Chromosome variation does not form new sexes, and such karyotypes represent the variation within males and females.

The fact that there are only two sexes in humans and most other higher order organisms does not exclude the existence of these chromosomal conditions.

THREAD https://twitter.com/Hood_Biologist/status/1333410791178072065

What do evolutionary biologists mean when they discuss 'male' and 'female'?

Male and female are not defined by chromosome composition but by gamete type, and it's the fundamental distinction between male and female in all anisogamous species.[1][2][3]

Cont. [4][5][6][7]

To understand whether a chromosomal condition produces a male or female, we look at the evolved phenotypic structures the fetus develops to support either large gametes (ova) or small gametes (sperm), such as gonadal tissue and internal and external genitalia.

Most of the time, the sex of the person is clear, but even in cases of apparent ambiguity, karyotype and genetic testing can be done, along with detailed imagery of the reproductive anatomy, and doctors can determine the person's development path.

For example, fetuses with 45,XO (Turner Syndrome) develop phenotypic structures organized around large gametes, such as ovaries, Mullerian structures, and female external genitalia; they are unambiguous females.[8]

Fetuses with 47,XXY (Klinefelter Syndrome) develop phenotypic structures organized around small gametes, such as testes, Wolffian structures and male external genitalia; they are unambiguous males.[9]

Even if some (much rarer) cases are objectively impossible to determine sex, this does not refute the existence of two sexes, as a new type of reproductive function (a new gamete type) would have to be formed.

The reality of two sexes does not mean there is no variation of anatomy, physiology, and behavior within the sexes; there is! [10]

There exists a wide range of biological diversity within males and females, and we should not prescribe narrow expectations onto bodies and behavior

At the same time, recognizing this biological diversity of sex development does not mean that we have to ignore the reality of male and female.

Understanding the full range of sex development broadens (rather than narrows) our conceptions of male and female body types.

We should support people's freedom to identify as they see fit (as the long as someone's actions does not infringe on the rights of others).

And we can also support and explore the fundamental reality of the two sexes (male and female), from which each of us was conceived.

[1] Lehtonen, J., Parker, G. (2014). Gamete competition, gamete limitation, and the evolution of two sexes. Molecular Human Reproduction, 20(12).

[2] Czaran, T., Hoekstra. R. (2004). Evolution of sexual asymmetry. BMC Evolutionary Biology, 4(34).

[3] da Silva, J. (2018). The evolution of sexes--a specific test of the disruptive selection theory. Ecology and Evolution, 8, 207-219.

[4] Charlesworth, B. (1994). The nature and origin of mating types. Evolutionary Genetics in Current Biology, 4(8).

[5] Lehtonen, J., Parker, G. (2014). Gamete competition, gamete limitation, and the evolution of two sexes. Molecular Human Reproduction.

[6] Epelman, M., et al. (2005). Anisogamy, expenditure of reproductive effort, and the optimality of having two sexes. Operations Research.

[7] Parker, GA. (2014). The sexual cascade and the rise of pre-ejaculatory sexual selection, sex roles, and sexual conflict. CSH Persp Bio.

[8] NIH. (2020). Turner Syndrome. Genetics Home Reference, National Library of Medicine.

[9] NIH. (2020). Klinefelter Syndrome. Genetics Home Reference, National Library of Medicine.

[10] Del Giudice, M. (2019). Measuring sex differences and similarities. Gender and sexuality development--Contemporary theory and research. Springer.