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Homosexuality isn’t a lifestyle choice, but is rooted in a person’s biology, according to a controversial new study.
Research from NorthShore University in Illinois claims 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 say, however, that the research could help them get closer to finding so-called ‘gay genes’.
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.
The latest study is unlikely to settle this debate, but it is by no means the first to try and the claim that homosexual men share a ‘gay gene’ created a furore in the 1990s.
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.
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.
In the latest study researchers at NorthShore University (NSU) HealthSystem’s Research Institute, based in Evanston, made the findings after conducting a genome-wide association study (GWAS).
This examined the sexual orientation of 1,077 homosexual and 1,231 heterosexual men.
Sexual orientation of participants in the study was rated based on their self-reported sexual identity and sexual feelings.
Men were asked to provide DNA by blood or saliva samples that were then analysed for variations in their genetic code.
They found two regions with multiple genetic variants most strongly associated with sexuality.
These were located on chromosomes 13 and 14, near genes that have functions which may be relevant to the development of sexual orientation.
Alan Sanders, a psychiatrist who studies behavioural genetics at NSU who led the study, said: ‘Because sexuality is an essential part of human life, for individuals and society, it is important to understand the development and expression of human sexual orientation.
‘The goal of this study was to search for genetic underpinnings of male sexual orientation, and thus ultimately increase our knowledge of biological mechanisms underlying sexual orientation.
‘What we have accomplished is a first step for GWAS on the trait, and we hope that subsequent larger studies will further illuminate its genetic contributions.’
The strongest associated region on chromosome 13 was located between the genes, SLITRK5 and SLITRK6.
SLITRK6 is a neurodevelopmental gene mostly expressed in a region of the brain called the diencephalon, which contains a region previously reported as differing in size in men depending on their sexual orientation.
On chromosome 14, the thyroid stimulating hormone receptor (TSHR) gene was found to span the most significant region that may offer clues into sexual orientation.
Genetic variants in TSHR may help explain past findings linking thyroid function and sexual orientation.
The authors note that the modest sample size in their study for a trait with complex genetics is a limitation.
So too is their focus on men of European ancestry, as well as limiting the study to just men.
Researchers emphasise that although the top two association regions provide interesting and perhaps trait-relevant examples with their closest genes, the potential connections remain speculative.
About the findings Professor Robin Lovell-Badge, group leader at The Francis Crick Institute biomedical research centre in London, who was not involved in the study, said: ‘Genome-Wide Association Studies (GWAS) are problematic and are frequently underpowered, meaning that the sample sizes are too small to allow any robust conclusions.
‘This is particularly so for traits like being gay that are likely to involve many genes and where « environmental » influences, perhaps both in utero and postnatally, can have a strong effect.
‘Moreover, correlation does not mean causation.
‘The new data on genetic associations they report here does not show sufficient statistical significance to make any formal link between a gene or chromosome region and being gay or heterosexual.
‘However, their data does tend to reinforce some previous findings of associations, which were also relatively weak, with a region of Chromosome 8 and the X chromosome.