Absence of "gay gene", influence of experience … A new study looks at what determines sexual orientation

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Like being small or big, or more or less intelligent, loving men or women is not defined by a single gene, but by multiple regions of the genome and, like any complex human trait, by elusive non-genetic factors.

Genetics alone can explain why you have brown or blue eyes. It does not, however, explain why you are attracted to men or women. A study of unprecedented scale, conducted by a group of American and European researchers, has just buried the theory that there is a "gay gene".

published Thursday, August 29 in the prestigious journal Science (Pdf) and relayed by the Washington Post (link in English, for subscribers)This study states that sexual orientation is a complex human trait in which multiple regions of the genome and elusive non-genetic factors are involved.

Franceinfo decrypts the main points of this work.

No "gay gene", but genetics play a role among others in sexual orientation

In 1993, American geneticist Dean Hamer and his team published in the journal Science a study that would skew, for more than twenty years, the perception by the general public of the role of genetics in the choice of sexual partners. At the time, genetics is in its infancy and, by studying a sample of 40 families, he thinks he has identified a gene (Xq28) involved in male homosexuality: a "gay gene". However, the study published Thursday, performed on half a million DNA profiles, refutes this simplistic model. If she is not the first to do so, she is the most complete analysis ever done on the subject.

"There is no single gay gene, but many small genetic effects distributed throughout the genome"says Benjamin Neale, a member of the Harvard Broad Institute and MIT, one of the many institutions from which the authors come. Thus, this new statistical analysis has revealed five precise positions on our chromosomes, called locus, which appear clearly related to sexual orientation. However, each of these positions exerts an influence "very small", says the scientist. Also, researchers believe that "8 to 25%" Sexual orientation differences in the population tested are due to genetic variation. But beware, this figure being a statistical concept about a population, it does not mean that 25% of a person's orientation depends on his or her genes. Similarly, a person with these genetic variations will not necessarily be homosexual.

For example, biologically, a marker is found to be associated with hair loss, suggesting 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's a complex behavior where genetics plays a role, but probably in a minority way," explains Fah Sathirapongsasuti, a scientist at 23andme.com, a DNA testing site that contributed to the study with genetic profiles of (voluntary) clients.

An elusive environmental component

For Fah Sathirapongsasuti, besides the influence of a myriad of genes, "the effect of the environment exists (in the development of a person's sexual orientation), but we can not measure it exactly. "

To understand how the environment in which a person evolves can have consequences on the biology, the researchers traditionally take the example of the size: the genetic effect is indisputable, since your size is linked to that of your parents, but of other external elements, such as nutrition during childhood, will have a significant impact. Ditto for cardiac risk: genes create predispositions, but your lifestyle, like your diet, have a bigger role.

Sexual orientation can not be "predicted" (let alone modified)

To quote Benjamin Neale: "It is de facto impossible to predict a person's sexual orientation according to his genome." Since genetic and environmental factors interact with one another, predicting a person's sexual orientation according to one or the other of these components seems impossible: "We have established that there are very diverse situationshe continues. And that makes our understanding (of homosexuality) more profound and nuanced today. "

Welcome nuance when it comes to studying a subject as complex as sexual orientation. The authors of the study also take care that their work is not recovered by defenders of homophobic theses: "Many people are suspicious of research aimed at observing sexuality from a genetic point of view because they fear that it will be used for discrimination purposes or that people are trying, with genomic editing technologies (such as CRISPR or the genetic sorting of human embryos) to influence sexual orientation ", thus prevent Steven M. Phelps and Robbee Wedow, co-signers of a platform in the New York Times (link in English) and members of the scientific team behind this study.

Similarly, the debate over the innate (genetic) or acquired (environmental) character of homosexuality has served over the years to fuel the arguments of a conservative fringe of society, which associates homosexuality with "choice", even at a "psychological deviance"Slate details in an article dated 2017 on the thorny issue of scientific work on homosexuality. "Pushed to the extreme, this assumption induces the setting up of the very worrying 'conversion therapies' aimed at 'healing' homosexuals", writes the article. If in spring 2018, the European Parliament adopted a non-binding text calling on states to ban "conversion therapies", France is working on a text to ban "forced modification of sexuality or gender identity", reported Le Figaro in July.

The Kinsey scale is irrelevant

Another result of this new survey calls into question 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, through 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 genetic markers affecting the number of partners of each sex.

Although more complete, the study has its limits

If the researchers put forward their methodology (detailed here on a dedicated site, in English) and the importance of their sample, they also point the limits of their study. Thus, the bulk of the analysis was made on men and women of the UK UK Biobank bank, mostly of European origin, which limits the scope of this work. In addition, researchers excluded transgender or intersex individuals from their study, confining themselves to studying the DNA of cisgender people.

Finally, the people tested answered the question: "Have you ever had sex with a person of the same sex?" But this question also has a limit. "We realize that using the terms 'never' or 'already' is an extreme simplification and that in this formula we combine several identities and sexual expressions," explain the researchers. And to admit that "our results remain imperfect and can in no way claim to speak for all sexual orientations and practices in their complexity."



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