The process of sex differentiation ensures that human fetuses are born with distinct external & internal genitalia (which is used to assign their sex at birth).

However, in a small town in the Dominican Republic, a community known as the "Guevedoces" (translating to "penis at twelve") undergo a different process of sex differentiation.

Many of them are brought up girls due to the absence a penis & testes & the presence of a vagina.

When puberty approaches, some of their male sexual organs grow, and their biological sex is reassigned as male according to their genitalia.

In the 1970s, Dr Julianne Imperato-McGinley, discovered that they have a genetic condition that causes a deficiency in an enzyme called 5-alpha-reductase, which converts testosterone into dihydro-testosterone.

Thus, male external & internal genitalia do not develop despite the presence of the Y chromosome & SRY gene, and they appear female when they are born.

At puberty, a surge of testosterone causes their body to develop internal & external male genitalia alongside secondary male characteristics.

Another thing that Imperato-McGinley discovered was that the Guevedoces tend to have small prostates.

This observation led to the development of the best-selling drug, finasteride, which blocks the action of 5-alpha-reductase, mimicking the lack of dihydro-testosterone seen in the Guevedoces.

Finasteride is an effective way to treat Benign prostate hyperplasia & male pattern baldness, hirsutism in women & is part of hormone therapy for transgender woman.

A final interesting observation that Imperato-McGinley made was that these boys, despite being brought up as girls, almost all showed strong heterosexual preferences. She concluded prenatal hormones play a bigger role in the development of sexual orientation compared to external influences.