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Let’s talk about biological sex

By Dr. Shannon Lee

When I was a student in my first biology class, we learned that sex is determined clearly by the sex chromosomes, X and Y. Those individuals with XX were female. Those individuals with XY were male. Easy to understand. End of story. Boy vs. girl.

Many of you probably learned the same and most young students continue to learn this because the introductory textbooks haven’t quite caught up with current scientific understanding. The biological underpinnings of gender, it seems, are not at all that simple.

A quick reminder that your unique genetic material (DNA) is made of genes. Genes are found on chromosomes. Humans have 23 pairs of chromosomes: numbers 1-22, and then the pair of ‘sex’ chromosomes. Humans therefore have 46 total chromosomes. Eggs and sperm each have 23, reductive cell division produces those gametes. When fertilization happens, egg + sperm coming together restores the 46 chromosomes typical for our species.

Non-typical reductive cell division is one way that the simple notion of gender can get blown out of the water. In some cases, the sperm or eggs can end up with more than one sex chromosome, or none at all, and any resulting embryos are different from ‘normal’. Those embryos could be XXX, XXY, XYY, or X0. It’s not very common, but it does happen and can have mild to profound consequences for the individual. For example, Klinefelter Syndrome (XXY) occurs in 1 out of every 500-600 individuals who have a Y chromosome. Another cell division event involves chromosomes ‘swapping’ their genes. This results in a chromosome not being exactly what it would seem. Crossing over can happen with the sex chromosomes, a little bit of X ends up on the Y, or vice versa.

But some of us have male parts and some have girl parts, right?

Separate distinguishable sexes are typical (wildtype) for humans. Individuals (unless sterile) have the capability to produce either sperm or eggs. And yet, there are humans who have both sets of reproductive organs in part (i.e. a man who successfully fathered children discovering later in life, via surgery, that he also possessed a uterus) and there are humans who have varying levels of sex ‘hormones’ or secondary sex characteristics (i.e. a woman with ‘male-style’ facial hair). Individuals can have a variety of anatomies including a combo of testicular and ovarian tissues. Ambiguous genitals at birth occur 0.02% to 0.05% of the time, the same range as the combo of red hair and green eyes (0.04% ocurrence). These conditions exist outside the norm for our species, and yet they do exist.

Sex appears through our wildtype condition to be binary, but the science shows us increasingly that these aspects are on a continuum for our species. The characteristics are bimodal, but individuals exist in the in-between. Intersex is very much biologically clear.

Why does this continuum exist?

This is a great question and the answer could fill an entire textbook on the genetics, endocrinology, neurobiology, and development of the human body. Let’s just focus on one aspect, one specific gene – SRY.

SRY is a gene located on the Y chromosome. It is just one of the genes involved with sex determination, but it plays a role very early on in the development of the embryo. If the SRY gene is not expressed specifically in a precise cluster of cells in a narrow window of time, the embryo will develop female characteristics or something in between traditional female and traditional male. If it is expressed, then the male characteristics (testes formation from that cell cluster) can occur.

So, if an embryo has a Y chromosome, then they have the SRY gene and therefore will develop testes, right?

Well, most of the time, yes but, an individual could have a Y chromosome that is missing the SRY gene or that SRY gene could be inactivated by either mutation or mis-regulation. On the other hand, you could have someone with no Y chromosome (i.e. XX or XXX) but due to crossing over they have a little fragment of a Y that swapped and carried with it the SRY gene! From the simple visualization of the chromosomes that individual would be labeled as female or superfemale but anatomically the individual could have testes, and other traditionally male characteristics.

This is just one of myriad examples that reveal how various genes, biochemical pathways, developmental regulation, hormone production, and brain neurodevelopment can lead to the continuum of sex/gender that is predicted by biological research and born out in our human populations.

The old view of binary chromosomal sex has been thrown around lately as a justification against transgender/ intersex persons’ rights/understanding. This is unfortunate and doesn’t reflect what is currently understood about sex/gender in the science of biology.

Shannon Lee, PhD, is a Glen Ellen resident and an instructor in the Biology Department at Sonoma State University. She has been a science educator for 20 years having taught previously at UCLA and California State University Northridge.