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.

The children, known as pseudohermaphrodite, will be explored in BBC2 series Countdown to Life – the Extraordinary Making of You.  Monday September 21 on BBC2 at 9pm.

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