A major new study analysing the genetic data of half a million people has concluded that there is no single gene that causes someone to be gay.

Looking at the data of individuals in the UK and US, the study found that there were some genetic variants among gay people but that genetic factors could only account for between 8 and 25 per cent of same-sex sexual behaviour within the general population at the most. 

The study, published in Science Mag, sought to build on previous research which found that genetics appeared to account for similarities in the sexual orientation of twins in 18% of cases for women and 37% for men.

The researchers from Harvard and MIT looked at the data of 409,000 people stored in the UK Biobank and 68,500 from the US-based genetics company 23andMe.

The study participants were asked about how many sexual partners they had had and whether these were of the same or opposite sex. 

The researchers were able to establish five genetic variants linked to same-sex behaviour, two that were found in both men and women, two found only in men, and another found only in women.

However, when considered altogether, they concluded that these variations explained less than 1% of the variation in same-sex behaviour among the study participants. 

Ben Neale, an associate professor in the Analytic and Translational Genetics Unit at Massachusetts General Hospital, who contributed to the study, said that while genetics was important, it could not tell the whole story of same-sex sexual behaviour. 

"Genetics is less than half of this story for sexual behaviour, but it's still a very important contributing factor," he said, according to the BBC. 

"There is no single gay gene, and a genetic test for if you're going to have a same-sex relationship is not going to work.

"It's effectively impossible to predict an individual's sexual behaviour from their genome."

David Curtis, honorary professor at the UCL Genetics Institute, University College London, said the study was evidence of "very large numbers of variants which have extremely modest associations".

"This study clearly shows that there is no such thing as a 'gay gene'," he told the BBC.

"There is no genetic variant in the population which has any substantial effect on sexual orientation."