When born, to all intents and purposes they appear to be female. But when the children referred to as “Guevedoces” in an isolated village in the Dominican Republic reach puberty, things start to change. They develop muscles, their chests start to grow, and by the age of 12, they sprout testes and a penis.

This extraordinary intersex condition, known medically as “5-alpha-reductase deficiency,” has been documented for a new BBC2 series “Countdown to Life,” which takes a look at how we develop in the womb and how early changes impact us for the rest of our lives. In the program, they meet one of these Guevedoces, which literally translates as “penis at 12,” called Johnny.

Talking to presenter and journalist Dr Michael Mosley, Johnny describes his childhood and being brought up as a girl called Felecitia by his parents. “I never liked to dress as a girl and when they bought me toys for girls I never bothered playing with them – when I saw a group of boys I would stop to play ball with them.” When he started to develop as male during puberty, he was taunted and bullied.

But Johnny is not alone. He is one of many children who live in the village of Salinas, located in the southern part of the Dominican Republic, with the condition. In fact, as many as 1 in 90 boys in the area were raised as girls until they went on to develop external testes and a penis. At birth, they have nothing except what appears to be a vagina, and it is only as they mature that it becomes apparent all is not as it seems.

The condition was first brought to light in the 1970s by Dr Julianne Imperato, who was working at Cornell University as an endocrinologist. By studying the Guevedoces, she determined the underlying cause for the astonishing condition.

In the first eight weeks of development, fetuses are neither male nor female. After this period, if the fetus is genetically male, then the internal glands known as gonads are instructed by the Y chromosome to turn into testes. At the same time, testosterone is sent to a particular structure called the tubercle. In males, the testosterone is converted into the more potent dihydro-testosterone which turns the tubercle into a penis. In females, dihydro-testosterone isn't made and the tubercle becomes a clitoris.

Dr Imperato found that while genetically male, in the Guevedoces lack the enzyme – 5-alpha reductase – that converts testosterone to dihydro-testosterone, meaning that the boys are born looking like females. When they then hit puberty, and the body is flooded with another surge of testosterone, the body responds this time and the penis develops and testes descend. The lack of this enzyme seems to be genetic, with the condition abnormally prevalent in this particular region of the Dominican Republic.

Once Guevedoces have hit puberty, then they seem to be able to live their lives as normal. The penis is a little undersized, but apart from that is normally fully functioning, with the testes even producing sperm.

[H/T: BBC Magazine]