In the 1970s, Dr Julianne Imperato traveled to the Dominican Republic upon hearing rumors about girls turning into boys upon hitting puberty in a small town somewhere in the country. The Cornell endocrinologist ultimately discovered the reason for this remarkable occurrence, the results of which led to another breakthrough in the medical field. 

During conception, babies inherit a set of genes that later on determine their genital structure. However, for the first few weeks of fetal development, the baby's internal and external genital structure are the same - all babies possess a protrusion, referred to as a tubercle. A baby with a Y chromosome will develop testicles from the gonads. The Y chromosome also gives testosterone to the tubercle, which then converts it into a penis, through the use of a hormone call dihydro-testosterone. If dihydro-testosterone is not present, the tubercle develops into the clitoris, thus making female the "default sex" for humans. 

Dr Imperatio studied the case of some girls in a certain area in the Dominican Republic called "Guevedoces," translated "penis at twelve," who, upon birth, possess what seems to be a vagina. Come puberty, however, and they begin to grow a penis and testicles. 

Johnny, from the said area in the Dominican Republic, during his pre-puberty years was called Felicita. He was raised as a girl and even wore a dress to school. Upon reaching 7 years old, he noticed a few changes that began to occur within himself. 

"I did not feel good, I no longer liked to wear a skirt, and I was no longer drawn to play with girls. All I wanted to do is play with toy guns and boys," he explained. The change was difficult for him, as he was teased at school, because, he shared, "It is hard to imagine a girl that is now a boy." 

Johnny is now in his 20s. 

"I'd like to get married and have children, a partner who will stand by me through good and bad," he shares. 

Dr Imperato's previous research of cases like Johnny's led to the discovery of a deficiency in 5-α-reductase. This enzyme is responsible for converting testosterone into dihydro-testosterone. The lack of it causes the children to appear female at birth, until they receive another testosterone surge which brings about bigger muscles, testes and a penis. 

Merck, a pharmaceutical company, eventually continued Dr Imperato's research and came up with finasteride, a drug which blocks the action of 5-α-reductase. This drug is now being used to treat male pattern baldness as well as benign enlargement of the prostate. 

Johnny's story is featured in the BBC2 series, "Countdown to Life."

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