5-alpha-reductase deficiency is a condition in which infants and children appear to be female until they reach puberty. At puberty, “they develop muscles, their chests start to grow, and by the age of 12, they sprout testes and a penis.”
Josh L Davis via IFL Science:
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When born, to all intents and purposes they appear to be female. But when the children referred to as “Guevedoces” in an isolated village in the Dominican Republic reach puberty, things start to change. They develop muscles, their chests start to grow, and by the age of 12, they sprout testes and a penis.
In the first eight weeks of development, fetuses are neither male nor female. After this period, if the fetus is genetically male, then the internal glands known as gonads are instructed by the Y chromosome to turn into testes. At the same time, testosterone is sent to a particular structure called the tubercle.