They call them the machihembras - the men who are born as girls. Some of them live in Salinas, an isolated village in the south-western Dominican Republic. They have a genetic abnormality that has stunned scientists.Despite appearing to be girls at birth, they are biologically male and at puberty develop male organs.Johnny is one of the many children affected. Instances of little girls turning into boys are so common in Salinas that it is not considered abnormal.Johnny, 24, was originally named Felecitia by his parents and brought up as a girl."I remember I used to wear a little red dress," he said."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. When they bought me girls' toys I never bothered playing with them. All I wanted to do was play with the boys."His story will be featured in a new BBC2 series, Countdown to Life - The Extraordinary Making of You.The rare genetic disorder occurs because an enzyme that converts the male sex hormone testosterone, in the womb, to dihydro-testosterone is missing.All babies in the womb, whether male or female, have internal glands known as gonads and a small bump between the legs called a tubercle.At around eight weeks, male babies, who carry the Y chromosome, start to produce dihydro-testosterone in large amounts, which turns the tubercle into a penis.But male babies lack the enzyme that triggers the hormone surge so appear to be born female.It is not until puberty, when another large surge of testosterone is produced, that the male reproductive organs emerge and their voices deepen.What should have happened in the womb happens around 12 years later.For Johnny, it happened at the age of seven. He claimed that he had never felt like a little girl and was far happier after he fully became a boy."When I changed I was happy with my life," he said.A little boy named Carla is currently going through the same transformation at nine.Although he was brought up as a girl, his mother noticed that, from the age of five, he was more inclined towards the rough and tumble play of boys. He has recently had his hair cut short after wearing plaits.Many decide not to change from their female names, so some men in Salinas have names such as Katherine.Also referred to as the Guevedoces - which translates to "penis at 12"- they were discovered for science by endocrinologist Julianne Imperato-McGinley, of Cornell Medical College, in New York, in the 1970s.Further cases have since been seen in the Sambian villages of Papua New Guinea, although the Sambians often shun the children, unlike the Dominicans, who celebrate the change.About one in 90 children in Salinas is affected and, although they resemble sexually normal males, subtle differences exist in adulthood. Most have sparse facial hair and smaller prostate glands relative to the average male.It is thought that the condition has persisted because of the isolation of the villagers. ..