1/ Getting some flack for this tweet, so let me clarify. Male and female, as a natural phenomenon, has been known long before the discovery of sex chromosomes. This is because the sex of any individual, whether it's a human, reptile, plant, etc., is based on reproductive... https://twitter.com/SwipeWright/status/1264444445891551234

2/ anatomy organized around the production of either small (male) or large (female) gametes. An organism doesn't have to actually be able to produce gametes to have a discernible sex.

3/ To conflate sex chromosomes with sex itself is an error. It's confusing *mechanisms* that typically lead to the development of males and females with the outcomes themselves. For instance, I am a male because I have male reproductive anatomy that can be objectively verified.

4/ There is nothing I could later discover about myself that would change the fact that I am a male. If I discovered tomorrow that my chromosomes were somehow XX, my sex would remain unchanged.

5/ When scientists discovered sex chromosomes, they discovered the proximate mechanism that causes bodies to develop into males and females. The only way it was possible for scientists to say "Hey, males appear to almost always be XY and females are almost always XX" is by...

6/ first knowing what males and females were apart from sex chromosomes, and observing the correlation. The discovery of sex chromosomes didn't give us any more information about what it means to *be* male or female, it simply revealed the proximate mechanism.

7/ I brought up the example of many reptiles having sex determined by the temperature at which they develop not to suggest humans are like reptiles, but to highlight the fact that we shouldn't confuse proximate mechanisms with sex itself.

8/ For instance, American alligators typically develop into males at temps 33C and above, while eggs incubating at below 33C typically develop into females. To confuse chromosomes with sex itself for humans would be like confusing temperatures above and below 33C with sex...

9/ itself in alligators. We don't determine the sex of individual alligators by knowing the temperature at which they developed, we determine their sex by their reproductive anatomy. If we later found out that a male alligator happened to develop at 30C, we wouldn't conclude...

10/ this alligator was now a female. It's still a male, regardless of the temperature at which it happened to incubate.

Regardless of the typical mechanisms for sex development, the sexes are not *defined by* those mechanisms.

11/ Biology can be messy, and there are usually rare exceptions to rules. For instance, it isn't the chromosomes themselves that trigger male or female sex development, but rather the SRY gene on the Y chromosome. This gene can get transposed onto an X, making XX males possible.

12/ By conflating chromosomes with sex itself, you're setting yourself up for a "gotcha" moment when someone points this out. Yes, almost every human male is XY and almost every female XX, but this isn't how we determine what sex people are.

13/ For every species with males and females, we determine the sex of individuals based on their primary reproductive anatomy. We know when a clownfish has developed into a female from a male because their sexual anatomy changes.

14/ We know plants are male and female based on their reproductive anatomy. And we can distinguish human males and females based on their reproductive anatomy.

We don't need to know *anything* about sex determining mechanisms to sort individuals into males and females.