I'm slogging through a book about genetic mutations as I finish up grading (I write this with two students still taking the Lit exam). Soon I'll have a big post about real, non-X-Man mutants but I read something so odd that I'll post a preview.
In Salinas, a small village in the Dominican Republic, the people coined a new word, "Guevedoche." Translated to English, this means "Penis at Twelve." This doesn't mean a nooner or a perversion. It's just that approximately one out of every 90 births (in a total population of 4,300 people) appear initially to be female but during puberty, transform to male.
If a developing XY fetus lacks an enzyme known as 5-alpha-reductase, it will not develop male organs despite having male DNA. However, in puberty as production of sex hormones increase, the higher amounts of testosterone (which both male and females make) causes the body to start following the XY outline.
Villagers trace the mutation to a single individual, one Altagracia Carrasco. It's unclear of the long-term psychological effects of going from girls to men but it would be quite the conversation starter.
A similar situation is found in the Sambia tribe in New Guinea. They have a word "kwolu-aatmwol" (changing to male thing).
The mutation could occur anywhere but those are the only two places where it has become so common.