https://phys.org/news/2018-06-non-coding-dna-genitals-youre-born.html > Mammals will develop ovaries and become females unless the early sex organs have enough of a protein called SOX9 at a key stage in their development. SOX9 causes these organs to become testes, which then direct the rest of the embryo to become male. > The amount of SOX9 produced is controlled initially by the SRY protein encoded by the Sry gene, which is located on the Y chromosome. This is why males, who have an X chromosome and a Y chromosome, usually develop testes while females, who have two X chromosomes, do not.