They found five sites– three in regions of “junk DNA”, the role of which is unclear, two in genes whose roles are relatively well established. They used an algorithm to search out gene regions in which methylation patterns differed significantly between the two groups. Next, Ngun and his colleagues looked at the genomes of homosexual and heterosexual volunteers. They ended up with a giant spreadsheet showing the levels of methylation across the genome of each twin, says Ngun. Ten of the twin pairs were both gay, while 37 pairs differed, with one brother identifying as gay and the other as straight. To investigate a potential role for epigenetics, Ngun and his colleagues looked for epigenetic modifications made to the genes of 47 sets of male twins. This might be down to epigenetic changes – the addition or subtraction of a methyl group to genes, which switches them on or off. A male pregnancy might leave some sort of marker behind that affects subsequent pregnancies. “It seems as though the mother’s body is remembering the sex of previous pregnancies,” says Tuck Ngun at the University of California Los Angeles.
The overall chance is still low, however, rising from around 2 per cent to just 6 per cent for a third son.
For every male pregnancy a woman has, a subsequent son has a 33 per cent higher chance of being homosexual, although no one knows why. Other observations also suggest a genetic basis for sexual orientation, such as the mysterious fraternal birth order effect. The same region has been implicated in other studies of sexual orientation since, although researchers haven’t been able to single out “gay genes”. Perhaps the biggest splash was made in 1993 by Dean Hamer’s team at the National Cancer Institute in Bethesda, Maryland, when they found that gay brothers tended to share a sequence of five genetic markers in a region of the X chromosome. Over the last two decades, several studies have suggested that sexual orientation is, in part, down to our genes. “The predictive test needs replication on larger samples in order to know how good it is, but in theory it’s quite interesting.” “The scientific benefit to understanding is obvious to anyone with an iota of curiosity,” says Michael Bailey at Northwestern University in Evanston, Illinois.