The publication by the journal "Science" of this study on 500,000 DNA profiles aims to bury the theory born in the 1990s that there is a "gay gene" to predict the sexual orientation of a person.

Loving men or women, just as being small or big, or more or less intelligent, is not defined by a single gene, but by multiple regions of the genome and by elusive non-genetic factors. This is the conclusion of an analysis performed on 500,000 DNA profiles by a group of researchers in Europe and the United States. Its publication Thursday, by the prestigious journal Science, aims to bury the idea born in the 1990s that there is a "gay gene" as predictable as what exists for the color of the eyes.

Can not predict sexual orientation

"It is de facto impossible to predict a person's sexual orientation according to his genome," said Ben Neale, a member of the Harvard Broad Institute and MIT, one of the many institutions from which the authors come. Sexual orientation does have a genetic component, researchers admit, but this component depends on a myriad of genes. "There is no single gay gene, but many small genetic effects distributed in the genome," says Ben Neale. To these is added an essential factor: the environment in which a person grows and lives.

To better explain this idea, researchers compare the definition of sexual orientation to determining the size of a person: the genetic effect is indisputable, since your size is linked to that of your parents. But genetics does not explain everything: your nutrition during childhood will have a significant impact. This is what scientists call "the environment".

Minority role of genetics

The new statistical analysis revealed five precise positions on our chromosomes, called "locus", which appear to be clearly related to sexual orientation, although each has a "very small" influence. Biologically, it turns out that a marker is also associated with hair loss, which suggests a link with the regulation of sex hormones. Presumably, there are hundreds or thousands of other markers, which future analyzes on larger DNA banks might one day discover.

"It is a complex behavior where genetics plays a role, but probably in a minority way.The effect of the environment exists, but we can not measure it exactly," says Fah Sathirapongsasuti, scientist 23andme.com , a DNA testing site that contributed to the study with genetic profiles of voluntary clients.

The bulk of the analysis was made of men and women from the UK-based UK Biobank, mostly of European origin, who answered the question: Have you ever had sex with a person of the same sex?

The Kinsey scale is an "oversimplification"

The authors are aware of the delicacy of the subject, and the two above researchers were careful to recall, in a conference call with reporters on Tuesday, that they themselves were gay. Wishing to avoid any misinterpretation, they went so far as to consult with LGBT associations on how to communicate their results, which they summarized on a special site, geneticsexbehavior.info ( in English ).

Another result challenges the idea that sexual orientation would be a continuum, according to the so-called Kinsey scale, of the name of the American biologist who defined it in 1948: from 100% homosexual to 100% heterosexual, by the way by bisexual. "Suppose that the more you are attracted to someone of the same sex, the less you are attracted to the other sex is an oversimplification," say the researchers, after comparing the genetic markers affecting the number of partners of each sex.