Boys in a small village in the Dominican Republic, called Salinas, will often be born without a penis and then grow the sex organ when they go through puberty at the age of 12, according to a new BBC documentary.

The children, known as Guevedoces, are biologically male in every respect but don’t grow a penis when they are in the womb because of a rare genetic disorder that stops a particular enzyme from releasing testosterone. This causes them to be brought up as females until they reach puberty, when the surge in testosterone activates the sudden growth.

Guevedoces are also sometimes called “machihembras” meaning ‘first a woman, then a man,’” said Dr Michael Mosley. “When Dr Imperato investigated the Guavadoces she discovered the reason they don’t have male genitalia at birth is because they are deficient in an enzyme called 5-α-reductase, which normally converts testosterone into dihydro-testosterone.

The results from the research carried out by Dr Imperato at the small village have since been used for other purposes. A pharmaceutical company has created a drug that can block the enzyme and it can be used to treat both prostate enlargement and male pattern baldness.

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