Researchers in the USA have developed an epigenetic test which they claim can predict whether a man is gay or straight with 67 percent accuracy.
The findings, which were presented at the annual meeting of the American Society of Human Genetics in Baltimore, Maryland, looked at patterns of DNA methylation across the entire genome in a small group of identical and non-identical twins. 'To our knowledge, this is the first example of a predictive model for sexual orientation based on molecular markers,' said Dr Tuck Ngun of the University of California, Los Angeles, who conducted the study.
Previous studies have found that when one identical twin is gay, there is a 20 percent likelihood that the other twin will be too. Additionally, the chance of being gay increases by one-third for every older brother that a male has, suggesting that that both genetic and environmental factors are likely to play a role in determining sexual orientation.
The current study looked at possible epigenetic influences – chemical changes to DNA that affect how genes are expressed, but not the information they contain. Epigenetic markers can be inherited but can also be modified by environmental factors.
The researchers collected DNA from 37 identical twins where only one was gay, and from ten identical twins where both were gay. From the patterns of DNA methylation, they identified five regions that were more prevalent in the gay twin when compared with his genetically identical straight twin. From this, a predictive algorithm was generated that was able to predict the sexual orientation of the research subjects 67 percent of the time.
The work has not yet been peer reviewed, and has received mixed reaction within the scientific community.
Professor Gil McVean, a statistical geneticist at the University of Oxford, said: 'The key issue here is that the authors have searched through the entire genome to identify some difference between discordant twins. Given the number of tests, it is likely that some regions will show up as differentiated by chance. Without validation of the result in an independent data set it is not really possible to know whether there is any substance in this claim.'
Professor Eric Miska of the Gurdon Institute at the University of Cambridge added that it was hard to draw any conclusions from the findings: 'Epigenetics is still a young science and although there is great potential very little is known about the mechanisms that shape the epigenetic landscapes of an individual. Simple correlations – if significant – of epigenetic marks of an individual with anything from favourite football player to disease risk does not imply a causal relationship or understanding.'
Dr Ngun acknowledged the limitations of the study and said his team hopes to replicate the findings in a different group of twins. 'Sexual attraction is such a fundamental part of life, but it's not something we know a lot about at the genetic and molecular level. I hope that this research helps us understand ourselves better and why we are the way we are,' he said.