Sex Determination

Gender is determined by the sex chromosomes from the parents (23rd pair). Each parent contributes a chromosome each, with the mother always providing an X chromosome and the father providing either an X or a Y chromosome. If the combination is XX then a female develops, if it is XY then a male develops.

A full (diploid) set of chromosomes

Gonad Development

Development of the female gonads (ovaries) is the default pathway, with the Y chromosome acting as a switch to trigger development of the male gonads instead. This is due to the sry gene present on the sex-determining region of the Y chromosome. This gene causes the primordial gonads to develop into testes which then begin to produce testosterone and Anti-Mullerian Hormone (AMH). If the gene is absent or mutated then the default female gonads develop.
Both sexes develop a Mullerian duct and Wolffian duct, one of which degenerates leaving the other to fully develop. The Mullerian duct, which females retain, develops into the uterus, fallopian tubes, and upper part of the vagina. The Wolffian duct, which males retain, develops into the epididymis, vas deferens, and seminal vesicles. In females the Wolffian duct doesn’t fully develop as it is testosterone-dependent, and in males the Mullerian duct degenerates due to AMH (however if testes fail to produce AMH then both ducts persist – this is called Persistent Mullerian Duct Syndrome).

The male and female development pathways for the gonads

CryptorchidismThe external genitalia develop from a common primordial structure which both sexes possess called the urogenital sinus. This develops into the clitoris and vulva for females and the penis and scrotum for males. Again, the female external genitalia develop as the default without any external triggers. For males, the testosterone produced by the testes is converted to 5α-dihydrotestosterone (DHT) by the enzyme 5α-reductase, which then triggers development of the male external genitalia. These androgens also induce secondary sexual characteristics during puberty such as hair growth and deepening of the voice.
Sex determination involves many steps, all of which can malfunction for several reasons. The most common fault for new-born males is cryptorchidism, which is failure of the testes to descend from the abdomen to the scrotum. This usually happens in the 7th month of gestation, and affects around 3% of live births with a much higher incidence for premature and low-birth weight babies. Take a look at the birth defects page for more information on development malfunctions.