I had a discussion with two friends recently about the genetic nature of homosexuality. This topic is often terribly misunderstood, so today I'll try to sum up all the information as concisely as possible.
First, though, it's important to distinguish the difference between the terms "genetic," "developmental," and "in-born." When we say that something is genetically inherited, we mean that a quality is determined by one's genetic code. A good example would be sickle cell anemia. The hemoglobin (Hb)in your red blood cells is coded for by a handful of genes; a particular mutation in one of these genes results in misshaped Hb, which then deforms your red blood cells and makes you sick. Sickle cell anemia is entirely genetic in origin.
To say that something is developmental is to mean that a quality is determined by the the environment an organism is exposed to during development. Your height, for example, is partially determined by what kind of foods you eat while growing up, your level of exercise, and other factors.
However, we still refer to one's height as something that is "in-born;" that is to say, a personal quality that is outside one's control. Something determined by one's own nature. Keep this in mind for later in the post...
Now, homosexuality poses a bit of an evolutionary paradox. If evolution favours traits that improve a species' reproductive success, how could a trait like homosexuality become so common? Surely any gene or genes for homosexuality would decrease the likelihood of its' possessor's reproduction, and be lost from the gene pool.
In a June 2008 paper, Andrea Ciani et al., propose an evolutionary mechanism called "sexually antagonistic selection" that could account for the prevalence of homosexual behaviour. To sum up (to the best of my abilities), their paper suggests that sexuality is determined by two genes, one of which is located on the X chromosome. Having the "gay" version of these genes (we could say "gay alleles") makes males gay but makes females more likely to reproduce. The gay alleles could be said to simultaneously decrease male fitness but increase female fitness. This provides a solution to our paradox, assuming that the advantage for females outweighs the disadvantage to males. Indeed, this genetic model is also compatible with other data, such as the fact that homosexual males' maternal aunts and grandmothers are found to be more fecund than baseline. 
This model adds to the growing data that support a natural cause for homosexuality. Previous studies have shown that homosexuality is at least 35% genetic in origin, and various other theories are being researched to understand the developmental components (handedness, brain anatomy, and hormonal exposure in the womb, for example).
However, as my friend and I agreed, while all this information is interesting from a basic sciences perspective, it serves almost no purpuse in terms of informing public opinion or policy concerning homosexuality. ...Or, rather, it shouldn't.
Public interest in the genetic/developmental origins of homosexuality center around the debate about whether or not being gay is a choice. While all the evidence so far points to a substantial "in-born" component to sexual preference, it doesn't matter anyway. There is nothing unnatural, immoral, or harmful about homosexual behaviour, so there should be no public recourse against it. And even if it were a choice to be gay (can you imagine choosing to be gay!?), lots of other life choices (such as religion, most notably), receive widespread public support. People who advance the "it's a choice" argument are either lacking integrity or honesty.
1. My apologies for the long time since my last post.
2 It is, in fact, quite common. See here for some examples of homosexuality observed in the Animal Kingdom.
3. Camperio Ciani A, Cermelli P, Zanzotto G (2008) Sexually Antagonistic Selection in Human Male Homosexuality. PLoS ONE 3(6): e2282. doi:10.1371/
4. This paper uses a LOT of wacky mathematics that make my head spin. I leave it to my more "numbered" readers to judge the merits of the authors' work.
9 months ago