And if so, does that mean there is also a straight gene? What about a bisexual gene?
No, there is no single gene. There may be a combination of genetic (and environmental) factors that contribute to sexual orientation, but it is by no means completely understood.
The causes of homosexuality, bisexuality, and heterosexuality are far from understood. As best we know, there seems to be an interplay between some genetic events and environmental factors which influence the orientation of an individual. A current hypothesis is that hormonal influences during the fetus stage affect gene expression which ultimately affects orientation. This hypothesis is supported a little by experiments done in mice in which researchers look at the masculinity and femininity of mice that are surrounded by opposite sexed siblings while in the womb.
It's been shown that a female mouse surrounded by male mice in the womb will have more masculine features, and a male mouse surrounded by female mice in the womb will be markedly less masculine in behavior and anatomy.
Another study shows that the more older brothers a man has, the higher his probability of being gay or bisexual. One possible explanation for this phenomenon is that a mother may develop 'immunity' or build up a stronger attack force upon recognizing the male 'non-self' fetus' Y-chromosome, and promote female gene expression on the X.
Does homosexuality really have anything to do with masculinity or femininity in mice though? I think it would be hard to make a tie between the behavior of a mouse in the womb to the development of a human during childhood. There are way too many differences.
I think you would also have trouble citing homosexuality to a specific gene because of the obvious impact this would have on the viability of the animals evolution.
If a animal or early human was to become homosexual this would have some serious implications on the offspring don't you think?
I think something could be said about the upbringing and environment of an individual in question.
If the question is: Is homo/bi-sexuality genetic? Then I'd say yes, with little doubt.
First you've got the opinion of people themselves, straight and gay, most of whom don't consider their sexual preferences a matter of choice. Second, the abject failure of psychological and other attempts at 're-orientation'. Third, the fact that homo/bi-sexual behavior has been observed in lots of other species. And fourth, because it'd seem unlikely that we wouldn't have evolved a basic 'programming' of sorts for sexual attraction, given how important it is for our reproduction (and hence evolution itself).
If the question is whether it's a single gene or not, then the answer's no, I doubt that.
It's really a typical example from a long line of assumptions about traits we humans think are massively important, for no particular good reason, and then project that percieved importance onto biology. As with racial theories, phrenology, etc. Those assumptions usually turn out to be false. The things we see as important often aren't, because biology's a lot more complicated than our superficial impressions lead us to think.
I don't think there's any good reason to assume there's simply a 'gay-straight' switch somewhere in the body. In fact, that speaks against itself in some ways, since there's no big survival benefit in itself, in having a portion of the population being homosexual.
To me, it seems a lot more plausible (given that it exists in other species, and thus would seem to have been around a while) that the mechanisms behind homosexuality are intrinsically linked to the mechanisms behind sexual attraction (at the higher level we know it; not talking about plant sex).
We don't actually know if nature had a 'choice' in the matter. It'd be entirely possible that you have a situation where you have a selection between a scenario where genetics gives you 95% straight people and 5% gay people, versus a situation where you have neither gay people nor a strong sex drive. In which case the former is a clear survival advantage for the population.
Or to put it another way: How do we know we're not wired in such a way that a certain likelihood of homosexuality isn't necessary for sexuality/sexual attraction as we know it? Could straight people exist without gay people?
Personally I think that's a more plausible scenario (and a more interesting question) than having a single 'gay gene' that turns homosexuality on or off.
That's just wrong at an intro-to-biology level. By that rationale, no lethal genetic diseases could exist. Genetics is not as simple as that.
Actually, that study was misrepresented by biophreak. I believe the work being referred to is vom Saal's work (I don't recall the year, but it was a long time ago). That work did demonstrate an effect of intrauterine environment on adult behavior, but it wasn't really a clear-cut masculinity/feminity effect. Rather, the behavior studied was aggressive behavior. Females that were located adjacent to males in utero were demonstrated to be affected by the locally higher testosterone concentrations (secreted by the males) and were more aggressive as adults than females located between two other females. The conclusions and implications of the work generally pertained to maternal behavior...that females gestated between other females might usually be better mothers, unless the mice experienced overcrowding conditions (i.e., a population growth), in which case the females gestated between males were better at defending their offspring and resources, so then had an advantage as mothers under those conditions.
And, as Monique indicated the answer to the question is there A gay gene is NO. Any genetic contribution is likely to be polygenic. There is too much variation and a large continuum of sexual orientation for it to be attributed to a single gene. There is also a good possibility of epigenetic influence, or intrauterine environment (that may be through epigenetic or other effects) such as sex hormone or glucocorticoid hormone levels.
One thing to be careful about is that there are those who try to argue that because something is not genetic it is not "natural." That bit is rubbish. There are biological factors that can affect things like post-translational modifications of proteins or alternate splicing that are not genetic, but still biological.
One person's work to look into if you're interested in learning more about the biological mechanisms is that of Fred Stormshak at Oregon (I think OHSU). He's been studying homosexual rams in a fairly systematic way.
I didn't misrepresent anything moonbear...you just haven't read http://dels.nas.edu/ilar_n/ilarjournal/45_4/pdfs/v4504vandenbergh.pdf [Broken]. Different article, not from a long time ago.
IMHO, the gene of intelligence is recessive when the gene of intolerance grows.
You should read your own article (and should have cited it in the first instance). First, it is not an original research article, it is a review article. And, indeed, the studies it is citing with regard to intrauterine position in mice is vom Saal's work.
You ARE misrepresenting that work to suggest it pertains to sexual orientation. As I pointed out, the masculinization and defeminization of behaviors that were studied in the original works had nothing to do with sexual orientation, but aggressive and maternal behaviors. Female mice with an intrauterine position between two male mice still mated with male mice. With any review article, go back and reread the original work before reading too much into the interpretation of it.
I see that what I had just posted in the Gay Gene thread has also been touched upon in this thread.
I believe that it should be clarified for those that don't understand that if there 'are' genes involved in determining sexuality that it most likely is only an 'increased probability' of causing said person to be homosexual. That's how most genes work.
For instance, if they do find that specific genes are linked to homosexuality and you find out that your brother had those genes, that doesn't imply that he is automatically a homosexual because of the genes. It would imply that he has a higher probability than a person without said genes of being gay.
I think this is the case IRL. Enviromental factors also play a very large role obviously too though.
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