It’s odd for me, as a long-time author of college biology textbooks, to witness governments rule on the nature of biological sex, perpetuating an oversimplified, binary definition.
A recent edition of The Week, in “The World at a Glance,” relates that Britain’s Supreme Court has defined a biological female: “While trans people are protected from discrimination under the 2010 Equality Act, the categories of ‘man’ and ‘woman’ in that act must refer to ‘biological sex’ because any other definition would be ‘incoherent and impracticable.’”
Ditto Hungary. A constitutional amendment officially recognizes only male and female, banning LGBTQ groups from holding public events. Prime Minister Viktor Organ claimed inspiration in his biological dictate by the US’s recent take on sex determination and gender identity.
“DEFENDING WOMEN”
The US president proclaimed the basis of binary biological sex in the January 20 executive order DEFENDING WOMEN FROM GENDER IDEOLOGY EXTREMISM AND RESTORING BIOLOGICAL TRUTH TO THE FEDERAL GOVERNMENT – CAPS his. The statement conjures Captain Picard, in Star Trek Next Generation, uttering “make it so.”
The January 20 order is infused with insults, name-calling, and assumptions, yet lacking utterly in the language of the science of biology.
Fortunately, the President’s DECLARATION of “biological truth” online has a search feature, so I typed in the basic terms for the science of sex determination. “No results” came back instantly for “XX,” “XY,” “chromosome,” and “SRY.” Was he absent that day in seventh grade biology? Or was it too long ago?
The terms from biology aren’t there because the document is utter nonsense. If anything, it is yelling. Make. It. So. The language is emotional, juvenile, angry, judgmental, and insulting – the antithesis of scientific exposition:
“ideologues who deny the biological reality of sex”
“The erasure of sex in language and policy has a corrosive impact not just on women but on the validity of the entire American system.” (WTF?)
“This unhealthy road is paved by an ongoing and purposeful attack against the ordinary and longstanding use and understanding of biological and scientific terms, replacing the immutable biological reality of sex with an internal, fluid, and subjective sense of self unmoored from biological facts.”
The Science of Sex
In contrast is my broader take on the “immutable biological reality of sex” in Human Genetics: Concepts and Applications,” one of my books with McGraw-Hill:
“Biological maleness or femaleness is determined at conception, when a biological female inherits two X chromosomes or a biological male inherits an X and a Y. Another
level of sexual identity comes from the control that hormones exert over the development of reproductive structures. Finally, biological factors and social cues influence sexual feelings, including the strong sense of whether we are female, male, or nonbinary—our gender identity.“
That definition has been in my textbook for decades.
Here, in a nutshell, is what happens to us as early embryos.
Sperm fertilizes egg, cell divisions ensue, and then tissues form and fold into organs. We all begin with a set of ambiguous structures that can follow either of two developmental fates, elaborating the organs associated with being biologically male or female. Genes awaken and steer development in one or the other direction – usually.
Early on, development revolves around the SRY gene, part of a Y chromosome. It encodes a type of protein called a transcription factor, which controls the expression of a bunch of other genes. SRY’s activation sparks a cascade of gene responses that ultimately sculpt a male body. Female structures predominate if SRY isn’t there or active. For years we XXs were considered a developmental “default” option because we do not have SRY, but a cascade of expression of other genes spearheads development of female body parts.
Of XX Males and XY Females – It Happens
We know that the SRY gene is critical because of what happens when it is missing or moved.
Share this article
Plop SRY into an X chromosome when sperm are forming, and an embryo could be XX yet elaborate male sexual structures because of the extra gene.
Conversely, an XY embryo developing from a sperm that’s jettisoned its SRY gene from a Y chromosome leads to an XY individual with female reproductive structures.
So one can indeed be XX with male structures or XY with female structures. A meme to this effect circulated instantly after the January 20 rewriting of biology, but I like to mull things over a bit before commenting. It takes me awhile to fathom the absurdity.
Genetic males with female structures and vice versa happen in other ways too.
• In androgen insensitivity syndrome, a mutation in a gene on the X chromosome blocks formation of receptors for testosterone. This cuts off signals to elaborate male structures. The person looks female, but is XY.
• In congenital adrenal hyperplasia (CAH), a blocked enzyme leads to buildup of testosterone. XY males enter puberty very early and XX females develop a swollen clitoris that resembles a small penis, with deepening voice and growth of facial hair. The individual doesn’t menstruate.
• In 5-alpha reductase deficiency (5-ARD), a person’s cells have a Y chromosome, a working SRY gene, and testes. Inside, organs are male. But the absence of the enzyme (5-alpha reductase) means that testosterone can’t react to form the molecule DHT. The child doesn’t appear male until puberty, when the adrenal glands start cranking out testosterone. Then, the XY individual, who thought she was female, starts to grow facial hair and elaborate male musculature, as the voice deepens. She doesn’t develop breasts or menstruate, as a small penis telescopes out from what appeared to be a clitoris.
A bit of history involving 5-ARD reveals the value of accepting human diversity.
In the Dominican Republic, the condition is more prevalent due to relatives having kids together. In the 1970s, 22 young girls reached the age of puberty and started to resemble boys. But instead of being shunned because they looked different, they were given their own gender name, guevedoces, which means “penis at 12.” They were fully accepted as whatever they wanted to be. Jeffrey Eugenides’ 2002 compelling novel Middlesex traces a family with the condition stemming from siblings forced to have sex while in hiding during the Armenian genocide during World War I. It’s one of my favorites.
On another front, transgender identity has been denied and belittled quite a lot since the beginning of 2025. That will not make the very real phenomenon go away. DNA Science covered the genetics of transgender identity in 2018, with an update in 2020.
CODA
The “unity and diversity of life” is a running theme that has stood the test of time in biology, appearing in the titles of many textbooks.
We all have DNA as our genetic material, and differ in only a very tiny smidgeon of its sequence. Yet at the same time, we are startlingly diverse, in appearance, talents, personality, health, and on and on and on. It’s a concept that requires knowledge and thinking – absent from the angry, emotional tone of the Presidential declaration concerning the biology of sex.
I can’t help but wonder what other biological facts will be distorted, denied, or silenced under political pressure in the years to come. I am thankful to have some control in revising my textbook, keeping the science intact. In the words of Tom Petty, I won’t back down.
Ricki Lewis has a PhD in genetics and is a science writer and author of several human genetics books. She is an adjunct professor for the Alden March Bioethics Institute at Albany Medical College. Follow her at her website or X @rickilewis
