A phenomenon which is very common in the West Indies is that at the age of 12-13 young girls become men; they begin to display testicles which were not visible before. They also develop more "manly" behavior; they develop stronger muscles and take up jobs typically performed by men in the Caribbean.
A celebrated research some years ago examined several families living in those islands. It turned out that the androgynous characteristic is caused by a mutant recessive gene so both parents must carry it for the child to be androgynous. The reason this phenomenon is so common (over there) is the islanders' tendency to marry close family members.
The researchers discovered that all the families of androgynes are descended from a single woman who had this special mutation eight generations ago. Until the age of 12-13 these "girls" look and behave just like girls, because they lack the enzyme which turns testosterone to hydrotestosterone.
Hydrotestosterone is a hormone responsible for male growth and the development of male sexual organs. Around the age of 13 their bodies -- for reason that are not yet known -- begins to produce this enzyme, eventually causing the sexual transition.