Description

Many often conflate two concepts in biology: how sex is defined versus how sex is determined. Here’s why the difference is important.

This animated video was based on a longer article at the Paradox Institute here.

Sources

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

[2] Lehtonen, J. (2017). Gamete Size. Encyclopedia of Evolutionary Psychological Science; Scharer, L. (2017). The varied ways of being male and female. Molecular Reproduction & Development, 84.

[3] Bhargava, A., et al. (2021). Considering sex as a biological variable in basic and clinical studies. Endocrine Reviews, 20(20).

[4] Holub, A., Shackelford, T. (2021). Gonochorism. Encyclopedia of Animal Cognition and Behavior. Springer.

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

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

[7] 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).

[8] Avise, J. (2011). Two sexes in one. In: Hermaphroditism: a primer on the biology, ecology, and the evolution of dual sexuality. Columbia University Press, 5.

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

[10] Beukeboom, L, Perrin, N. (2014). The evolution of sex determination. Oxford, UK: Oxford University Press.

[11] Kimball, J. (2020). Sex chromosomes. LibreText.org; Ellis, PJ., Erickson, RP. (2017). Genetics of sex determination and differentiation. Fetal and Natal Physiology (Fifth Edition), Elsevier.

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

[13] NIH. (2021). 46,XX testicular disorder of sex development. MedlinePlus Genetics.

Transcript

Many often conflate two concepts in biology: how sex is defined versus how sex is determined. Conflating these two things can create absurd conclusions, so it is important we differentiate them to understand what male and female are and how they develop in the womb.

Defining sex: What are male and female? The male sex is the body plan that produces sperm, and the female sex is the body plan that produces eggs.[1] This applies to all species that reproduce through two gametes of differing size, including humans.[2]

Based on this definition, we know whether an organism is male or female by looking at the sexual body plan: the structures that produce and release either gamete type.[3] For human males, it includes the testes, epididymis, vas deferens, seminal vesicle, and penis. For human females, it includes the ovaries, Fallopian tubes, uterus, cervix, and vagina. Some species have the male and female roles in a single individual, but in humans, individuals are either male or female throughout their entire life cycle, a sexual system known as gonochorism.[4]

Determining sex: How does an individual develop into a male or female? In humans, sex is determined by genes.[5] In biology, determining sex does not mean “observing” or “identifying sex” in the normal sense. Instead, determining sex is a technical term for the process by which genes trigger and regulate differentiation down the male or female path in the womb.[6] This determines the structures that produce and release either gamete type, and thus, the individual’s sex. Thus defining sex describes what the sexes are, while determining sex describes how the sexes develop.

There’s two main reasons why the mechanisms that determine sex (like genes) are not used as the definition of sex. First, sex determination mechanisms vary across species.[7] Mammals typically use the X-Y system to determine sex, birds typically use Z-W, and many species of reptiles and fish use a range of temperature values.[8] Despite all these different mechanisms to determine an organism’s sex, what unites all males across species is that they develop the body plans that produce sperm. And what unites all females across species is that they develop the body plans that produce eggs.[9] Thus, while there are different mechanisms that determine sex across species, there are still only two endpoints: male or female.[10]

The second reason why the mechanisms that determine sex cannot be used as the definition for sex is because random mutations or genetic errors can occur: sometimes sex determination mechanisms result in a sex opposite of what you would expect from the chromosomes. For example, in humans, 1 in 20,000 males are born with XX chromosomes, rather than the typical XY. This occurs when the SRY gene, the gene that triggers differentiation into a male, translocates onto an X chromosome during cell division in the father’s sperm cells.[11] When the fetus is conceived, they receive two X chromosomes, one with the SRY gene. SRY then 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.[12] Though they cannot produce sperm, since this requires the AZF region from the Y chromosome, XX males are defined as male because they develop the body plan that produces sperm (determined by genetics).[13] Defining this person as female simply because of the XX chromosomes would be absurd, because they develop the body plan that produces sperm, not eggs. Thus, because sex determination mechanisms vary across species and can result in genetic errors during development, they cannot be used as the definition for sex.

It should be clear now that males and females are not defined by the mechanisms that develop them in the womb. They are defined by the body plans that produce either sperm or eggs. For humans, this is determined by genetics. Defining sexes this way does not mean sex is a spectrum or that one can change sex. Sex is determined at conception by the individual’s genetic profile, developed in the womb, and immutable. If we wish to ascertain the full picture of a person’s sex, we must analyze their genetically determined body plan: the structures that produce and release either gamete type. Defining male and female with respect to gamete type is the only way forward for an accurate and consistent definition of sex.

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