In the largest study ever to analyse the possible link between genetics and homosexuality, researchers from the NorthShore Research Institute in the US have investigated the genetic make-up of 409 gay twins over five years to discover that male sexual preference likely has a genetic basis.
The team, led by geneticist Alan Sanders, identified two regions of the human genome that have clear ties to homosexuality - one located on the X chromosome and another on the middle 'twist' of chromosome 8. According to Andy Coghlan at New Scientist, the region on the X chromosome, called Xq28, was first identified as having a possible link to homosexuality in 1993, and has previously been linked to sexual behaviour in animals. The link between homosexuality and the other region, called 8q12, was first suggested in 2005.
Now, in a study that's three times larger than any that preceded it, these genetic regions have been singled out once more. "It erodes the notion that sexual orientation is a choice," Sanders told Coghlan.
Sanders and his team have spent the past five years collecting blood and saliva samples from non-identical male twins belonging to 384 families. The advantage of using non-identical twins is that they share approximately 50 percent of their genetic material, which means they offer greater statistical significance across a relatively small sample size than regular people. Identical twins are even better, as they're genetically the same, but there are less of them available for analysis.
The team used genetic markers to identify single nucleotide polymorphisms (SNPs), which are the most common type of genetic variation in the human genome, and measured how much each of them were shared by the twin brothers.
"The only trait unequivocally shared by all 818 men was being gay. Because the twins were non-identical, so don't have the same genes, all other traits, such as hair colour, height and intelligence, varied by different degrees between each twin in a pair and between all sets of twins," says Coghlan. "Therefore, any SNPs consistently found in the same genetic locations across the group would most likely be associated with sexual orientation."
The team identified five SNPs, and the ones that were most commonly shared across the twins were Xq28 on the X chromosome and 8q12 on chromosome 8.
While the researchers are careful to add that this doesn't mean we've found individual 'gay genes', they suggest that these regions could be part of a combination of factors - both genetic and environmental - that determine a person's sexuality. Perhaps the presence of these regions in a person's genome just predispose them to homosexuality.
"The most pleasing aspect is that the confirmation comes from a team that was in the past somewhat sceptical and critical of the earlier findings," Andrea Camperio Ciani from the University of Padua in Italy told New Scientist.
Genticist Robert Green from Harvard Medical School in the US told the The Associated Press that the new study was, "intriguing but not in any way conclusive".
The results were published in the journal Psychological Medicine.