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.
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.
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.