[chris (OP)]

I was asked today whether there is any evidence for the existence of a "gay gene" ?

Your thoughts?

Chris

[SteveFish]

Any human behavior is very complex, not like genes for hair and skin color or the color of pea plants. If there are actually allele sets that bias an individual to be homosexual, they would probably require gene numbers in the hundreds. The real question is-- Other than scientific curiosity, why does anyone care. Steve

[Geezer]

I think it's a great pity that "gay" has become a euphemism for a particular sexual orientation.

Some tend towards the heterosexual while some tend towards the homosexual. Anyone who pretends that there is some arbitrary dividing line between the two does not know much about sex.

[RD]

Seems there must be more than one mechanism for homosexuality.

The birth order correlation for gay men is environmental: it depends how many male fetuses had preceded them.

But that immune mechanism wouldn't explain gay women.

[chris (OP)]

My feeling is, if there is a genetic basis to homosexuality, then the allele(s) responsible must be clusters of genes that bias behaviour rather than genes that directly determine sexual orientation. Otherwise pro-gay genes would coninually be removed from the population because homosexuals are far less likely to reproduce than heterosexuals. The reported level of homosexuality in the human population - at 5-10% - is therefore far higher than would be expected if such a scenario were true.

I think it therefore fits better with a model in which a group of genes load the developmental dice, rather than directly determine outcome, which means that the environment - and chance - also play a role. The roles of the alleles involved would also extend beyond homosexuality, hence explaining their conservation at high level in the gene pool.

[graham.d]

This is not my subject so I may be way off here, but isn't sexual orientation something that is determined post-conception. That is there is nothing in the genes in the sperm or unfertilised egg that predetermine the sex of a child, but the appropriate route taken during the early stages of embryonic growth. Is it this mechanism that is being varied, maybe by environmental influences, that determines the sex. We may say it has gone wrong when people are born with one set of genitals but with a brain that tells them they should have the other set. Some people seem to fit this category but I doubt that this is true of all homosexuals; there seems to be quite a number of people who "swing both ways".

Where the genetics comes in is that this mechanism for determining sex, which is genetically passed down, is clearly not 100% doing, what we may think of, as the right job. Maybe this is just the how it is, and if there is no significant disadvantage, there would be no selection to remove it. However, as is pointed out, homosexuals are less likely to breed so why would such a "flawed" mechanism gradually not be selected out. One possible reason is that a society with sexual diversity is somehow beneficial to the society as a whole. I can think of some reasons why this may be so but it would be hard to prove scientifically.

[rosy]

Graham:

This is not my subject so I may be way off here, but isn't sexual orientation something that is determined post-conception. That is there is nothing in the genes in the sperm or unfertilised egg that predetermine the sex of a child,

Do you mean sexuality? Gender? Or do you really not know that the sex of a child (plus or minus a very small number of anomalous individuals) is absolutely determined genetically by whether they do, or do not, have a Y chromosome passed on from the father?

The thing about genetics is that a gene isn't always just "favourable" or "unfavourable" to reproduction, even for something that can be attributed to a single gene. Having one copy of the sickle cell gene provides some protection against malaria (a definite benefit, if you live in  malarial region). Having two copies gives you sickle cell disease (not favourable, wherever you live). If someone's got one copy they're more likely to breed, and so even if some of their descendents have sickle cell they're quite likely to have more descendents in total so the gene stays in the population.

With something as complicated as sexuality, and bringing in environmental factors too, having a gene, or several, which tends to predispose someone to being attracted to the same sex might have advantages for reproductive success... and although an individual with several of these genes and/or the relevant environmental factors (being a younger son, for example) might be less likely to reproduce, the genes might on average confer an advantage.

Of course, there is also the fact that being gay doesn't actually prevent people reproducing... if society says loudly enough that you get married and have kids, very many people will get married and have kids regardless of their personal preferences (possibly some gay men just couldn't, but this doesn't apply to women, especially in societies where what the woman wants isn't held to be particularly important).

[QuantumClue]

I was asked today whether there is any evidence for the existence of a "gay gene" ?

Your thoughts?

Chris

I'm not a biologist, but I would guess it would be due to levels of tetesterone during the development in the womb. Some studied have suggested maybe even before this.

[graham.d]

Yes, now you mention it Rosy, I did know that at some time. I was talking bo**ocks (or perhaps something with less well determined gender). Oops, can't always be right :-) I did say it wasn't my subject.

I thought there was something about sexual development that happened post conception though. I can't have just imagined it.

[SteveFish]

Graham.d:

All of development occurs post conception. The primary controls for this, in utero, are provided by the genetic code. What you are probably thinking of are the hormonal interactions that control sexual development, but they are just part of the program that will occasionally go awry. I don't think that there is any evidence that the part of the code that guides sexual development (anatomical) have any affect on ultimate gender preference (behavioral).

Steve

[Geezer]

Aren't there examples of identical twins having different sexual preferences? If so, would this tend to indicate that the influencing factors are "post natal"?

[rosy]

If so, would this tend to indicate that the influencing factors are "post natal"?

Well, it might... it seems to me (intuitively, and totally unscientifically) likely that at least some of them are, in any case.

But not necessarily. The experience of twins, even identical twins, even in the womb, can be very different because usually one has a better positioned placenta (or something) than the other. All the pairs of identical twins I've ever known (small sample size, but this is more-or-less a counterexample argument anyway..) had very different birth weights, and as they grew continued to be noticeably different in height and weight at least for several years. It's not obvious to me that two foetuses of significantly different sizes, even if exposed to the same hormonal signals across the placenta, will get the same effect from it (especially with something that is such a broad spectrum as sexuality).

[RD]

It's not obvious to me that two foetuses of significantly different sizes, even if exposed to the same hormonal signals across the placenta, will get the same effect from it

Or there is a critical period in prenatal brain development during which sexual orientation is defined and the "identical" twins, developing at different rates, passed through their respective critical periods at different times when conditions in the womb were different, e.g. the exposure of one "identical" twin to a hormone during its critical period is above threshold, the other below.   

[Geezer]

Chris raised an interesting point about selection. If all males were heavily biased towards masculinity, the human race would probably have wiped itself out some time ago due to wars. A "spectrum" of masculinity may be a good thing for survival.

[CPT ArkAngel]

If there is a gene associated to it, it should occur by mutation because this gene should be eliminate by natural selection. Gay people have less children than heterosexual in average so after a long period of time, their percentage would regress.

I think it is a matter of social and psychological interactions with others in a sexual perception context, mainly during adolescence and preadolescence... In some context, like a high density population, the percentage of gay people can grow. It could be a kind of defence mechanism against overpopulation...

[Geezer]

Er, frankly, I think this may be a load of bolloc baloney.

The notion that there is "gay" requires the concept of "ungay". Ungays would have absolutely no interest in the opposite sex except for one thing, and that would not get them very far. Sexual success encourages some amount of empathy with a partner, and that requires a degree of "gayness".

I think it all about shades of grey.

[JP]

But you do agree with the idea of a spectrum between only being interested in the opposite sex and only being interested in the same sex? 

In that case, would genes have an influence on your position on that spectrum?

For the record, from what I've heard I'm fairly convinced there's a spectrum, although I'm unsure of the evidence for genetic influence. 

I have heard, however, that the mother's genetics and biology may effect sexuality, through the amount of testosterone the fetus is exposed to in the womb.

[Variola]

But not necessarily. The experience of twins, even identical twins, even in the womb, can be very different because usually one has a better positioned placenta (or something) than the other. All the pairs of identical twins I've ever known (small sample size, but this is more-or-less a counterexample argument anyway..) had very different birth weights, and as they grew continued to be noticeably different in height and weight at least for several years. It's not obvious to me that two foetuses of significantly different sizes, even if exposed to the same hormonal signals across the placenta, will get the same effect from it (especially with something that is such a broad spectrum as sexuality).
 

The phenotypcial differences can be explained in some part by the prevalence on TTTS in identical twins, the degree of severity of TTTS and stage in development can both play a part in this.


In regards to the 'gay gene' which has been subject of much discussion and research, so far all the studies done have implicated different genes, or different chromosomal linkages  [] So none of it is particularly conclusive.
In my opinion, if there is a genetic component then it will be more akin to the APOE4 allele in AD, with environmental factors and hetro/homozygocity  plays a bigger influence that possession of a gene. 

[yor_on]

It seems as much social as genetic to me?
Like a bell shaped curve in where you will find most gay's having a blend and some, especially as a guess those transforming themselves into women going the whole way so to speak, truly having a genetic bias. And maybe all of those genes found in some combinations can create predispositions?

I mean, why are most of the human population heterosexual?
Genes?

 

[techmind]

I don't know the answer to the original question, but there was research published in the past week or so about some mammal? species which was dying out somewhere and it was found that ?lead? (or other heavy-metal) poisoning was causing the males to mate with males instead of females, which was causing the population to die out over successive generations even though the ?lead? wasn't causing acute symptoms.


There was also a story a few years ago about some flies that were specially bred/modified so that they switched from hetero- to homosexual behaviour when the ambient temperature was changed. That this is possible must tell us something quite deep... but I'm not sure what!

On many discussion groups people will argue that there is a big difference between physical sexual preferances/activities and who they'd have a long-term emotional relationship with. I don't know how "real" this distinction might be, or whether it's more a case of sexuality being a sliding scale (which may be 'sampled' by a succession of physical encounters) and who you would choose for a long-term partner, influenced to a greater extent by societal pressure and/or desire to have children?