The Y chromosome contains a gene called the SRY gene, which codes for a protein that is essential for male development. The SRY gene is located on the short arm of the Y chromosome, and it is present in all males but not in females.
When the SRY gene is turned on, it triggers a cascade of events that leads to the development of male reproductive organs. These events include the formation of the testes, the production of testosterone, and the differentiation of the external genitalia into male structures.
In the absence of the SRY gene, the embryo will develop into a female. This is because the female reproductive system is the default developmental pathway for embryos. In females, the ovaries develop and produce estrogen, and the external genitalia differentiate into female structures.