The list of bad arguments forwarded by activists designed to undermine the reality of there being two and only two sexes is very long and constantly growing. I dealt with some of these common arguments for Quillette in 2020. However, I came across a tweet by Dr. Shay-Akil McLean responding to Zach Elliott claiming “there are precisely two sexes (no more, no less).”
Zach Elliott @zaeleftyThe biological sciences overwhelmingly support the claim that there are precisely two sexes (no more, no less) in almost all complex multicellular life. Here are 40 peer-reviewed journal articles from biologists on the evolution of sex and why there are only two sexes. https://t.co/j02seUIdXh
McLean, who prominently lists five degrees after his name and claims to be a human evolutionary geneticist, took issue with that claim and forwarded a very common—and very wrong—portrayal of biological sex: that different sex chromosome compositions beyond the standard XX and XY each represent their own unique sex. In fact, Dr. McLean appears to suggest in his tweet that there may be as many as 10 biological sexes!
Such extreme misinformation would be concerning coming from anyone, but especially so coming from someone with a PhD in human evolutionary genetics who. The 4000+ retweets his tweet received demonstrate just how fast misinformation can spread. However, a glance at his Google Scholar reveals he has authored several papers on racism and only one paper that discusses some topics in genetics yet contains no actual genetic analyses whatsoever. His claim to being a geneticist (of any kind) is therefore dubious. But credentials don’t determine whether someone is right or wrong, and I am sure it wouldn’t be hard to find a real geneticist making the same argument, so let’s look at the argument itself.
The argument that individuals with sex chromosomes that deviate from the typical (46, XX and 46, XY) arrangements, such as those with Klinefelter syndrome (47, XXY) or Turner syndrome (45, X0), is common and usually used to argue that there are 6 sexes, though other numbers are frequently thrown around as well. Searching Twitter for the term “6 sexes” returns countless tweets similar to the one below.
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This argument results from a fundamental misunderstanding about what distinguishes males and females biologically, which is the size of the gametes (sex cells) they produce. Males produce small gametes (sperm) and females produce large gametes (ova). But on an individual level (since not all individuals may be able to produce gametes) an organism’s sex corresponds to the type of primary sex organs (testes vs ovaries) and individual has developed.
In mammals, which includes humans, the Y chromosome carries a gene (SRY) that encodes a testes-determining factor. If an individual has a Y chromosome with a functional SRY gene, they will develop testes and therefore will be biologically male. Absent a Y chromosome and functional SRY gene (unless the SRY gene has been transposed to an X chromosome), an embryo will develop ovaries and will therefore be biologically female. What’s important to note is that the presence of a Y chromosome, or two, or three, etc., all result in the development of testes and therefore these individuals are biologically male. Likewise, individuals with additional or fewer X chromosomes, in the absence of a Y, all develop ovaries and are therefore biologically female.
With this in mind, the chart in the above tweet can more accurately be rewritten as:
X – Female
XX – Female
XXY – Male
XY – Male
XYY – Male
XXXY – Male
So no, these different chromosomal compositions are not new sexes, but rather represent natural variation within males and females. To illustrate by way of analogy, a person with Klinefelter syndrome (47, XXY) isn't a new sex in the same way that a person with Down's syndrome (who have 3 instead of 2 copies of chromosome 21) isn't a new species.
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