I received an excellent question about sex determination, and how biologists classify organisms.

Here's the question: "Is it a consensus among biologists that sex is determined by gametes? If so, why is the chromosomal explanation for sex so popular?"

THREAD...

It's a common misunderstanding that sex is defined by chromosomes. That's why activists use chromosomal variations (like X or XXY) to claim these are new sexes.

But chromosomes are simply the genetic instruction which lead to the development of one's sex. They are not what defines sex, because other animals do not use XX and XY, or sex chromosomes at all, and we can still determine their sex.

So, how is sex defined, objectively, across all sexually reproducing species?

The answer is gametes.

Sex IS gametes. Gametes are the mechanism by which new individuals are produced, and so the ultimate definition of sex is found in gametes.

In fact, gametes are the objective definition of sex because they are the one trait that consistently differentiates male from female in all anisogamous species. Even if there are a wide diversity of sex characteristics across species, the one unifying trait is gamete type.

The reason why biologists studying sex don't use a sex characteristic as the ultimate determinate of sex, for example, is because species that reproduce have a wide variety of sex characteristics.

For example, human males have penises; male plants have stamens. How can we identify the male in both cases?

Both human males and male plants have the structures which support small gametes.

The decision in utero of what sex one will be is controlled by a variety of different mechanisms across different species. In humans, it's XX and XY (barring any developmental anomalies). In ducks, it's ZZ and ZW (males are ZZ). And in many turtles, it's determined by temperature

But the one unifying trait that all humans, ducks, and turtles share is that we reproduce through the fusion of two gametes of differing size. And this is why gamete type is the ultimate objective definition of sex.

So, the question arises: how do we determine sex if we cannot presently observe gametes being produced or released?

The answer is in the phenotype (body morphology / reproductive anatomy) an organism develops for either large or small gametes.

Biologists studying different animals can "sex" an organism by looking at how the phenotype correlates to the gamete type it will produce, or is producing, or has produced.

Thus, sex IS gametes, and an accurate marker for sex is the organism's body morphology.

This peer-reviewed paper has a great overview of this concept, and its discussion about why we only have two sexes represents the large consensus of evolutionary biologists for the last five decades: that gametes are the ultimate definition of sex. https://academic.oup.com/molehr/article/20/12/1161/1062990

For more on this: https://twitter.com/zaelefty/status/1274369571814858754?s=20

And the video response to SciShow's claim that there are more than two human sexes:

And here is my sex spectrum rebuttal thread master list. https://twitter.com/zaelefty/status/1312933370121289729?s=20