Activists often conflate chromosomes with sex, claiming that biologists only thought sex was binary because male and female were defined by XY and XX.

Here's why this is false.

The argument is a strawman because scientists who studied the biology of sex never thought male and female were defined by chromosomes, nor did they think the discovery of chromosomes was the ‘end of the story.’
Chromosomes were only discovered in 1882, the Y chromosome in 1905, and the sex-determining region Y gene (SRY) in 1990. Scientists, and especially biologists, already understood what male and female were well before any of these discoveries.
In fact, the only reason they could identify chromosomes like X and Y as sex chromosomes, or the SRY gene as a “male” sex determining gene, is because they already had a concept of male and female independent of these systems.
Thus, while male and female became associated with certain chromosomes, they are not and never were chromosomes.
While chromosomes include the genes that determine the development path a fetus will go down, and thus, their sex, chromosomes are not the defining feature of male and female.

They simply hold the genetic mechanisms that lead to male or female development.
The biological definition of male and female is not found in chromosomes, since sex determination mechanisms vary across species.

Instead, male and female are universally defined as the phenotypes that produce small and large gametes, respectively.
For more on what sexes are and how they are defined:…

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More from @zaelefty

15 Nov
People with Down syndrome have an extra copy of chromosome 21. That’s why the syndrome is called Trisomy 21.

You wouldn’t think someone with an extra copy of the 21st chromosome would be anything other than male or female.

So why would you do it with XXY, XO, XYY, and so on?
These are chromosomal disorders just as much as Trisomy 21 is. They just so happen to impact the sex chromosomes, not the autosomes.

But because sex determination in mammals is binary, these atypical karyotypes do not result in anything other than males or females.
Read 4 tweets
14 Nov
I find it interesting to think about how the Sex Spectrum has similarities to Young Earth Creationism (YEC) in its pseudoscientific argument techniques.

One example is through the use of "ad hoc rescue", the use of evidence-less rationalizations to dismiss counter-evidence.
When a person's beliefs are challenged with evidence, each challenge is often met with a specific rationalization to dismiss the evidence.

This rationalization will often change each time a different challenge to their belief is forwarded. The rationale will be baseless.
For example, YEC claims that the universe was created roughly 10,000 years ago.

When cosmologists show that the Hubble Space Telescope can see objects billions of light years away, YEC claims that the light must've been created in transit or sped up to "appear" old [AD HOC].
Read 11 tweets
14 Nov
This evolutionary biology paper on the evolution of two sexes, by Lehtonen & Parker, is the gold standard for explaining why there’s only two sexes in anisogamous species.

I started sharing it in June 2020. It’s great to see it being shared so much!…
There’s even a section that tracks every time it’s shared on Twitter and shows the tweets referencing it.…
Geoff Parker has been a critical researcher in the evolutionary origins and maintenance of male and female since at least the 1970s.

His papers have been cited thousands of times and make up a critical part of the extensive body of research on the evolution of anisogamy.
Read 8 tweets
2 Nov
Sex determination in mammals is binary. Every karyotype in humans results in a male or female.

Genetic errors, such as the unequal distributions of chromosomes (nondisjunction), do not create new sexes.
This shows us that karyotype (the collection of an individual’s chromosomes) is conceptually independent from what sexes are.

It is true that 46:XX and 46:XY are tightly linked to females and males, respectively, but it is not true that this is the defining feature of F and M.
In other words, you can have a karyotype outside of the norm, and you will still be a male or female.

Ex: XXY = male; XO = female
Read 10 tweets
27 Oct
Gender, as described by theorists and activists as an expression of behavior, is practically “personality” at this point.

Are there personality sex differences? Yes. Does this mean a masculine girl is actually a boy? No.
This is what a typical sex difference between males and females in a Big Five personality trait looks like. The averages are very close together.

Traits like this show us there is plenty of variation within males and within females to reject rigid behavioral categorizations.
Trait Agreeableness.

Males and females, on average, are far more alike than different in this trait.

Extremes of the bell curves show little overlap, however (Ex: The MOST disagreeable people are almost always men.)
Read 8 tweets
18 Oct
Across species, it does not matter whether male and female exist in the same individual (hermaphroditism) or in separate individuals (gonochory); sex is binary in any species that has male and female reproductive functions.
For example, in sunflowers, like many other flowering plants, individuals are both male and female, and so the species is hermaphroditic.

In humans, like all other mammals, individuals are either male or female throughout their life cycle, and so the species is gonochoric.
Sex in the sunflower and sex in humans requires only two roles: a male and female, and so sex is binary in both species.
Read 10 tweets

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