.
Step-by-step explanation:
The process of sex determination in Drosophila melanogaster (fruit flies) differs significantly from sex determination in humans. In Drosophila, the primary determination relies on the X chromosome-to-autosome (X/A) ratio, with the Sex lethal (Sxl) gene being the key determinant. If this ratio equals or exceeds 1, which occurs in females with two X chromosomes, Sxl is transcribed leading to a series of splicing events that result in female-specific proteins. In males, with only one X chromosome, the Sxl gene is not transcribed, resulting in default splicing and the production of male-specific proteins. This process is cell-specific and does not involve circulating sex hormones.
In contrast, human sex determination is governed by the presence of the XY chromosome system, in which the presence of a Y chromosome typically results in male development (XY), and the absence of a Y chromosome in the XX configuration results in female development. The human system is not dependent on the X/A ratio or on splicing events for sex determination, and it relies instead on the presence or absence of the Y chromosome and its SRY gene, which initiates the development of male characteristics.