A new study in Psychological Science, Genome-wide scan demonstrates significant linkage for male sexual orientation, is getting breathless coverage in the press. Representative: “A genetic analysis of 409 pairs of gay brothers, including sets of twins, has provided the strongest evidence yet that gay people are born gay.” As a matter of fact I don’t think this is the strongest evidence that people are “born gay.” The study is decent, and better than what has come before, but the authors themselves in the text acknowledge issues of statistical power. These results could be right, but I doubt this is going to end up being a robust signal.* That being said, at some point in the next ten years I’m pretty sure we’ll localize the genes which carry variants which do result in a higher than typical likelihood of an individual exhibiting homosexual orientation. It’s a matter of time, not if. Behavioral genomics was way too optimistic in the interval 2000 to 2010. I suspect we’re starting to become too pessimistic in the interval 2015-2025.
But the bigger point is that we already know homosexuality has a heritable component. We don’t need to know what genes, we just know that related individuals exhibit a propensity for the trait in direct proportion to their relatedness. Heritability is just the proportion of the variation of the trait (e.g., homosexual vs. heterosexual) within the population that can be explained by the variation of the genes in the population. Heritability of homosexuality is modest, but it is there nevertheless, so there is some biological component.** We’ve known this for a long time. A modest linkage study doesn’t really shift the need much at all. It’s asking and exploring somewhat different questions. It assumes heritability, and is looking to uncover its genetic architecture.
The problem here is that the public and the press conflate the concrete biophysical instantiation of genes with the abstract concept of the gene. The latter pre-dates the former by about 50 years. For two generations geneticists developed their field without a precise understanding of the biophysical mechanism of inheritance. But that’s because all Mendelian, and evolutionary, genetics requires is that the units of inheritance follow regular laws across the generations. Quantitative genetics, arguably a branch of applied statistics, is even less tied to the concrete unit of genetic transmission in the form of the DNA molecule.
Concrete physical locations of genes as structures in the material world are important data. In a field like biomedicine it has changed the whole game. Genomics as an enterprise wouldn’t really be possible in a practical sense without our understanding of the physical basis of inheritance in DNA. But that doesn’t make DNA necessarily a game change in understanding whether a trait is heritable or not. Rather, it adds detail and specificity to how a trait is heritable. For applied science the “how” is essential. But for basic research it is not the be all and end all.
* Two reasons that I’m skeptical. First, large effects like this often don’t pan out for behavioral traits. Second, I doubt it’s so simple as a common large effect variant because homosexuality almost certainly decreases fitness directly. For a variant to get moderately common with this sort of effect it had to have another outcome which was strongly favored.
** Note that genetics does not include all biological factors. E.g., developmental stochasticity or some early environmental perturbation in utero with lasting consequences.