Saturday, 16 May 2015

On Homosexuality

Where it all begins
A few weeks ago I wrote a post about how homosexuality could be maintained in a population even though it appears to violate some central tenets of Darwinian selection. I proposed two simple models which could conceivably explain why homosexuality is maintained, albeit at relatively low population levels. In this post, I'll be reviewing why homosexuality occurs at all. Although, if I'm being totally honest, the answer should be: 'We simply don't know'. Of course, because we are ignorant, doesn't mean we shouldn't try to unravel this intriguing and most obtuse of conundrums. 

As a professional geneticist, I'm fascinated by human sexuality and especially the mechanisms involved in determining our sexual orientation. Most of us are heterosexual and sexually attracted to the opposite sex. This makes sense from a pure biological perspective. But sexual preference is a complex matter and a minority of individuals are homosexual (perhaps as many as 8% of people), whilst an even smaller category are attracted to both sexes- twice the fun with only half of the equipment.

Homosexuality can be an emotive topic. Luckily, in the civilised West, most folk have become enlightened and homosexuality is no longer seen as a 'disease' begging for a cure, except amongst the fundamental Christian fraternity. This change in attitude is a relatively recent development. A practising homosexual in Britain a mere 50 years ago faced the reality of being sent to gaol. This is not to say homophobia has disappeared from the mind of man. There will always be people who express distaste for homosexuality even in the most civil of societies. As for the rest of the world- no comment.

After that rather long preamble, I can move onto the interesting science (really Flaxen?). The early embryo in the womb is sexless. Sexual differentiation does not kick in until the 8th week of gestation. If the embryo is destined to become male, testosterone is produced by the gonads which act on various locations in the developing embryo sending it scurrying down the male pathway. If the foetus is destined to be female no testosterone is manufactured and the outcome is a girl. Therefore, the default pathway is always female. Something has to happen and 'signal' maleness in order for the foetus to get a willy.

At the current state of knowledge scientists are unsure of the mechanism(s) underlying homosexuality,  however, there are biological clues. You can learn a lot about biological processes by observing what happens when it goes wrong.

There is a condition called Testicular Feminisation (TF). In TF, although the chromosome sex is male (XY complement), the embryo develops along the female developmental  pathway. A gene on the Y chromosome  (SRY) causes the primitive gonads to differentiate into testes which then go on to produce the androgenic hormone testosterone. Although testosterone is produced as expected the end point organs are insensitive to the hormone, due to a genetic mutation, and so the embryo is feminised rather than masculinised.  At birth, they look like perfectively normal girls. As they grow and develop nothing seems amiss. They undergo puberty and develop breasts. Generally they get discovered when they are referred to an obstetrician due to infertility or because of failure to menstruate. On examination, they are found to have a blind ending vagina and a total absence of ovaries. But they do have testes which are present in the abdomen. This is the only vestige pointing toward their true gender. And what is interesting is that they are psychosexually attracted to men. To be accurate, this should be called homosexuality as from a chromosomal perspective they are male. Studies have shown that there is a lower incidence of sexual preference for women in TF individuals than is observed in the normal female population. Again, to be strict, an attraction to women in TF individuals is a heterosexual orientation - confusing isn't it?

All these women have Testicular Feminisation

I've outlined a model below which is entirely consistent with what we know to date. However, a caveat must be applied: how homosexuality is determined is mostly unknown and my proposal should be seen as a plausible theory, although a simplistic theory at that.

Evidence from studying TF individuals suggest that testosterone has an important role in defining our sexual preference. A pulse of testosterone at a critical stage in embryonic development assigns our sexual identity and our sexual preference. Remember, TF individuals produce testosterone but the hormone has no impact on the developing embryo.

Embryonic development is a complex choreographed interaction between tissues and hormones. Any slight deviation results in embryonic malfunction. I suspect that the pulse of testosterone comes either too late or too soon subtly influencing the brain of the embryo. The resultant outcome is that the  individual becomes attracted to members of the same sex. All this sounds way too simple. If there is one thing I've come to realise as a practising biologist is that biological processes are mind numbingly complex. Therefore, I suspect there are other factors involved (multi-factorial) and this is particularly true for highly complex behavioural traits. Positing that sexual preference is down to testosterone alone, just wont do. I suspect other hormones have a role to play and I also suspect that maternal chemicals entering the foetus via the placental connection are important factors as well. Furthermore, compounding the situation, hormones may also interact with each other resulting in a bewildering interplay of influences (my head hurts).  

One thing we do know is that sexual orientation is fixed in the womb, or shortly after birth, and we have no choice when it comes to our sexual orientation. Once set it does not change. The religious nuts who think they can 'cure' homosexuality should take heed.  When male babies with deformed genitalia are raised as girls and given female hormone therapy and even surgery to produce female genitalia, they almost always grow up being sexually attracted to women.    

The brain is the seat of sexual preference and the brain is the most complex organ we have. How it forms and how it functions is not well understood. Translating the grey matter to human behaviour such as sexual attraction is a labyrinthine quest and I expect we won't unravel all its mysteries sometime soon if at all.

Two sensible posts in a week FS, either you are losing an already tenuous grip on reality or you have finally teetered and fallen into the abyss of madness. Arse.  



  1. "...There will always be people who express distaste for homosexuality even in the most civil of societies..."

    Well I find the thought of sexual encounters with another man distasteful: but then I find Gangsta Rap distasteful, Association Football distasteful, the Labour Party distasteful, most banks distasteful - oh, the list is almost endless.

    I don't believe that it makes me unenlightened or narrow-minded as (with the possible exceptions of Bankers & the Labour Party) I wouldn't dream of legislating or discriminating against those who don't share my distaste or who even indulge. It's their business and as long as they respect my right to disagree, without calling me all sorts of "-ist", then there's no problem.

  2. I have no problem with other people's sexual orientation. So long as all parties are consenting, why should I care?

    Alice Roberts' book 'The Incredible Unlikeliness of Being' explains the normal complexities of gestation in a manner that is fascinating, witty, and understandable. A damn good read.

    Hey Flaxen, I bet you could do similar!