What specific observations (evidence) support the conclusions about sex determination in Drosophila and humans?

Question: Discuss the specific observations or evidence that support the conclusions about sex determination in Drosophila and humans. Answer: Evidence for sex determination in humans comes from studies on individuals with sex chromosome abnormalities such as Klinefelter syndrome, Turner syndrome, and XYY karyotype, demonstrating that the presence of the Y chromosome is sufficient to determine male sex. In Drosophila, experiments involving chromosomal manipulation, such as Calvin Bridges' work in 1921, provided evidence for the role of the X:A ratio in determining sex. The identification of key genes involved in sex determination, such as the SRY gene in humans and the Sex lethal, transformer, and doublesex genes in Drosophila, also supports their respective mechanisms. Comparative studies between species, like the similarities between the fruit fly gene "doublesex" and the human gene "DMRT1," further demonstrate the involvement of evolutionarily related genes in different sex-determination systems.