Chromosomes Are Not Sexes

Description

Some claim that rare chromosome combinations prove there are more than two sexes, yet these combinations still result in males and females, not new sexes.

Sources

[1] Scharer, L. (2017). The varied ways of being male and female. Molecular Reproduction & Development, 84.

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

[3] Lehtonen, J., Parker, G. (2019). Evolution of the two sexes under internal fertilization and alternative evolutionary pathways. The American Naturalist, 193(5), 702-71.

[4] Bachtrog D, Mank JE, Peichel CL, Kirkpatrick M, Otto SP, Ashman TL, et al. (2014). Sex Determination--Why So Many Ways of Doing It. PLoS Biol, 12(7).

[5] Eggers, S., Sinclair, A. (2012). Mammalian sex determination--insights from humans and mice. Chromosome Res, 20.

[6] Kimball, J. (2020). Sex chromosomes. LibreText.org.

[7] Sekido, R., Lovell-Badge, R. (2009) Sex determination and SRY, Down to a wink and a nudge. Trends in Genetics, 25(1).

[8] Rey, R., Josso, N., Racine, C. (2020). Sexual differentiation. In--Endotext. South Dartmouth, MDText, Inc.

[9] Witchel, S. (2018). Disorders of sex development. Best Practice and Research in Clinical Obstetrics and Gynecology, 48.

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

[11] NIH. (2020). Triple X Syndrome. Genetics Home Reference, National Library of Medicine.

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

[13] NIH. (2020). 47:XYY Syndrome. Genetics Home Reference, National Library of Medicine.

[14] NIH. (2020). 48:XXXY Syndrome. Genetics Home Reference, National Library of Medicine.

[15] NIH. (2020). Tetrasomy X. Genetics Home Reference, National Library of Medicine.

Transcript

In almost 100% of births, every human has either XX or XY chromosomes. Sometimes, however, rare genetic errors during cell division result in humans with additional sex chromosome combinations beyond the typical XX and XY, like X or XXY. Some people claim these rare combinations prove there are more than two sexes. There’s one problem, however: chromosomes are not sexes. Here’s the difference.

Sexes are reproductive strategies that produce two differently-sized gametes.[1] The male sex is the phenotype that produces the smaller gametes, and the female sex is the phenotype that produces the larger gametes.[2][3] Chromosomes hold the genes which determine sex, whether you will develop into a male or female.[4] Chromosomes are the input, and sexes are the result. In humans, even if there are missing or duplicate chromosomes, the genes within the chromosomes still produce a male or female.[5] Thus, additional chromosome combinations are not additional sexes.[6]

The key to this remarkable consistency is found in the SRY gene.[7] Located on the Y chromosome, it triggers a cascade of genes leading to male development: gonadal differentiation into testes, which then leads to the development of male internal and external genitalia.[8] In its absence, other genes like WNT4 and RSPO1 lead to female development: gonadal differentiation into ovaries.[9] The absence of testicular hormones like testosterone and anti-Mullerian hormone then leads to the development of female internal and external genitalia. This is why the SRY gene is known as the master switch sex determining gene for mammals. In its presence, the fetus develops into a male, and in its absence, the fetus develops into a female, regardless of missing or duplicate chromosomes.

Knowing this, we can now return to the atypical combinations and easily predict the resulting sex:

45:X – Female

47:XXX – Female

47:XXY – Male

47:XYY – Male

48:XXXY – Male

And finally, 48:XXXX…you guessed it…Female.

Those who conflate atypical chromosome combinations with additional sexes are using a sleight-of-hand trick. One moment, they are discussing chromosomes, the next, sex—without defining the difference. By conflating the two, they incorrectly argue that chromosome variation forms additional sexes. And yet none of these chromosome variants results in a third role in reproduction. They still result in just two: either male or female. In fact, while most chromosome variants beyond XX and XY result in infertility, when individuals with these conditions are fertile, they produce either sperm or eggs, not a third gamete type. Thus, they are not additional sexes.

END

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