Here's why:
* Sex-linked genes are located on the sex chromosomes (X and Y).
* The Y chromosome carries the SRY gene, which determines maleness.
* The X chromosome carries many genes that are not related to sex determination.
* Hormones play a crucial role in sexual development, but they don't directly control the expression of genes located on the X or Y chromosomes.
Here's a breakdown:
1. Genetic Basis: The presence of a Y chromosome triggers the development of a male phenotype, regardless of hormone levels.
2. Hormonal Influence: While hormones are essential for the development of secondary sexual characteristics (like body hair, muscle mass, etc.), they don't directly influence the expression of genes on the X or Y chromosome.
Example:
A male with an X and a Y chromosome will inherit the Y chromosome-linked gene for SRY, leading to the development of testes. This will then trigger the production of testosterone, which drives further male development.
However, the expression of the SRY gene itself is determined by the presence of the Y chromosome, not by testosterone levels.
In conclusion:
While hormones play a critical role in sexual development, they don't directly control the expression of sex-linked genes. This is determined by the presence or absence of the Y chromosome, which carries the SRY gene responsible for maleness.
