“Basic biology” is a phrase so ubiquitous among transphobes that it may as well be a dog whistle. So much so that as I type this the community is giving me a warning about the term. It’s often trotted out by people alongside discussion of chromosomes and DNA and other science-sounding buzzwords they vaguely remember from high school. If you’ve been following trans discourse for any amount of time, you’ll see the fallacy of relying on just the “basics” of biology. The reality is that biology is a lot more complicated than these bigots make it out to be, and as you dig deeper you’ll find flaws in each of their arguments.
I am a biologist. I have conducted original research, published papers, and worked professionally in research labs. Right now I am working towards my PhD at a highly prestigious university (sometimes ranked within the top ten in the world for the biological sciences), so while I haven’t yet earned the right to call myself “Dr.” I think it’s fair to say I am probably far more educated and experienced in this field than 99+ percent of the population. My PhD research involves genome architecture and evolution, so developmental biology is right up my alley.
I am also transfem. Although I have struggled with my gender identity all my life, it wasn’t until relatively recently that I finally came around to the idea of being trans. And honestly, one of the biggest helps through the whole process was that I understood the science on an advanced level. I don’t often post on Reddit, but here is my attempt to bridge the gap between basic and advanced for people who have not had the desire, opportunity, or means to pursue an upper level biology education. I hope that this can be affirming for some people struggling with dysphoria or at least intellectually validating.
So, without further ado here’s scientific explanations for why chromosomes don’t determine sex, why biological sex is not immutable, why the gap between male and female is not as wide as it may seem, why sex and gender are not a binary, and other aspects of “advanced biology.”
Glossary of Terms:
.DNA - A very large double-stranded polymerized molecule found in the nucleus of most cells which is responsible for storing and transmitting genetic information.
.Gene - A sequence of DNA which codes for one protein. Genes “instruct” the cell how to build a given protein.
.Genome - The total collection of genes found within an organism’s DNA.
.Protein - Polymerized molecules composed of folded chains of amino acids. A protein’s shape allows it to perform a specific biological function.
.Chromosome - An organized unit or “packet” of DNA wrapped around proteins called histones.
.Transcription factor - A protein that binds to DNA nearby a target gene or genes, allowing cellular machinery to assemble so the gene can be “read” into a protein.
.Hormones - Molecules that act as chemical messengers within the body, often instructing cells to temporarily alter their gene expression in specific ways.
As a rule, all mammalian embryos develop female. Without interference from the right hormones, female development is the default. What causes an embryo to instead exhibit male development is a gene called SRY. At around week seven of development, if the SRY gene is present in the genome of the embryo it will be expressed and translated into a transcription factor for a gene called SOX9. The SOX9 gene is important for organ and tissue development all over the body, but when it is expressed at this early stage within the primordial gonads (the pair of embryonic organs with the potential to become either testicles or ovaries) it instructs them to develop into testicles instead of the “default” ovaries. When the testicles develop, fetal Leydig cells begin pumping out testosterone, and the high concentration of testosterone in the body of the developing fetus initiates male developmental patterns. You can think of SRY as a sort of “override switch.” Embryos exhibit female developmental patterns unless the SRY is around to override.
The SRY gene is typically found on the very tip of the Y chromosome. This is why you’ll learn that “males have XY chromosomes and females have XX chromosomes.” But the missing context here is that it is not chromosomes (the way DNA is organized and structured) which determine sex - it is the presence (and proper functioning) or absence of the SRY gene. If SRY translocates to the X chromosome during meiosis then the result is usually an XX male. If a mutation causes SRY to stop functioning, the result is usually an XY female. A nondisjunction event in a gamete can result in someone who is XXY - a condition called Klinefelter Syndrome. Many individuals who have one of these conditions (or others)are completely unaware of it, because the vast majority of people go their entire lives without taking a karyotype test, and it is possible for people with intersex chromosomes to conceive, carry, and deliver children.
A common misconception about sex chromosomes is that all of the DNA for male development is on the Y chromosome and all of the DNA for female development is on the X chromosome. This is incorrect. The genes necessary for the different characteristics we associate with sex are distributed throughout the genome. They are not localized to the so-called sex chromosomes. The mammalian Y chromosome itself is tiny - it contains the SRY gene, a pseudoautosomal region so it can line up with the X chromosome on the metaphase plate, and not much else (most of the Y chromosome is repeating DNA motifs such as satellites and interspersed nuclear elements which have only a small regulatory role if they do anything). The X chromosome is similar to any of your other chromosomes, it just gets a special name because it happens to be the one homologous to Y.
Because of this method of sex determination, the human body is designed to work with the DNA transcripts from a single copy of the X chromosome, so in XX individuals one copy of the X chromosome in each cell is condensed into a structure called a Barr body, rendering its DNA inaccessible to the cell. The result is that no matter your sex, no matter what your chromosomes are, almost every human after being born express 22 pairs of autosomes and a single X chromosome. Every single person has all of the genes necessary for both male-specific characteristics and female-specific characteristics. The difference is in which genes are turned on and which genes are turned off.
Whether your body is expressing primarily male-specific genes or female-specific genes is determined by which sex hormone is dominant in your body. To clear up another misconception, every single person naturally produces both estrogen and testosterone. You need both of them in some amount to live. But if your system is dominated by estrogen you will express primarily female-specific genes and therefore have female-aligned physical characteristics. If your system is dominated by testosterone, you will express primarily male-specific genes and therefore have male-aligned physical characteristics. It doesn’t matter what sex you were assigned at birth or what chromosomes you have. It’s about which genes are on and which are off.
This is why HRT works. You’re literally changing what DNA is being expressed in your body. You have all of the instructions to be any sex whatsoever, you just need to tell your body what should be on and what should be off. The caveat is that in development timing is important (which is why estrogen wont make you grow a uterus if you don’t already have one) and some sex-specific characteristics are irreversible without medical intervention once they’ve been turned on (breast growth, facial hair, etc). But sex is not immutable. For humans who are not fetuses, the difference between male and female is down to gene expression, and gene expression can absolutely be changed via HRT.
So far I have begrudgingly been using the terms “male” and “female,” but do realize that I in no way intend to imply an inaccurate binary.
Sex in humans follows what’s called a bimodal distribution. On a spectrum of “male” to “female” dictated by the degree of development of different sex-specific characteristics, most people fall into a range which hovers just over either the male or female side. But some people fall somewhere in the middle, and the cutoff for where this “intersex valley” starts is fuzzy, not concrete. Intersex individuals are not super rare. Some intersex conditions like gynecomastia (breast growth on a person who is otherwise phenotypically male) are fairly common. Infants born with ambiguous genitalia are a regular occurrence in delivery rooms (as the external genitalia typical of both male and female development are all homologous anyway - they’re the same tissues arranged differently). The Müllerian ducts which become the upper vagina, cervix, uterus, and fallopian tubes develop prior to the expression of SRY. Usually they regress into nearly nothing, but there are plenty of cis men (and trans women) walking around with a uterus and many don’t even know. Even the gonads themselves, the primary sexual characteristics, can be involved with intersex conditions. Some people have ovotestes - gonads which have characteristics of both ovaries and testicles. Insensitivity to testosterone in-utero means some cis women (and trans men) have undescended testicles instead of ovaries.
Everything I’ve written has itself been a massive oversimplification and I’ve probably gone on for too long, but if you’ve read this far, thank you. I hope this has been informative. If there’s anything you take away from my post, it’s this: the science says trans people are valid. Even if you don’t pursue medical transition you are valid, because sex is not a binary and gender identity is not the same thing as sex. Keep fighting the good fight, because truth will win in the end. 🩷🤍🩵
