Gay Genes? part deux
Bobby is more likely to be gay than Greg.
Last blog I talked about some studies that link homosexuality and genes. The most powerful studies are those that compare identical twins to fraternal twins. These studies show that both twins in an identical pair are more likely to be gay than are both twins in a fraternal pair. Since identical twins have the same DNA, this suggests that something genetic is going on.
But no study showed that if one identical twin was gay, then the other one was always gay as well. We'd expect both twins in an identical twin pair to share a purely genetic trait 100% of the time. Because they don't, the environment definitely plays a role. But not like you might think.
By environment I don't mean certain family situations (although these sorts of factors probably contribute as well). What I am referring to are environmental factors that can affect brain development. Factors like viruses, hormones, or maybe even antibodies.
We know, for example, that the more older brothers a man has, the more likely he is to be gay. Even if he doesn't live with the older brothers.
This suggests that something biological is going on. It is as if the mother's body remembers how many sons she has had. One way this might happen is through her immune system.
Perhaps when a mother has a son, she makes antibodies to something having to do with carrying a male child. The more sons she has, the more antibodies she makes. At some point, she makes enough antibodies to affect brain development and the younger son is now gay.
Of course, not every youngest son is gay– he is just more likely to be homosexual. Other factors have been reported to increase the chances that someone is gay too. These include being left handed, having a counter-clockwise hair whorl and maybe even different finger lengths. All of these traits are associated with differences in brain development.
There are apparently many paths to a homosexual brain. This isn't surprising as human sexuality is much too complex to be due to a single gene or environmental factor. Most likely, it is the result of many factors all working together.
Some gay men may have inherited genes that made environmental factors more likely to affect their sexuality. And some gay men may have been exposed to multiple environmental effects that affected their sexuality despite their genes.
I think you can appreciate how these kinds of complex interactions can make finding "gay" genes incredibly complicated. And why it is hard to pinpoint the environmental effects that contribute to becoming homosexual as well.
Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.
latitude 37.3316, longitude -121.89
3 Comments
Are there similar studies looking at twin daughters?
Yes there are and they give similar results. The studies are a little harder to interpret (at least the ones I have read are) because female sexuality is a bit different than male sexuality.
Human sexuality is really a continuum ranging from completely heterosexual to completely homosexual. Men's sexuality tends to be more bimodal–they tend to be either straight or gay. (Of course this is just a tendency–there are plenty of exceptions.) Women tend to span the continuum more so it gets harder to define gay and straight…the study results depend on where you draw the line.
Has human sexuality ever been compared to the diversity that describes handedness. Left-handed, right-handed, ambidextrous. As Kinsey describes 7 levels of sexuality (flaming homosexual to flaming heterosexual with several gradations in between.) Could the similarity be as simple as left handed equals flaming homosexual, right handed equals flaming heterosexual and ambidextrous equals bisexual? That might explain why a person can eat or hit a ball with either hand just as a person might choose homo or hetero, depending on the social or environmental situation. I realize this is not scientific thinking, it's just my gut observation and feeling.