The concordance of sexual orientation in twins

In 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 spotlight

With 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.

The last biological factors influencing sexual orientation which are being studied by scientists are the epigenetic factors. “Epigenetics involves the modification of histones, protein-based constituents of chromosomes, or parts of the DNA which permanently change the expression of certain genes”, explains the researcher. “We know that the synthesis of sexual hormones and their actions are controlled by epigenetic factors. In addition, before these hormones appear in the embryo, dozens of genes are expressed in a differential manner in the male and female. This can only stem from a genetic control from sex chromosomes that induce the modification of the expression of other genes by epigenetic mechanisms”, affirms Jacques Balthazart. In humans, initially, genital structures are non-differentiated at the early embryonic stage. It is only by means of the action of testosterone that the external genetic organs are formed. “When we consider the levels of testosterone in embryos, there is more in the males than in the females but there are superimpositions of these hormone levels. The females who have the most testosterone are at the same level as the males who have the lowest amount of the hormone”, explains the Professor. If this was the only factor controlling the differentiation of genital structures, there would be many individuals born with intersex genital structures but this is not the case. “In reality, testosterone only acts after being transformed by an enzyme and this transformation is epigenetically regulated differently in males and females. There is therefore more testosterone in active form (5-alpha dihydrotestosterone) in males, independently of the levels of testosterone”. This explains why, despite the overlapping levels of testosterone, in almost all cases, a sex difference in genital structures is observed. Concerning the question of the influence of epigenetics on sexual orientation, American researchers recently developed a mathematical model making it possible to analyse the contribution of the epigenetic phenomenon to the sexual orientation of males and females. “This model could help to partly explain the deviations that can occur in certain individuals”.