The genetic sex is determined at the time of fertilization by the delivery of an X or Y chromosome by the sperm. The normal female is 46XX and the normal male is 46XY. The medullary portion of the indifferent gonad develops into a testis under the influence of the protein product of the SRY (sex-determining region of the Y chromosome) gene, a product of the short arm of the Y chromosome, also known as the testis-determining factor. In the absence of the SRY protein, the cortex of the indifferent gonad develops into an ovary by default. By the seventh week of gestation the gonadal sex has been established.

By the sixth week of gestation, two paired internal duct systems derived from the mesonephric renal system exist side by side. The mesonephric (wolffian) duct will give rise to male structures, and the paramesonephric (müllerian) duct will become female structures. Male differentiation is dependent on testicular production
of müllerian-inhibitory substance (MIS) from fetal Sertoli cells and testosterone secretion by fetal Leydig’s cells. Following regression of the müllerian duct under the influence of MIS, the wolffian duct will develop into the epididymis, vas deferens, and seminal vesicles under the influence of testosterone. In the absence of MIS and testosterone, the müllerian duct will evolve into the fallopian tubes, uterus, and upper third of the vagina, and the wolffian duct will passively degenerate.

The external genitalia develop from the urogenital sinus, genital tubercle, genital folds, and genital swellings. Male development is dependent on the ability of these tissues to convert testosterone into dihydrotestosterone by the enzyme 5-α-reductase. Female differentiation is essentially the unmodified embryologic state that persists in the absence of these hormones.