How do we know that Drosophila utilizes a different sex-determination mechanism than mammals, even though it has the same sex-chromosome compositions in males and females?

Verified step by step guidance

1. Step 1: Understand the sex chromosome compositions in both mammals and Drosophila. In mammals, females are XX and males are XY, while in Drosophila, females are also XX and males are XY.
2. Step 2: Recognize that despite having the same chromosomal notation (XX for females and XY for males), the mechanism that determines sex differs between the two organisms.
3. Step 3: Learn that in mammals, the presence of the Y chromosome, specifically the SRY gene on the Y chromosome, triggers male development, making the Y chromosome the key determinant of sex.
4. Step 4: In contrast, Drosophila determines sex based on the ratio of X chromosomes to sets of autosomes (the X:A ratio), not simply the presence or absence of the Y chromosome. The Y chromosome in Drosophila is not the primary sex determinant but is required for male fertility.
5. Step 5: Conclude that the difference in sex-determination mechanisms is evident because mammals rely on a dominant Y-linked gene, whereas Drosophila relies on the X:A ratio, demonstrating that identical sex chromosome compositions can lead to different sex-determination systems.

Key Concepts

Sex-Determination Mechanisms

Sex-determination mechanisms are biological systems that dictate an organism's sex. While mammals use the presence of the Y chromosome (XY males, XX females) to determine sex, Drosophila relies on the ratio of X chromosomes to sets of autosomes, not just the presence of a Y chromosome.

Chromosomal Composition in Drosophila vs. Mammals

Both Drosophila and mammals have XY males and XX females, but in Drosophila, the Y chromosome is not the primary determinant of maleness. Instead, the X:A ratio (number of X chromosomes to autosome sets) controls sex, whereas in mammals, the Y chromosome carries the SRY gene that triggers male development.

Genetic and Molecular Evidence for Sex Determination

Experimental studies, such as chromosome manipulation and gene expression analysis, show that altering the X:A ratio in Drosophila changes sex phenotype regardless of Y presence. In contrast, mammals require the SRY gene on the Y chromosome for male development, demonstrating distinct molecular pathways despite similar chromosome types.