The largest study of sexual orientation has concluded that sexual orientation is not influenced by one gene, but by many

"I am pleased to announce that there is no gay geneIt was October 2018 and Andrea Ganna was speaking at the annual meeting of the American Society for Human Genetics. Many people spoke at that convention, but neither that phrase nor that presentation was one more.

Homosexuality is, needless to say, one of the themes of our time. And what Ganna was exhibiting on that stage was intended to be a key piece in understanding the biological mystery behind human sexual behavior, identity, and orientation. But, of course, extraordinary claims require extraordinary evidence and today, finally, Science publishes that evidence.

In search of the heritability of sexual behavior

Although the genetics of non-heterosexual sexual behavior is relatively little studied, the studies that have been carried out have made it clear that this type of behavior, orientation and identity was inheritable. Or what is the same, that sexual behavior with same-sex partners would be partially influenced by genetic variants.

That something is inheritable is not uncommon. In this case, studies with twins have shown a heritability of 30 or 40%; a level of heritability very similar to that of other traits related to people's behavior or personality. To give us an idea, the extroversion or the number of years that a person will spend in school have a similar heritability. The important question is how much and how.

That is what Ganna's study was trying to clarify. For example, in 1993, a small genetic study suggested that a stretch of DNA on the X chromosome was linked to hereditary homosexuality. In other words, the analysis clearly indicated a specific and defined genetic variation as the cause of homosexuality. The problem is that the techniques were still very imprecise and the sample was scarce, more was needed.

To overcome those limitations, Ganna and her team needed an enormous amount of data. The central difficulty was that they not only needed DNA profiles, but also information on the behavior, orientation and sexual identity of these people. Finally they collected more than 470,000 DNA samples from various banks such as the UK Biobanck or 23andMe.

Using those data, they conducted a study a Genome-Wide Association Study (GWAS). These types of studies, with their limitations, are able to find genetic markers associated with behavior, examine the biological pathways associated with it, and estimate to what extent the identified genetic differences were related to other different traits.

What did they discover?

First, researchers have discovered five genetic markers associated with same-sex sexual behavior. Each of them has a very small effect individually. This, again, is very common in complex behavioral traits; that is, in traits in which, although there is a certain association with specific genetic variants, it cannot be said (at all) that this trait is determined by those variants.

This is important because much of the debate around this study has hovered around that concept of genetic "determination". A concept that is often obscure. When we talk about something being genetically determined, we are referring to traits (such as blood type) in which the genetic variant is necessary and sufficient to define them (completely).

In the complex traits we are talking about, it cannot be said that they are determined because the genetic variant is necessary, but not sufficient. It is the difference between being "genetically influenced" and being "genetically determined." Normally, these types of traits are made up of thousands of genetic variants that combined (and in a given social environment) are expressed in a specific way.

The news is that, compared to what studies such as the 1993 and the so-called "gay gene" theory pointed out, non-heterosexual behavior is a complex feature. This means that "we can confidently affirm that" there is neither a single genetic determinant nor a single gene for sexual behavior, orientation or identities ": there are many.

That does not mean that genetics is not important. In fact, the researchers found that all of those markers together accounted for 8-25% of individual differences in people's sexual behavior. This means that it is a more complex trait than is commonly thought in public debate.

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