Genetic Basis for Sexual Orientation?

Here we go people, I am about to dive into the great debate:

Is homosexuality a choice? Is it Nature or Nurture? In other words, is it genetic or does it come from how we are raised?

I think I will answer the first question in tomorrow's post, so I will start by looking at the Nature versus Nurture debate. I recently read this interesting article in The Economist. I highly suggest all of you read it, but I will detail the most interesting parts here:

The first sentence of the article provides our thesis and a bit of evidence, "The evidence suggests that homosexual behaviour is partly genetic. Studies of identical twins, for example, show that if one of a pair (regardless of sex) is homosexual, the other has a 50% chance of being so, too."

The article goes on to explain what biologists deem "heterozygote advantage." For those of you that don't recall much from your high school biology class, I give a brief explanation of genetics. We all have genes that determine our traits (duh!). For each gene, we have 2 alleles, one from mom and one from dad. These two alleles work together to produce our traits. The theory is that some alleles are dominate to other alleles, so the dominate allele is what appears in our phenotype.

So, the recessive (weaker) allele will henceforth be called "r" and the dominate allele will be called R.

RR is a fully dominate person, Rr and rR are going to have the same traits as RR, but they have different genes, and rr will have a completely different trait.

However, it doesn't always work that way. Sometimes, Rr (or rR) have different characteristics than RR does. A well documented example of this is the gene that causes sickle-cell anemia. If you are RR, you have normal blood, if you have rr, you are infected, but if you have Rr or rR, then you have a heightened resistance to malaria. This explains why Africa has one of the highest rates of sickle-cell anemia in the world, because the environment selects for the Rr people, and when two Rr people mate, there is a 25% chance of having a rr child.

Woah, confusing, huh? Might want to take a moment and reread through that.

So, back to the article. The article basically proposes the theory along the lines of sickle-cell anemia. Replace sickle-cell with homosexual and you have the explanation for how such a reproductively catastrophic trait can remain in the population. The article says that people who are rR for the homosexual gene may be more attractive to the other sex or may be better parents. It details a study done with 2452 sets of twins (fraternal and identical) and had them fill out a questionnaire about sexual orientation and number of opposite sex partners that they have been with.

The results were as follows: "the more feminine a man, the more masculine a woman, the higher the hit rate with the opposite sex." The final paragraph detailed the overall findings:

"According to the final crunching of the numbers, genes explain 27% of an individual’s gender identity and 59% of the variation in the number of sexual partners that people have. The team also measured the genetic component of sexual orientation and came up with a figure of 47%—more or less the same, therefore, as that from previous studies. The idea that it is having fecund relatives that sustains homosexuality thus looks quite plausible."

So, it seems that homosexuality may have a partial genetic basis. Of course, this is just a hypothesis, and much more evidence is needed before we can come up with a more definitive answer.

Oh, I want to share with you my favorite line of the article: "More directly, the study showed that heterosexuals with a homosexual twin tend to have more sexual partners than heterosexuals with a heterosexual twin." I liked this line the most because I have a twin brother. We are fraternal (non-identical) so I guess he has me to thank for all the women he will eventually get. You're welcome bro!

Source: http://www.economist.com/sciencetechnology/displayStory.cfm?story_id=12465295