Follow the science: There are only two (count ‘em) two sexes! Part Two

Okay folks, here is the transcript from yesterday’s video entitled: Why Sex Is Binary. If you missed yesterday’s post, I recommend you go back and read it. Hey! It’s free.

Why Sex Is Binary


A binary is a system composed of two parts–a duality, a pair. In developmental biology, sex is binary. Here’s why. An organism’s sex is defined as the type of gamete its reproductive anatomy is organized to produce: Male bodies develop towards the production of small gametes (sperm), whereas female bodies develop towards the production of large gametes (ova). Because there are no intermediate gametes between sperm and eggs (such as the often joked about sperg or speggs), there are, therefore, only two sexes.

In humans, this sexual dimorphism is so consistent that 99.98% of births are unambiguously male or female.

As developmental biologist, Dr. Emma Hilton and evolutionary biologist Dr. Colin Wright note, “The evolutionary function of these two anatomies is to aid in reproduction via the fusion of sperm and ova. No third type of sex cell exists in humans, and therefore there is no sex ‘spectrum’ or additional sexes beyond male and female. Sex is binary.”

To understand how this binary system is produced, let’s explore the process of sex determination. Sex in humans is genetically determined at conception solely by the presence or absence of a functioning SRY gene, which is located on the short arm of the Y chromosome. With SRY activation, the gonads differentiate into testes, and the fetus develops anatomy to support the production of small gametes.

In the absence of SRY activation, the gonads differentiate into ovaries, and the fetus develops anatomy to support the production of large gametes. This is why developmental biologists refer to SRY as the “master switch” gene for mammalian sex determination because, without its activation, the fetus develops as a female.

The critical importance of SRY for sex determination can be seen in differences in sex development. Here are three examples:

On very rare occasions, a fetus may develop with two X chromosomes and one Y, or three Xs and one Y, or even four Xs and one Y. Despite the extra X chromosomes, all these cases develop as males thanks to the presence of SRY on the Y chromosome.

Sometimes, a translocation of the SRY gene results in it being placed on an X chromosome in a fetus with two X chromosomes. Thanks to the presence of SRY, the fetus develops testicular tissue despite having no Y chromosome.

In exceptional cases, a fetus may develop a female phenotype with an XY karyotype. Because the SRY gene remained inactive, they developed as females.

Thanks to the master switch SRY gene, we can see that sex determination is entirely dimorphic. However, this does not exclude variation within the binary system. Sex differences between males and females are commonplace. There is a variation in chromosomes, gene expression, gonadal tissue, hormone production, genital morphology, height, weight, voice pitch, muscle mass, bone density, and more.

But, no matter the variation of traits, the principle remains: sex is defined by the type of gamete your anatomy is organized to produce. Two gamete types, two sexes. As Hilton and Wright note, “Not everyone needs to be discretely assignable to one or the other sex in order for biological sex to be functionally binary. To assume otherwise–to confuse secondary sexual traits with biological sex itself–is a category error.”

Such boundaries in biology can be fuzzy, but the boundaries of gametes are clear. Or, as evolutionary biologist Dr. Heather Heying writes, “The boundaries between species are almost always fuzzy. The moment of change, when one species becomes two, is rarely known. It is this fuzziness, in part, that explains why we have so many species concepts. So many species concepts, but only two types of gametes. In animals and humans, the borders between gamete types aren’t fuzzy. Gametes are always male or female. There is no in-between.”

This is why sex is binary–not because there is no spectrum of human body types; there is. Sex is binary because there are only two gamete types bodies can be organized around: sperm and eggs. If, however, you happen to find the mythical intermediate gametes spergs or speggs, let us know.


I don’t know about you, but I’m convinced that the science is correct, and the fools and charlatans are simply doing their best to gaslight the rest of us. What do you think? 

Here’s a link to the video, in case you missed it: Why Sex Is Binary:

And for those who demand to know the sources of the scientific information contained in this transcript, here you go:


[1] Kumar et al. (2019). Anisogamy. Encyclopedia of Animal Cognition and Behavior.

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

[3] Cox, P., Togashi, T. (2011). The Evolution of Anisogamy, A Fundamental Phenomenon Underlying Sexual Selection. New York Cambridge University Press. 17.

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

[5] Kodric-Brown et al. (1987). Anisogamy, sexual selection, and the evolution and maintenance of sex. Evolutionary Ecology, 1, 95-105.

[6] Sax, L. (2002). How common is lntersex? A response to Anne Fausto‐Sterling.

[7] Wright, C. & Hilton, E. (2020). The Dangerous Denial of Sex. Wall Street Journal.

[8] Jones, R., Lopez, K. (2014). Chapter 5: Sexual differentiation. Human Reproductive Biology, 4th edition. Elsevier. 95.

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

[10] Kashimada, K., Koopman, P. (2010). Sry, the master switch in mammalian sex determination. Development, 137.

[11] Kimball, J. (2020). Sex chromosomes.

[12] Gilbert, SF. (2000). Chromosomal sex determination in mammals. Developmental Biology, 6th edition. Sunderland (MA), Sinauer Associates.

[13] Wizemann, TM., Pardue ML, editors. (2001). Sex begins in the womb. Exploring the biological contributions to human health: Does sex matter? US Institute of Medicine.

[14] Carlson, B. (2014). Chapter 16, Urogenital System. Human Embryology and Developmental Biology. 404-407.

[15] Witchel, S. (2017). Disorders of sex development. Best Practice and Research in Clinical Obstetrics and Gynaecology, 48, 90-102.

[16] Heying, H. (2019) Boundaries in biology are fuzzy, but gametes aren’t. Twitter.


[17] Graham, C. (2019). Is sex a spectrum? Sex determination and differentiation. MRKHVoice.

[18] Bachtrog, D., et al. (2014). Sex determination, Why so many ways of doing it. PLOS Biology, 12(7).

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About Ronald E. Yates

Ronald E. Yates is an award-winning author of historical fiction and action/adventure novels, including the popular and highly-acclaimed Finding Billy Battles trilogy. Read More About Ron Here

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