Homosexuality may be triggered by environmental factors during childhood after scientists found that genetic changes which happen after birth can determine whether a man is straight or gay. The finding is highly controversial because it suggests that some men are not born gay, but are turned homosexual by their surroundings. It also raises privacy concerns that medical records could reveal sexuality.
The new research by the University of California has not yet been published but is being presented at the annual meeting of the American Society of Human Genetics in Baltimore. Scientists studied 37 sets of identical male twins, who were born with the same genetic blueprint, to tease out which genes were associated with homosexuality. In each pair, one of the twins was gay. Only 20 percent of identical twins are both gay leading researchers to believe that there must be causes which are not inherited. They found that it was possible to tell whether a man was gay or straight by monitoring tiny changes in how his DNA functions after birth – a field known as epigenetics. Where DNA works as an overall instruction manual, epigenetics act as another layer of information highlighting which parts of the text are important and which can be ignored.
Epigenetic changes are known to be triggered by environmental factors such as chemical exposure, childhood abuse, diet, exercise and stress. Researchers identified nine areas in the genome where genes functioned differently when a twin was homosexual. And the scientists say that they can predict with 70 per cent accuracy whether a man is gay or straight simply by looking at those parts of the genome.
"To our knowledge, this is the first example of a predictive model for sexual orientation based on molecular markers," said lead author Dr Tuck Ngun. "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."
British scientists said the work was intriguing but should be treated with caution until the scientific paper was published.
Prof Tim Spector, Professor of Genetic Epidemiology, King's College London, said: “It has always been a mystery why identical twins who share all their genes can vary in homosexuality. Epigenetic differences are one obvious reason and this study provides evidence for this. However the small study needs replicating before any talk of prediction is realistic.”
Prof Darren Griffin, Professor of Genetics, University of Kent, added: “While there is strong evidence in general for a biological basis for homosexuality my personal impression has always been one of a multiple contributory factors, including life experiences. “My gut feeling it that, as the complete story unfolds, the association may not be quite as simple as suggested. To claim a 70 per cent predictive value of something as complex as homosexuality is bold indeed. I wait with baited breath for a full peer-reviewed article.”
The US researchers are now planning to try out their genetic test on a larger population of men. They have not yet carried out any work on women. Dr Eric Miska, Herchel Smith Chair of Molecular Genetics at the University of Cambridge, said: “Epigenetic marks are the consequence of complex interactions between the genetics, development and environment of an individual.”