This is a framework that makes intuitive sense to almost everyone because it correlates exactly with sexual dimorphism – there are those with penises, and those with vaginas, and with a bit of luck combining the two means we end up with even more humans that each have their own penises or vaginas.
But like the platypus, it’s crucial not to think the taxonomy more important than the reality it’s meant to describe.
But doctors have long known that some people straddle the boundary – their sex chromosomes say one thing, but their gonads (ovaries or testes) or sexual anatomy say another.
Parents of children with these kinds of conditions – known as intersex conditions, or differences or disorders of sex development (DSDs) – often face difficult decisions about whether to bring up their child as a boy or a girl.
The scientific process often involves tweaking taxonomies.
Humanity saw distant objects above, and the taxonomy we built was simple: two entries, one labelled “planets”, the other “stars”.
Sex can be much more complicated than it at first seems.
According to the simple scenario, the presence or absence of a Y chromosome is what counts: with it, you are male, and without it, you are female.
In reality, there are extremely few sexual characteristics solely controlled by the presence or absence of a Y chromosome – and just as there are plenty of characteristics controlled by genes found on other chromosomes, the “sex” chromosomes also carry genes that determine traits that have nothing to do with sex.
Some researchers now say that as many as 1 person in 100 has some form of DSD. Since the 1990s, researchers have identified more than 25 genes involved in DSDs, and next-generation DNA sequencing in the past few years has uncovered a wide range of variations in these genes that have mild effects on individuals, rather than causing DSDs.