Girls in an isolated Caribbean village become male and grow penises when they hit puberty due to a rare genetic disorder.

It is estimated that one in 90 children born in Salinas in the Dominican Republic make the transition by the time they reach 12.

It is so prevalent there that it is no longer considered abnormal and the children are simply referred to as the ‘guevedoces’ – which literally translates as ‘penis at 12’

The children, known as pseudohermaphrodite, are explored in BBC2 series Countdown to Life - the Extraordinary Making of You.

Catherine and his cousin Carla are what are known as 'guevedoces' in the Dominican Republic

'They didn't know what sex I was'

The documentary makers meet Johnny, who appeared female at birth and was known as Felicitia.

As a child, he did not have a penis and was brought up as a girl. Now 24, he is physically and biologically male - a process that started when he was seven years old.

He said: 'I remember I used to wear a little red dress. I was born at home instead of in a hospital. They didn't know what sex I was.

Johnny, known as Felicita, appeared to be born female at birth but developed into a boy at 7

Dr Michael Mosley visits the Caribbean for the new BBC2 series Countdown to Life - the Extraordinary Making of You

'I went to school and I used to wear my skirt. I never liked to dress as a girl.

'When they bought me girls toys I never bothered playing with them. All I wanted to do was play with the boys.'

Missing enzyme

The rare genetic disorder is caused by a missing enzyme which prevents the production of a type of male sex hormone - dihydro-testosterone - in the womb.

All babies in the womb, whether male or female, have internal glands known as gonads and a small bump between their legs called a tubercle.

At around eight weeks old, male babies who carry the Y chromosome start to produce dihydro-testosterone in large amounts - this turns the tubercle into a penis. For females, it becomes a clitoris.

A panoramic shot of Salinas in the Dominican Republic where girls become boys (file photo)

But some male babies are missing the enzyme 5-α-reductase which triggers the hormone surge. This means they appear to be born female with no testes and what looks very much like a vagina.

Then when puberty hits, a large surge of testosterone is triggers the male reproductive organs to grow. This causes their voices to deepen and the development of a penis.

Essentially, the development that should have taken place in the womb with these children happens around 12 years later.

Countdown to Life – the extraordinary making of you airs Monday September 21 on BBC2 at 9pm.