"This means that I have XY [male] chromosomes," she says. "However, the SRY gene or one of the other testes-determining regions on the Y chromosome is nonfunctional, so when I was a fetus, my proto-gonads -- the structures that become ovaries in typical women and testes in typical men -- didn't become testes. They just stayed proto-gonads. Now, the differentiation of the proto-gonads is the first step in the masculinization process. The next step should be that the testes release testosterone and other androgens into the fetus' body and cause it to develop typically male structures. When that doesn't happen, however, the 'default' development is female. So I developed female physical structures, aside from my undifferentiated proto-gonads, and I was born looking physically female."
Spoiler: None of this is going to be as cut and dried as looks.
Others have no discernible cause, like MRKH syndrome, which CJ says "is where a child born female is born without a womb," among other things, while still others are the result of your genes and your hormones violently disagreeing with each other, as is the case with congenital adrenal hyperplasia, the condition Allison has.
"Because of something to do with the adrenal gland, I make far too many androgens, so what it comes down to is I have full female anatomy but I cannot produce estrogen and only produce androgens and testosterone," they explain. "The overproduction of androgens and testosterone gave me what doctors refer to as 'ambiguous genitalia.' In my case, because I didn't have adequate estrogen production in puberty, my vagina is basically the vagina of a prepubescent girl. It's very small and not very elastic, and my clitoris is very large, which is referred to as virilization."