Androgen insensitivity syndrome

Androgen insensitivity syndrome (AIS) is a sex-linked mutation which causes a lack of sensitivity to androgens, the male sex hormones. Males who are born with it develop as females, but have no uterus; the vagina dead-ends. This is known as a blind pouch[?]. Females who are born with it, if there were any, would develop almost normally (females do produce some testosterone, but not much).

Unlike other sex-linked traits, which are merely very rare in women, AIS is extremely rare in XX humans. For it to happen, an AIS carrier would have to have a child with a man with AIS - but he developed into a woman who can't reproduce. Or else she would have to have a child in which the other X chromosome mutated as well.

People born with AIS thus are almost always genetically male, even though their appearance and personality tends to be more "feminine" than most normal women due to the lack of androgenic effects. Such individuals tend to be rather tall and muscular and, curiously, the syndrome often results in an enlargening of the breasts.

The effects of AIS are defined by the severity of the condition. Partial AIS males will respond to limited amounts or types of androgens, while complete androgen insensitivity syndrome (CAIS) will result in a biological male who will appear fully female and have a vagina, but will not be able to reproduce due to the lack of female gonads and internal sex organs. The AIS male possesses undescended testes, which provide estrogens for development of female secondary sex characteristics, but will usually need to be surgically removed due to the cancer risk. Following a bilateral orchidectomy, or castration, hormone replacement therapy using estrogens will be needed.