Homosexuality and Genetics

I believe that exactly this hypothesis has been advanced by some of the folks studying this issue. I don't keep up with the literature in this field, but I know that I've seen references to this on one of these discussion forums in the past.

Pink Sasquatch: Yes, I see what your getting at, and I do not disagree that environmental factors most likely do play a role in development. You gave the example that humans exposed to dioxins may be born with a cleft palate. Ok..fine, I agree. But I doubt that this is the case for homosexuality. Even if something the mother is exposed to somehow stops (or starts or interferes with) however many genes are involved in determining sexual prefernece, then the mother of every homosexual out there would had to have been exposed to the same stuff. Homosexuality is not exactly rare and occurs on a global scale. Ok, how do I address this? The evidence strongly suggests that genetics does seem to be important in determining if an individual will be a heterosexual or a homosexual. Your speculative answer says that maternal hormones may have an effect on fetal brain development such that the child will develop into a homosexual. But again, I still say it's the genes of the individual first. Plus, this also would mean that the mothers of all homosexuals would had to have experienced similar hormonal "problems". How likely is that?Here's my speculative answer. Homosexaulity is determined in manner similar to skin pigmentation. Maybe it's determined at multiple loci and if the interaction of all the genes at these loci reaches some arbitrary threshold, then...boom...homosexuality. If it's sub-threshold, then perhaps we see bisexuality or a preference for one sex but not a repulsion of the other. And it can have lessening effects as we get further and further from this threshold level. I'm sure it's a bit more complicated but I'm running short on time here (almost time for dinner and my wife is a great cook!). And hey, just so ya all know...I'm headed out for the weekend so you won't get another response from me till at least Monday.

I fear you are oversimplifying things a bit - and a maternal effect is still genetic.Many (most, I would suggest) phenotypes are highly modified by maternal genotype, rather than solely influenced by individual genotype.By my dioxin example, I was not implying that all homosexuality is caused by exposure to a specific toxin - I'm not sure how you read it as such. But back to the dioxin example, dioxin causes birth defects that can be caused by lots of other toxins or biological anomalies - even though these causes are different, it is often the same molecular pathways that are being modulated. Again, you are oversimplifying. A uterine hormonal imbalance is not the same as an overt hormonal "problem" with effects on the woman. Its frequency could be quite common (and unknown) if it has no obvious effects on the woman.Imagine that a pregnancy-related circulating factor is regulated by interaction of the maternal and/or embryonic genomes. If that factor reaches a certain threshold, perhaps a particular "homosexual" neurological pattern is turned on or off. How would we know whether this was happening or not? Why? The maternal genotype effect is every bit as genetic as the individual phenotype.I understand your polygenic 'threshold' model - but threshold of what? a gayness index? a molecular or neural signaling pathway? the effect on a developmental endpoint?It is just as likely that a threshold effect on a developmental phenotype is modulated by maternal genotype. I'll pose an important question to you:Do you think homosexuality is reversible?If you are arguing that a threshold is maintained in homeostasis by polygenic interaction, than sexual preference should be switchable by pathological condition or drug treatment effecting the gene products. Skin pigmentation is a trait with these characteristics, by the way.I would favor a developmental model resulting in a fixed sexual preference endpoint, instead.

How could homosexuality be genetic? Wouldn't natural selection weed that mutation out?

Kin selection. It turns out that the gene that makes men gay also winds up making their sisters very, very fertile. So, your sister has a bunch of kids; you have none, so with nothing better to do, you're a good uncle to your kids. That's three adults providing resources to the kids (both parents and Uncle Pimps), so they tend to survive better than the two-adult household in the next cave.Because your sister shares half of your genes, her kids carry a bunch of your genes. It's in your genetic advantage to help raise her kids, because they have a bunch of your genes.

So, you're saying the gene that makes people gay also makes people very fertile? So it's negative selection effect is negated by a positive one?

I guess what I'm saying is that, on the genetic level, there is no "negative effect" at all. The gene spreads throughout the population because its successful at getting organisms to make copies of themselves.This message has been edited by crashfrog, 10-23-2004 04:11 PM

That would explain all the hours and days I used to babysit all my nephews and neices before I started college.

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He's not dead. He's electroencephalographically challenged.

Moreover, it would explain why you have all those nephews and neices in the first place.

Only 7... well... ok, there are a lot of them. Well, I have 4 brothers and sisters, though.

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He's not dead. He's electroencephalographically challenged.

Sorry for the delay in replying. Got some really interesting data for one of my projects and have been distracted by actual work

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By that I mean that those individuals that do not have the gene will not exhibit the trait, regardless of any environmental factors to which they are exposed (if you do not have the genes to be six feet tall, then you will not be six feet tall). This is not the same as saying that %100 of those that have the gene will express the trait. The environment will/could make the difference. But still, they first have to have the gene(s). It is in this sense that I made my comment the genes are the root cause to all behaviors. So with homosexuality, individuals much first posses the correct genetic sequenceagreed?

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There may be genetic backgrounds that prevent a certain outcome (like becoming six feet tall) as you say. But a root cause is a bit too extreme. Since development is so flexible and there are other mechanisms (some heritable such as genomic imprinting) that also profoundly affect the outcome, are those root causes as well? The only root cause is you need genes to exist. But that does not help with determining which of many possible outcomes will be reached given a specific genotype.

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As such, I still have a hard time believing that the environment will have an affect on this particular behavior. What possible environmental factor(s) could there be that would result in an individual becoming gay?

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There are many enivronmental factors. How much androgen exposure you recieved in the womb. The sex hormone levels as you grow up. Cultural practices where you grow up i.e. are gays tolerated or not. Imagery that is considered attractive in your culture. All sorts of obvious and not so obvious factors will shape your behaviors including sexual orientation.

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I don’t buy it. How can this be? It would mean that every homosexual person on the planet somehow was exposed to the same environmental condition(s) that triggered the gay gene sequence to be expressed.

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The problem I see with this is how are you defining gay? This also will affect studies of the genetics of sexuality. Someone who is bi-sexual, homosexual, slightly attracted to both sexes but only pairs with the opposite sex etc. may all have a different genetic associated pre-disposition (again, the problem with studying complex traits). Couple that with variable environment and you get quite a range. If you could really carefully examine peoples attraction to the same or opposite sex, you would probably get a normal distribution of the homosexuality trait in the population which would look very much like the distribution of height (for example) in a given population i.e. another genetic and environment influenced trait. But homosexuality is usually defined by its most extreme manifestation.

Only if:a) Homosexuality prevented reproductive relationships with women (which it didn't in almost all historical examples) - bisexuality is common historically, exclusive homosexuality isn't.
b) It was caused a single gene rather than a combination of genes, who's individual effects could be positive (in reproductive terms).An adaptive suggestion for bisexuality is that it helped form strong bonds between males that allowed them to operate more effectively as a unit and thus win females from males who weren't bi.

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I guess what I'm saying is that, on the genetic level, there is no "negative effect" at all.The gene spreads throughout the population because its successful at getting organisms to make copies of themselves.

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I am not sure I agree with this. I think there is a threshold where there is a positive effect that when exceeded could have a very negative effect. Think of it like mimicry. Mimicry only works if the percentage of mimics (say mimicing a poisonous animal) is low. If everybody mimics then predators will not be at a disadvantage by playing the odds and eating everything that crosses their path. If the number of strictly homosexual members of a population reaches a very high frequency, there will be less individuals contributing to the next generation and fitness for the population will decrease (as seen from a genes eye view). So as long as there are enough individuals to enable the benefit to their kin (and thus themselves) but not so many that the effective population decreases, selection will likely maintain the trait by positive selection....I could be wrong, but the "no negative selection" struck me as unlikely.

Pink Sasquatch:You wrote:As I said from the get go, I am over simplifying itand I’m doing it for two primary reasonsI am not a geneticists and it (genetics) is not exactly my cup of tea. I should also reiterate that I am getting in way over my head here. Again, I agree with this. But I am still of the notion that the genes of the fetus must be correct in order for there to be an effect. By that I guess I mean that some people have the potential to be gay while others do not. Are you saying that it is possible for the mothers’ genes to be the underlying cause of homosexual behavior in her offspring? Do the genes of the offspring matter at all? Umm, my first guess would be to say no, but who knows. Isn’t anything reversible if the causes are known and can be reversed? How’s that for a cop-out As I said at the beginning of this thread (and my previous thread)I was way oversimplifying the idea of polygenic inheritance. I do not mean to imply that it’s as simple as skin pigmentation (but hey, maybe it ishas anybody looked?). I suppose there could be linkage groups or pleiotropic effects as well. I don’t knowhelp me out here. Care to elaborate on this a bit? I’d be truly interested in what you (or anyone else that wants to help get me off the hook here) would have to say. Honestly though, this is kind of what I have wanted to say all along. I did ignore the biological environment at first, but now I see that it could very well play a role. Stilllet me say this before I send this one off. I will grant you that the biological environment probably does play a significant role in homosexuality. I have no idea how (you’ve brought up some interesting points), but it certainly seems reasonable to assume that it does. Having said that, let me now add this. Way back at the start of this thread I said that I think a person is indeed born gay and that there is a gay gene (so to speak). Let me now elaborate and add that it’s not a single gene but instead some sort of genetic sequence(s) at multiple sites that, if the biological environment plays out in a certain way, then that individual will be born gay. Does that make sense? There is no gay gene per say, but rather a genetic predisposition to be born gay if certain things happen early on during development. By the same token, if an individual does not possess the proper genetics, then they will not (despite the biological environment) be born gay. That’s kinda what I mean by a gay gene and/or being born gay.

SirPimpsalot:Let me turn the tables on you a bit. Do you think heterosexuality is genetic? Assuming you're a heterosexual, did you make a choice to like girls, or is that just the way you "turned out"?