Sexual orientation in all its aspects
The concordance of sexual orientation in twinsIn parallel with the study of the influence of hormones on sexual orientation, much research has focussed on the influence of genetics on homosexuality. “We know that this influence exists because, if we look at family lines where a first individual is seen to be homosexual, the probability that other members of the family will be homosexual also is increased compared to the average observed for the population”, explains Jacques Balthazart. Naturally, we could all tell ourselves that it is simply a question of education or family environment, but this would be to ignore a certain number of studies conducted on dizygotic and monozygotic twins. “The concordance of sexual orientation of twins is higher in “real” twins than in “false” twins”, states the Professor. “There is a 65% rate of concordance of sexual orientation in monozygotic twins but only 18% in dizygotic twins”. But genetics does not explain everything because there is “only” 65% concordance in two people who have a virtually identical genome…Whether they involve hormones, genetics or epigenetics, the biological factors do not by themselves provide a full explanation for the biological origins of homosexuality, but together, they constitute a set of arguments showing that the biological influence is very important. The X chromosome in the spotlightWith regard to the genes involved in a predisposition to homosexuality, different research groups have been focussing on the question but no clear evidence has yet been established. “It seems that there is not only one gene responsible but that there are many and they are not very penetrating, that is to say that while they do not directly determine sexual orientation, they can influence it”, indicates the researcher. However, scientists agree to say that masculine sexual orientation is inherited from the mother. In fact, when there is a homosexual male in a family, there is a higher chance that other men will be homosexual in the maternal branch of the family. From this starting point, researchers have hypothesized that the genes responsible for sexual orientation are likely to be found on the X chromosome. “They realized that at the end of this chromosome, there is an area called XQ28 in which they found genetic markers associated with homosexuality”, says Jacques Balthazart. “Another study shows that this zone contains a gene known as MAGE A11, which controls sensitivity to testosterone in males. This gene could be modified in some individuals and affect the sensitivity of their brain to testosterone”. But this has yet to be demonstrated. |
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