You probably know this, and I certainly do, but it’s worth repeating.
We treat the X and Y chromosomes in a way we’d never think of treating other physical characteristics – many people who would think it absurd or rude to tell a stranger that “really you’re male, though” because they have short hair, or a penis, or excess body hair, nevertheless think nothing of doing so when it comes to having XY chromosomes. Sex Itself is the story of how some scientists became convinced that there was something in the body, and then the cell, and then the genome, that would literally be “sex itself” – the only thing that truly mattered for sex, the thing that was its true source and the thing that finally allowed for a simple, causational definition of sex. It’s also the story of how the premise of that entire argument was wrong from the start. […]
By the end of the 1930s, the metabolic theory had been discarded in favour of this new model, where the genetic sex (XX/XY) causes the developments of either testes or ovaries, which in turn create the sex hormones that take care of the rest. This two-stage process was “a powerful mutually self-reinforcing framework for the biology of sex”, and the foundation upon which later work – like the idea that sex is biological and fixed, and gender social and malleable – was built.
This can been seen in how the sex chromosomes began to influence debates on gender and sexuality. There was speculation that the Y chromosome “represses” the feminine X, or that femaleness is the “absence” of maleness; or, that the “greater intellectual variability among males” (ie, why male researchers thought men were smarter than women) was down to the lottery of having a single X chromosome. With two Xs, unusual recessive traits would be more commonly repressed, but with one, rare genes presumed responsible for genius must be allowed through. And, similarly, early women’s rights activists and feminists, as well as male writers like the anthropologist Ashley Montagu, seized on the idea of having twice as much X as men as the scientific justification for what was really women’s “biological superiority” over the male. The idea of the X and Y carrying “sex itself” was entrenched, helped by the fashionable eugenics of the time that saw biology as the justification for a range of racist, sexist and classist prejudices.
It’s amazing how a nearly century-old pseudoscience can live on, so long as it conforms to societal bias. Anyway, I recommend you read the full story.