Human sex is determined by the chromosomes: Y is the male chromosome, and X is the female chromosome. A normal male chromosome pattern is XY, and female is XX.

The phenotype (physical appearance) is determined by the SRY gene, located on the short arm of the Y chromosome.

• If the SRY gene is absent or inactive due to mutations, the phenotype is female, though such women are sterile.
• If the SRY gene is translocated to an X chromosome, the phenotype is male, even with an XX chromosome set (XX male).

In embryonic development:

• Gonads develop as testes under the influence of the SRY gene.
• Sertoli cells produce Anti-Müllerian Hormone (AMH), which causes the regression of the Müllerian ducts (structures that would otherwise form the fallopian tubes, uterus, and upper vagina).
• Leydig cells in the testes produce testosterone, which binds to the androgen receptor or is converted into dihydrotestosterone (DHT) via the enzyme 5-alpha reductase.

Testosterone and DHT Effects:

• Testosterone promotes the development of the internal male reproductive tract (Wolffian ducts).
• DHT causes external male virilization (development of external male genitalia).