Right, we’re still struggling with the idea that there’s only two sex classes and that intersex people do not prove the ability to change sex, or do not exist on a spectrum. So, let’s look at sex determination and differentiation…
Sex is determined at the point of conception. This is known as primary (or gonadal) sex. In mammals, primary sex determination is chromosomal. In most cases, the female is XX and the male is XY. Every individual must have at least one X chromosome. These chromosomes then dictate development. The presence of a Y chromosome is a crucial factor for determining sex in mammals as it carries a gene that encodes a testes determining factor. In other words, it tells the gonads to develop into testis rather than ovaries.
To be really clear about this, even a person with XXXXY chromosomes would be male because of the Y. Someone with only one X (X0) chromosome would still be female and their body would begin making ovaries (although usually they would not develop fully as the second X chromosome is needed). From this we move onto secondary sex determination, or sex differentiation.
Secondary sex determination affects the bodily phenotype outside the gonads. A male mammal has a penis, seminal vesicles, and prostate gland. A female mammal has a vagina, cervix, uterus, oviducts, and mammary glands. In many species, each sex has a sex-specific size, vocal cartilage (aka Adam’s apple), and musculature. These secondary sex characteristics are usually determined by hormones secreted from the gonads. However, in the absence of gonads, the female phenotype is generated.
If the Y chromosome is absent, the gonadal primordia develop into ovaries. The ovaries produce oestrogen, a hormone that enables the development of the Müllerian duct into the uterus, oviducts, and upper end of the vagina. If the Y chromosome is present, testes form and secrete two major hormones. The first, AMH, destroys the Müllerian duct. The second hormone, testosterone, masculinizes the foetus, stimulating the formation of the penis, scrotum, and other portions of the male anatomy. This also inhibits the development of the breast primordia. Thus, the body has the female phenotype unless it is changed by the two hormones secreted by the foetal testes.
Now, I know someone will be quick to pop up and say, “but MRKH, there are XX males and XY females), this is where the SRY gene (sex determining region of the Y chromosome) comes into play, so let me explain….
The major gene for the testis-determining factor resides on the short arm of the Y chromosome. Individuals who are born with the short arm but not the long arm of the Y chromosome are male, while individuals born with the long arm but not the short arm are female. Right on the end of the short arm is where we find the sex determining region Y gene. There is extensive evidence that it is indeed the SRY gene that encodes the human testis-determining factor. SRY is found in normal XY males and in the majority of rare XX males, and it is absent from normal XX females and from many XY females. Another important gene in sex determination is SOX9. XX humans who have an extra copy of SOX9 develop as males, even though they have no SRY gene.
Secondary sex determination in mammals also involves the development of the female and male phenotypes in response to hormones secreted by the ovaries and testes. The formation of the male phenotype involves the secretion of two testicular hormones. The first of these hormones is AMH, the hormone that causes the degeneration of the Müllerian duct. The second is the steroid testosterone. This hormone causes the urogenital swellings to develop into the scrotum and penis. The existence of these two independent systems of masculinisation is demonstrated by people having AIS.
These XY individuals have the SRY gene, and thus have testes that make testosterone and AMH. However, they lack the testosterone receptor protein, and therefore cannot respond to the testosterone made by their testes. Because they are able to respond to oestrogen, made in their adrenal glands, they develop the female phenotype. However, despite their distinctly female appearance, these individuals do have testes, and even though they cannot respond to testosterone, they produce and respond to AMH. Thus, their Müllerian ducts degenerate. These people develop as normal but sterile women, lacking a uterus and oviducts and having testes in the abdomen.
In summary, because this is a long enough thread, primary sex is determined at conception, the foetus then follows one of several differential routes. This may be a complicated process with lots of variables, but it does not a sex spectrum make. We’re all just male or female, this is decided before we are even born, and science is able to explain it and categorise us accordingly. The end.
If you want to read about this in more detail, I recommend this paper (it’s long but fascinating). Also this paper, which is quite technical and heavy going but has links to lots of other research and some handy illustrations.
Addendum: Many medical journals have recently responded to the use of intersex conditions, by trans activists, as “proof” that human sex is not dimorphic. Such as The British Journal of General Practice, The BMJ and The Lancet.