If you ever get the chance, I recommend the book A Cabinet of Medical Curiosities: A Compendium of the Odd, the Bizarre, and the Unexpected by Jan Bondeson Mutants: On Genetic Variety and the Human Body by Armand Marie Leroi (2003). Please read the reviews.

The book talks at length about medical conditions, including the human tail, the cleft pallet and also covers intersex. It talks about XY female androgen insensitivity, SRY gene transposition/deletions, the güevedoce males from Dominican Republic who are indistinguishable from females until about the age of 12 when their testes drop, and the prevalence of more subtle forms of intersex that go under-diagnosed. It also touches on fetal development and general genetics including the inversion of sexual chromosomes in birds and reptiles.

It’s a great dive into the complexity of biology and particularly sexual development. I suspect you won’t be so sure of what you think is normal after exploring its barrage of edge cases that deeply contemplate the nature of genetic sex that creates these deviations under a basic tenet: Nothing in biology is set and it’s all subject to change.

98.5% of those have a gender identity that conforms with their biological sex.

There are many more people today who have incorporated a hybrid gender precisely because they don’t fit into neat categories. People call them femboys and tomboys because everything about their gender expression is mixed. You can’t tell me with a straight face they’re just pretending. The whole category is called “gender non-conforming”.

E: Sorry, I got the wrong book off my reading list somehow!