General Question

cockswain's avatar

Could you link me the studies that show homosexuality is likely genetic?

Asked by cockswain (15181 points ) February 14th, 2011

My buddy and I were talking about homosexuality and evolution. He’s a PhD biochemist and had “heard” (but wasn’t adamant) that there wasn’t a genetic link between homosexuality, and was hypothesizing that possibly it was the result of different environmental factors during development, likely in the womb, but not necessarily due to gene expression. I have read some brief, not overly detailed news articles putting forth that there are indeed genes that are tied to homosexuality, and told him I’d see if I could find them to send him.

Can you provide me links to such articles? Or if you know otherwise, send me those links instead.

It was definitely an interesting conversation. He hypothesized that possibly the link wasn’t genetic, as over millions of years it would have bred itself out of existence, so therefore it must occur for other reasons. I suggested that perhaps, as there are multiple genes that code for skin color or height, there may be multiple genes that produce a “range” of sexual tendencies, so there may have been plenty of reproduction of individuals that were not 100% strictly homosexual.

We also weren’t certain if animals that display homosexuality are generally bisexual, which led us to further wonder if maybe pure homosexuality has become a purely human phenomena stemming from bisexuality first.

I certainly hope I’m not offending anyone with the guesses we made during our conversation. If I have said something rude, please point it out so I can address it. The last thing I want to do is piss someone off about their sexuality.

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12 Answers

nikipedia's avatar

(1) Let’s define what you/he mean by “genetic.” You mention “genes” and “gene expression”—these are two separate things. Genes themselves are the wording, so to speak, of the genetic code. Gene expression, however, involves the RNAs and proteins that are the results of those genetic codes, and can be modified by other genes, or by the environment and experience.

(2) Because gene expression is such a complex process, and behavior is so complex, it’s very difficult to get a 1:1 relationship between genes and behavior. So if you are looking for a single gene that, when present, makes someone gay, and when absent, makes someone not gay, you are probably not going to find it.

(3) That said, abundant evidence points to the existence of some genetic component to homosexuality:

(a) Twin studies: perhaps the most recent evidence is this paper published in 2010, finding that genes account for 34–39% of the variance in sexual orientation in adult twin men, and 18–19% in adult twin women.
(b) Genetic linkage studies: several studies have pointed to a gene at locus Xq28 that seems to be related to adult male homosexuality, although there is some dispute. The last study linked, however, suggests a potential epigenetic effect at locus 10q26.

(c) Some people also believe that differences in brain structures between gay and straight people are evidence of a genetic basis for homosexuality. The problem with this is that the brain is modified by both genes and experience (although as I pointed out above, so is the rest of gene expression). With that caveat, a famous study from 1991 found significant differences in the size of one nucleus of the hypothalamus when comparing gay and straight men—of particular interest because the hypothalamus is involved in sexual behavior.

How’s that?

DancingMind's avatar

Kate Bornstein: Gender Outlaw
I know it’s not exactly what you’re looking for, but you and your friend might find it interesting. While it’s talking mostly about gender, and that’s not the same thing as sexuality, they are a bit related.
(Since @nikipedia‘s got the links you were looking for, I thought I’d share this.)

crisw's avatar

Some of the most interesting research posits that genes linked to male homosexuality are also linked to female fecundity. Therefore, even though the males may be at a reproductive disadvantage, the advantages to the females outweigh it.

As for animal studies, almost all nonhuman animals that show homosexual behavior are not exclusively homosexual. Here’s a good review.

cockswain's avatar

Excellent! Thanks, these are really solid answers. This is going to take a decent amount of time for me to digest.

@nikipedia Ironically, my friend and I discussed that studying sexuality in identical twins would be an interesting comparison to determine the likelihood of a genetic component.

I don’t think there is going to be only one gene responsible. As I mention in the details, I think it would be more likely if multiple genes were responsible for a range of sexual tendencies.

And I do understand the difference between genes expressing themselves vs not. The company for which my friend and I work specialize in RNAi utilitizing siRNA. This is a method by which the mRNA is chewed up by an enzyme before translation occurs. I’m not mentioning that to show off, I’m mentioning it because I love what we do and am proud of being a part of it.

@crisw Thanks for those links. That is quite interesting indeed that animal sexuality isn’t exclusively homosexual. There are many thoughts I have about this, such as the fact that if there is a range of genes involved resulting in many shades of bisexuality, the animal would feel less inhibited to act on its impulses without societal influence and pressures.

If homosexuality is controlled by a range of genes, all of us are likely bisexual to some degree, and “true” genetic hetero or homosexuality would be less common than we might assume.

Just some random thoughts. We also discussed the fact that societal repression over the last few thousand years has probably acted to force those who would have preferred to be happily homosexual into having a wife and reproducing. Due to the fact that there weren’t techniques available until fairly recently that would allow homosexual couples to reproduce, possibly this has had a significant effect on the current gene pool.

mattbrowne's avatar

Take a look at this

http://en.wikipedia.org/wiki/INAH_3

INAH 3 has been reported to be smaller on average in homosexual men than in heterosexual men, and in fact has approximately the same size as INAH 3 in heterosexual women.

I’m not sure about the role of INAH 3 genes versus the genes influencing prenatal hormones which could in turn influence the growth of INAH 3.

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augustlan's avatar

[mod says] Please remember: This question is in the General Section. Responses must be helpful and on-topic.

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