What is biological sex?


How do you define one’s biological sex?

Sex, as opposed to gender identity, usually refers to one’s biology. Most people reduce this to chromosomes and genitalia, but the reality is that biological sex is a spectrum and not a binary. Most people fall into two prototypical categories of male and female. That is, most people with XX chromosomes have internal female genitalia (uterus, ovaries, cervix, etc.), external female genitalia (vagina, clitoris), estrogen as their primary hormone, and identify as female. Most people with XY chromosomes have internal male genitalia (testicles, vas deferens, etc.), external male genitalia (penis, scrotum), testosterone as their primary hormone, and identify as male.  However, not everyone does. And the folks that do not are called “intersex,” meaning, “between sex.” (Read more about what intersex means from the Intersex Society of North America.) Biological diversity exists everywhere – including within biological sex. So, biological sex is, too, a spectrum.

How is “biological sex” a spectrum?

There are five main factors of biological sex:

  1. Sex Chromosomes: generally, this means XX or XY, but there is also XXY, XXX, XYY, X. These are called sex chromosomal abnormalities, but that doesn’t necessarily mean there is anything wrong with these folks. Sex chromosomal abnormalities are common and the results are far less severe than nonsex chromosomal abnormalities such as Trisomy 21 (Down Syndrome.) For example, individuals with XXX chromosomes are fertile and healthy. Some individuals are also born with “Mosaic Genetics” wherein some cells are XY and some are XX. See more here: https://www.ncbi.nlm.nih.gov/books/NBK115545/. [Hormones & Behavior, Lecture, Professor Hooven, 2018]
  2. Hormones: usually testosterone and estrogen, including their derivatives like estradiol and progesterone, dihydrotestosterone and other androgens. (Androgens are the class of hormones that include testosterone, and testosterone is probably the main hormone in this group, and certainly the most well known, but it is only one of several androgens that are usually considered the main “male” sex hormone.) Keep in mind that hormones are important for so many more things other than sexual maturation, though! [Hormones & Behavior, Lecture, Professor Hooven, 2018]
  3. Expression of hormones: in order for the effects of a hormone to take place, you need to have the proper and functioning hormone receptors for that hormone. Some folks don’t have these. [Hormones & Behavior, Professor Hooven, 2018] For example, there is a syndrome called Complete Androgen Insensitivity Syndrome (CAIS) in which a person who is born with XY chromosomes does not have any androgen receptors which means the person receives NO effects of testosterone whatsoever. I’ll explain more about this in a little bit.
  4. Internal genitalia: these generally include testes, vas deferens, fallopian tubes, uterus, ovaries, etc. In order to have testes, you have to have a Y chromosome. [Hormones & Behavior, Lecture, Professor Hooven, 2018]
  5. External genitalia: these generally include penis, vagina, clitoris, scrotum (not necessarily with descended testes. Testes remain internal until puberty when, most natal males, they descend. [Hormones & Behavior, Lecture, Professor Hooven, 2018] However, sometimes they do not and they remain internal.) More about this here: http://www.embryology.ch/anglais/ugenital/diffmorpho04.html.)

Okay, so what does this mean? What does it mean that there is a spectrum?

Most folks exist along two binary “opposites” of these factors. That is, most folks who were assigned male at birth have the following make up: XY, primarily testosterone, functioning receptors, testes & vas deferens, and penis and scrotum (with descended testes after puberty). And most folks assigned female at birth have the “opposite” make up: XX, primarily estrogen, functioning receptors, ovaries, fallopian tubes & uterus, and vagina and clitoris. However, many people have a variation of these biological parts of sex that do not exist inside these two binaries, making biological sex a spectrum. And, there isn’t anything wrong with these folks, they just have different biology that is part of the normal human experience of biological diversity. Chromosomes don’t always indicate everything about sex, neither do internal/external genitalia.

Let’s revisit the person with CAIS that I discussed in number 3 of my list. First of all, most people with CAIS don’t love the term “syndrome” because it makes them sound sick or damaged somehow, which actually isn’t the case. A lot of these folks like to call it Complete Androgen Insensitivity. So I’m going to use that here — CAI. People with CAI have an XY chromosomal make-up which means they have testes. (Y chromosome usually means that testes form. The formation of testes is based solely on the presence of the Y chromosome.) Functioning testes produce testosterone. In folks with CAI, the testes are functioning and therefore produce testosterone but because they have no androgen (testosterone) receptors, their bodies exhibit NO effects of that testosterone. Reception of testosterone is necessary for the formation of external genitalia, otherwise known as a penis and scrotum. So, folks with CAI are almost always sexed as female at birth because they have a vagina and clitoris, and appear to be “completely female” on the outside at birth. They also almost always identify as female, and are women, despite having a different biological make up. They are usually not diagnosed with CAI until late puberty, and this is only because their period never comes. Also, because testosterone actually eventually converts to estrogen (a process called aromatization), during puberty, these folks go through a very typical female puberty, and folks with CAI actually appear hyper feminine because they have no body hair, very smooth and classically “nice” skin, are not muscular, etc, because they have no effects of testosterone, just estrogen. Watch a fantastic video about CAI here: INTERSEX EXPLAINED! | Complete Androgen Insensitivity.

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References & Further Reading





https://en.wikipedia.org/wiki/Sexual_differentiation — check out the citations at the bottom of this article for a deep dive into the biology if you’re interested


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