Sex chromosomes aren’t “simple science” - they’re what we call “lies to children”. That means a very, very simplified and not-quite-true version of reality which we teach to little kids to get them onto roughly the right page for learning the real stuff. Unfortunately people who never *did* learn the real stuff spend their lives thinking molecules really are like pingpong balls linked by cocktail sticks, and that sex is binary.

All mammal embryos start out female. In therian mammals like us, the development of testicles is usually triggered by the presence of a gene called SRY which is usually on a Y chromosome. The testicles then produce androgens, and the embryo usually responds by developing other male characteristics. Usually.

But there are Y chromosomes which don’t have a functioning SRY gene. There are X chromosomes which *do* have an SRY gene, or a different gene which has mutated to act like an SRY. There are embryos which don’t respond normally to the presence of androgens produced by their own testicles. There are embryos which respond to androgens produced by a male twin. And there are a *lot* of people who developed some male characteristics, but not all. That’s without even mentioning XYY, XXY and XXXY individuals, who all have varying degrees of abnormal development, or chimaeras, who are fusions of two fraternal twin embryos who may have been different sexes.

So while *most* people are either XX females or XY males, there are also XX males and XY females - sometimes *fertile* XY females, able to get pregnant. There are people who are female in all other ways, but have a penis. There are people who have a vagina and big breasts and undescended testicles and male muscle. There are people born with a vagina who suddenly grow w pneis and balls at puberty. There are males with very feminine bone structure and musculature. There are people who have one ovary, one testicle, a penis, a vagina, half a womb, one big breast, one small breast and a face which is masculine on one side and feminine on the other. There are people who have ovotestes - gonads which can produce both ova and sperm.

In the specific case of transgender individuals, the science currently says that most of their body became male but the neurological wiring needed to connect their brain to their body properly stayed female; or in the case of F2M trans people, *only* their neurological wiring became male while the rest of their body stayed female.

Throughout history, humans have always known that not everybody fits the male/female binary - for example the Talmud lists eight sexes.