Two regions of the human genome have been linked to homosexuality in the largest study on the topic to date.
Researchers assessed DNA from 409 pairs of homosexual brothers and found that regions on the X chromosome and chromosome 8 might influence sexual orientation. However, the research has not yet identified any specific genes.
The study follows over 30 years of research started by Dr Dean Hamer, who was not involved in this study, who first identified the region on the X chromosome in a small study on 40 gay men in 1993. 'When you first find something out of the entire genome, you're always wondering if it was just by chance', Dr Hamer told Science magazine.
Dr Kenneth Kendler, editor at Psychological Medicine, which published the study, told Science: 'In my circles, it was seen as "Oh, another false-positive finding". Findings in this general area of human behavioural genetics were at that time really plagued by concerns about replicability.'
Over the years, several other studies had tried to replicate Dr Hamer's original study, but only one has identified the region on the X chromosome as a potential link to homosexuality. However, another region on chromosome 8 did come up in some studies.
'I thought that Dean did a fine but small study,' Professor Michael Bailey, a psychologist at Northwestern University and senior author on the current study, told Science. 'If I had to bet, I would have bet against our being able to replicate it.'
The current study, which is the largest yet, found evidence for both previously identified sites, though results were only statistically significant for the region on chromosome 8. However, no specific genes were identified, only regions that contain hundreds of different genes.
Research attempting to identify a 'gay gene' is controversial and in the current paper, the researchers highlight that the development of sexual orientation is likely a complex process involving both genetics and environment. They also stress that a prenatal 'test for homosexuality' would be unlikely to work due to the small effects they found.
Professor Bailey's team is working on extending the current study with the aim of attempting to identify specific genes. 'It looks promising for there being genes in both of these regions', Professor Bailey said. 'But until somebody finds a gene, we don't know.'