[_cenw]

>The single biggest genetic difference among humans is the presence of a Y chromosome.

That is actually incorrect. You can be born with a Y chromosome and zero functional testosterone[1]. There are a lot of moving parts in between having a Y chromosome and getting testosterone expression. The later of which is a much better discriminator.

And also, these deviations aren't rare. Though they are usually unknown, when they don't cause obvious developmental deviation (and thus labeled DSDs). They also are more common in some populations.

[1]: https://en.wikipedia.org/wiki/Complete_androgen_insensitivit...