American researchers in behavioral neuroscience have identified two genes linked to the sexual orientation of men. But the conditional is required.
This would be the first time that genes linked to sexual orientation have been identified. The find, if it is confirmed, is therefore of importance. Published in Scientific Reports, it results from a study conducted by the team of Prof. Alan Sanders, from the behavioral neuroscience department of NorthShore University (near Chicago), on a sample of 2308 men of European origin, including 1231 declaring themselves homosexual.
As much to raise the hare from the start: the researchers did not discover “genes of homosexuality”. If such genes existed, as with eye color, they would have been discovered a long time ago. In all likelihood, sexual orientation results from an interaction between a set of genes and the environment, biological and cultural. But we know that it is partly transmissible, with a heritability of 30 to 40% depending on the studies, and that it results in men by a strong predisposition for heterosexuality or often exclusive homosexuality.
Two genes linked to the brain
The first gene identified, carried on chromosome 13 (SLITRK6), arguably plays a role in sexual differentiation within the hypothalamus, a deep structure of the brain which varies in size (in English) according to sexual orientation. The second, present on chromosome 14 (TSHR), codes for a receptor in the thyroid, the gland responsible for secreting the thyroid hormone, which is also suspected to be linked to sexual orientation. via mother (in English).
To identify these two genes, the researchers used the first genome-wide association study in the field. Appeared thanks to high-throughput sequencing, these studies make it possible to analyze the entire genome of subjects, without a priori on regions of interest. If DNA analysis were a war, the genome-wide approach would be a weapon of mass destruction. There remains one catch: to be valid, such studies must relate to very large samples.
Results to be confirmed
With only 2300 subjects when it would probably take ten times as many, the results of Sanders and colleagues are subject to caution, and will have to be replicated to be considered final. Genetics, which rely on statistically significant associations in an attempt to explain eminently complex traits, lend themselves to reporting results sometimes anticipated, all the more so in a field that arouses as many passions as that of sexual orientation.
Many researchers in neuroscience and genetics believe that male homosexuality is primarily due to biological factors – a view often considered controversial, but based on a large body of clues (in English). But determining the underlying mechanisms, hereditary and environmental, is very difficult. A popular theory, popularized in France by Prof. Jacques Balthazart, sees prenatal exposure to testosterone as one of the major determinants of the future sexual orientation of the fetus. If the role of the SLITRK6 and TSHR genes is confirmed, this theory may well be reinforced.
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