Debate has long raged among the scientific community over whether sexual orientation is biologically determined, the result of environmental causes or a combination of factors.
In 1993, American geneticist Dean Hamer found families with several gay males on the mother’s side, suggesting a gene on the X chromosome.
He showed that pairs of brothers who were openly gay shared a small region at the tip of the X, and proposed that it contained a gene that predisposes a male to homosexuality.
Hamer’s conclusions were extremely controversial.
He was challenged at every turn by people unwilling to accept that homosexuality is at least partly genetic, rather than a ‘lifestyle choice’.
Debate has long raged among the scientific community over whether sexual orientation is biologically determined, the result of environmental causes or a combination of factors
Gay men were divided. It vindicated the oft-repeated claims that ‘I was born this way’ but also opened frightening new possibilities for detection and discrimination.
Similar studies gave contradictory results. A later search found associations with genes on three other chromosomes.
More recently, in February 2014, scientists announced they had found two stretches of DNA linked to homosexuality in men.
In the study, Chicago University researchers analysed the DNA of more than 400 pairs of gay brothers, recruited at Gay Pride festivals at marches over several years.
This flagged up two pieces of DNA that seem to be linked to homosexuality.
And in October 2015, experts claimed they could predict whether someone is gay or straight with up to 70 per cent accuracy by looking at their DNA.
The study involved 37 pairs of twins in which one brother was homosexual and the other heterosexual, and 10 pairs in which both were homosexual.
Using a computer program called Fuzzy Forest they found that nine small regions of the genetic code played the key role on deciding whether someone is heterosexual or homosexual.
Earlier this month research from NorthShore University in Illinois claimed to have found genetic markers that can reveal if a man is gay.
Scientists looked at the DNA of more than 2,000 people and located two regions of chromosomes that may be linked to sexuality.
The study has been described as ‘weak’ by independent scientists, and the study’s authors admit the link is ‘speculative’.
They said, however, that the research could help them get closer to finding so-called ‘gay genes’.