There is scattered evidence that genes are behind sexual orientation.
As specific genes determining homosexuality are still to be found, scientists only have been able to draw some hypotheses to explain the evolutionary causes of homosexuality. "During the 1990s there was a short surge of interest by a small number of labs in finding major genes that might mediate homosexuality," said scientist William Rice.
"I think that-when studying humans-many people shy away from studying sexual phenotypes in general and homosexuality in particular. If a firm genetic foundation for homosexuality in humans were established, then many people would view this fascinating human phenotype more objectively."
Findings point to the fact that male homosexuality appears to be inherited more often from the mother than from the father. This gene might hold on because it also increases the fecundity of the other sex.
Recent data shows that female maternal relatives of gay men have higher than average reproduction capacity. Previous research shows that a male's chance to be homosexual increases with the number of biological older brothers he has, even when he does not grow up with his older male siblings. Each male fetus produces an antigen that provokes in his mother's body an increased immunization, and this antigen likely plays a role in masculinizing the brain.
Research seems to indicate that homosexuality may be determined by a polymorphic gene that has more than one different form, so homosexual inheritance is very complex. Scientists have developed several statistical models to predict the possible factors responsible for the polymorphism of genes influencing homosexuality. "We know that homosexuality (gay or lesbian) can be caused by simple genetic changes in fruit flies, and since so many reproductive and neurological genes are shared by flies and humans, it seems highly likely that there are major genes influencing homosexuality in humans," said Rice.
"However, we also have firm evidence for a birth-order effect on male homosexuality, and discordance in the expression of homosexuality of identical twins, so clearly there is also an environmental influence on the trait."
Two main genetic phenomena may explain homosexuality's genetics: overdominance and sexual antagonism. Overdominance renders homosexuality from the combination of two different genes for heterosexuality. Sexual antagonistic traits are those that are advantageous in one sex, but may cause homosexuality in the other. These genes stand on the human gene pool because the benefits for one sex must outweigh the damages for the other.
Certain gene types (e.g. autosomal or sex-linked, recessive or dominant, with small or large effects) favor either overdominance or sexual antagonism under different conditions. "I think that it is too early to decide which of our models (or one yet to be formulated) is most feasible. However, based on the abundance of sexually antagonistic variation found in fruit flies, the sexually antagonistic variation seems like a probable candidate process leading to polymorphism for homosexuality." said Rice.