Girls living in a Caribbean village turn into boys by growing a penis when they hit puberty as a result of a rare genetic disorder.
Around one in 90 children born in Las Salinas, in the Dominican Republic's Barahona province, are born with the condition which sees them naturally transform from females to males during puberty.
Referred to as "guevedoces", which translates to "penis at 12", the rare disorder featured in the BBC series Countdown to Life: The Extraordinary Making of You in which youngsters were interviewed.
Among them was Johnny, a guevedoce, who was once known as Felicita.
Johnny, known as Felicita, appeared to be born female at birth but developed into a boy
Johnny said he did not have a penis when he was born and was brought up as a girl in which he remembers attending school wearing a dress.
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.
Two children - Catherine and his cousin Carla - in the Dominican Republic
"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."
The mystery condition is caused by a missing enzyme which stops the production of a male sex hormone, dihydro-testosterone, in the womb, The Sun reports.
It means boys appear as females when they are born.
But when testosterone starts to surge upon reaching puberty, their voices break and they grow a male sexual reproductive organ.
Dr Michael Mosley visits the Caribbean for the new BBC2 series Countdown to Life: The Extraordinary Making of You
The development should have taken place in the womb but affects these children 12 years later.
Dr Michael Mosley was among those to visit Las Salinas as part of the series.
Writing in The Telegraph , Dr Mosley said: "Guevedoces are also sometimes called "machihembras" meaning "first a woman, then a man".
"When they're born they look like girls with no testes and what appears to be a vagina.
"It is only when they near puberty that the penis grows and testicles descend."
He said guevedoces live out the rest of their lives as men, although their prostates remain small.
He said Dr Julianne Imperato, a Cornell endocrinologist, was among the first to study guevedoces when she travelled to the Dominican Republic in the 1970s.
Dr Mosley said her research was later used by US pharmaceutical giant Merck to create a drug which is used to treat benign enlargement of the prostate and male pattern baldness.
Countdown to Life: The Extraordinary Making of You first aired on the BBC in September 2015.