A rare genetic disorder, that is prevalent in a remote village in the Dominican Republic, sees children who appear to be girls when they are born grow penises when they reach puberty.
The children are referred to as "guevedoces", which translates literally to "penis at twelve".
According to a new BBC documentary Countdown to Life – the extraordinary making of you, one in 90 children in the village of Salinas are born as guevedoces and many of them are brought up as girls.
But between the ages of seven and 12 when they hit puberty, they "grow" a functional penis.
Children in the village of Salinas
The condition is so common in Salinas, that among people who live there it is now considered to be normal.
Johnny, 24, was brought up as a girl before he grew a penis at the age of seven.
Speaking to Dr Michael Mosley as part of the documentary, Johnny 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. I went to school and I used to wear my skirt. I never liked to dress as a girl."
The Telegraph explained the disorder is caused by a missing enzyme called 5-α-reductase, which triggers the production of a specific form of the male sex hormone - dihydro-testosterone - while still in the womb.
Usually at eight weeks into a pregnancy, a male foetus starts to produce dihydro-testosterone, which helps the formation of the penis.
For guevedoces, who lack the 5-α-reductase enzyme, this doesn't happen and they are born with no testes and what appears to be a vagina.
However, during puberty, a new surge of hormones triggers the production of dihydro-testosterone and the male reproductive organs then emerge.
Further cases have since been seen in the Sambian villages of Papua New Guinea.
The discovery of the disorder has given a pharmaceutical company more insight into how the missing enzyme can affect men over the world.
"They used [the] discovery to create a drug called finasteride, which blocks the action of 5-α-reductase.
"It is now widely used to treat benign enlargement of the prostate and male pattern baldness."
Countdown to Life – the extraordinary making of you, will be aired on BBC2 at 9pm, 21 September.