Is Sex a Spectrum? Sex Determination and Differentiation

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 PracticeThe BMJ and The Lancet

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  1. You however overlooked one aspect: The brain masculination. It is an active process of the androgenes to transform a brain into male and effect the behavior accordingly. As research has found, the effect of estrogenes on the brain is minor, deactivation of female hormones doesn’t affect a change in brain and behavior development but deactivation of male hormones has a huge effect.

    CAIS women, the ones with the XY chromosome but deactivated androgen receptors, develop feminine behavior and are mostly heterosexual (means, they are attracted to men). The fact, that female hormones don’t effect the brain as much as male ones also explains, why there are more mtf transsexuals than ftm transsexuals. The brain is a complex organ, it’s development is a complex process and in details there are many aspects where hormones influence that process That goes from small, normal variations in behavior such as males with *some* stronger developed female traits as well as females with *some* stronger developed male traits (e.g men that are interested somewhat more in social activities and females that are somewhat more interested in mechanics and technology) and goes into sexual preferences (homosexuality for instance) and to the extrem into the feeling, that someone is in the “wrong body” (transsexuality).

    1. The brain isn’t relevant when talking about sex determination and differentiation. In determining the sex of intersex infants, many experts are used,but none from neurology. This isn’t about the validity of trans identities, bur purely about reproductive development.

      1. Hi Claire, I am interested in this subject because a. I know people who are transitioning and am concerned by the arguments b. because I once worked at the business end of female reproductive organs and saw the distress of people who were told they did not have the right equipment. (I use the term lightly, it was devastating for them.) c. clearly understanding of developmental issues has really come a very long way, and you have a good understanding of why and how. d. my primary interest is social, and the legal and logical ways to protect people with difference without encroaching on the rights of others.

        I am curious to know why you would dismiss the role of the developing brain in human sexuality. I understand it is not one of the primary organs, but without attraction a lot of reproduction simply would not take place. The brain plays a crucial role in that, and if indeed as another poster indicates that process starts very early and is definable the I am surprised that you dismissed it completely. Neurologists do not seem to engage that much with the more messy parts of the body, which is nearly everything else. Well that is a generalisation, And I know it is! So I think they would very rarely be called in to look at people with chromosomal variations, if there were things they were concerned about it would be more likely a psychologist or psychiatrist would advise. So maybe neurology lages behind genetics in this respect? I hear your arguments as regards some of the politicisation of such issues, and agree that intersex people have been co-opted, or even weaponised to pad out the arguments. When I was a student a very important emphasis was placed on context, i.e. placing things where they had a verifiable and logical relevance. I find a lot of the arguments do not have that, and also leave out the fact that once the arguments are done, no matter where that takes us, there will still be real human individuals struggling with their own reality. Most of them will just want a meaningful and peaceful life and a place in the world. All the X and Y stuff might help them understand, but they will still be who they are, unless somebody inducts them into some kind of -ology about who they are. So I think the brain part of it should not be dismissed in favour of the chromosomes they are both important. And valid. Your arguments are really good to deconstruct some of the more toxic ones, but I want to see them helping to construct better ones that are not oppositional but inclusive. Because outside of the ectreme end of things there are genuinely dysphoric people many of whom have not been coerced at all by the agendas of other, but who have been grappling over many years with how to exist as the person they are. So we need the good arguments to be as complete as possible. And that has got to include the brain.

        1. Hi, the reason I don’t include anything about brains and dysphoria is because these are separate issues to reproductive development. My blog is here to help people to understand what DSDs are/are not, and how they occur. As the blog isn’t about sexuality or identity, and brains are not part of the diagnosis and management of DSDs, they are really irrelevant to the subject.

  2. Hi. q: what is the most precise definition we can have of “male” or “female”? what exactly does “small gamete producer” constitute & how does it apply to males who don’t produce any sperm at all? Another version of the question is: what are the sufficient set of conditions required to be male (for humans)?

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