Question

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

Short answer

Answer: In Drosophila, sex determination is based on the ratio of X chromosomes to autosomes, with the crucial gene Sxl controlling sexual development. In humans, sex determination relies on the presence or absence of the Y chromosome, specifically the SRY gene, which initiates male sexual development. Evidence supporting these conclusions includes mutations in Sxl leading to intersex or sex-transformed individuals in Drosophila, gene dosage experiments affecting sex determination, and the direct manipulation of Sxl expression. In humans, the presence of XYY males, cases of XX males and XY females, and experiments involving the addition of the SRY gene to XX mice, all support the role of the SRY gene in sex determination.

Step-by-step solution

Basic introduction to sex determination

Sex determination is the process by which an organism's sex is established. It can be a genetic or environmental process. In most animals, including Drosophila and humans, sex determination is based on the presence or absence of specific sex chromosomes.

Sex determination in Drosophila

In Drosophila melanogaster, sex determination is based on the ratio of X chromosomes to autosomes. Males have one X chromosome and one Y chromosome (XY), while females have two X chromosomes (XX). A critical gene on the X chromosome, called "Sex-lethal" (Sxl), controls this process. When Sxl is active, the female sexual development occurs, and when it is inactive, the male sexual development occurs.

Evidence for Drosophila sex determination

Several observations and experiments support this conclusion about sex determination in Drosophila: 1. Mutations in the Sxl gene can lead to intersex or sex-transformed individuals. 2. Gene dosage experiments showed that increasing or decreasing the number of X chromosomes can alter the sex of the individual. 3. Manipulation of Sxl expression through transgenic techniques can lead to the transformation of one sex into another, providing direct evidence for its role in sex determination.

Sex determination in humans

In humans, sex determination is based on the presence or absence of the Y chromosome. Males have one X chromosome and one Y chromosome (XY), while females have two X chromosomes (XX). The SRY (Sex-determining Region Y) gene, located on the Y chromosome, is responsible for initiating male sexual development.

Evidence for human sex determination

There is numerous evidence supporting the conclusion about sex determination in humans: 1. Individuals with an extra Y chromosome (XYY) are male, supporting the idea that the Y chromosome contains the essential genes for male development. 2. Cases of XX males and XY females show that the presence or absence of the SRY gene can determine the individual's sex, regardless of the chromosome composition. 3. Experiments involving the addition of the SRY gene to XX mice resulted in male sexual development, providing direct evidence for the role of the SRY gene in sex determination. By analyzing the evidence and observations in both Drosophila and humans, it is clear that the mechanisms of sex determination vary significantly in these species. The presence or absence of key sex-determining genes, such as Sxl and SRY, plays a crucial role in determining the sexual development of these organisms.