A study of nearly half a million genomes has identified five DNA markers associated with sexual behavior, but none of them alone determine a person's sexuality. The results show how complex human sexuality is. Another challenge for researchers is how to explain the nuances of such a delicate topic to the general public.
The largest study of the genetic basis of sexuality to date has identified five markers in the human genome associated with same-sex sexual behavior, but none of them can be considered a reliable indicator of sexuality.
The results of the study were published August 29 in the journal Science and are based on genetic data from nearly 500,000 people. They are in line with the findings of earlier studies of smaller coverage and confirm the suspicions of many scientists: although sexual preference is partly genetically determined, no single gene has a determining influence on orientation.
"There is no 'homosexual gene'," says lead scientist Andrea Ganna, a geneticist at the Broad Institute in Cambridge, Massachusetts, at MIT and Harvard University.
Ganna and his colleagues concluded that up to 25% of sexual behavior is due to genetics, and the rest is the result of environmental and cultural influences. Similar estimates were previously given in smaller-scale works.
"This is serious research," said Melinda Mills, a sociologist at the University of Oxford, UK, who studies the genetic basis of reproductive behavior.
At the same time, she warns that the conclusions do not reflect all of humanity - this is recognized by the authors themselves. The lion's share of genomes came from UK research program Biobank and 23andMe, a consumer genetics company headquartered in Mountain View, California. Their databases contain genetic information and medical records of predominantly age individuals of European descent. UK Biobank members were between 40 and 70 years old at the time of the study, and the average customer age in 23andMe's database is 51 years old.
The study authors also note that, in accordance with the terms of the genetic analysis agreement, they did not include people whose biological sex is at odds with their sexual identity. As a result, sexual and gender minorities (LGBT community), such as transsexuals and intersex people, were left out of the study.
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More data needed
Scientists have long believed that sexual orientation is at least in part due to sexual orientation. Studies in the 1990s showed that the sexual orientation of identical twins coincides more often than that of fraternal twins or, even less, half-siblings. Others have concluded that a particular segment of the X chromosome - the so-called Xq28 region - is somehow related to the sexual orientation of biological males. Subsequently, however, these conclusions were doubted.
All of these studies, Mills notes, had a very limited sample, and moreover, they were dominated by men. Thus, scientists may well have missed a number of genetic variations, one way or another associated with sexual orientation.
In a recent study, Gann and colleagues used genome-wide analysis (GWAS) to scan the DNA of hundreds of thousands of people for "single letter" changes or single nucleotide polymorphisms (SNPs). The principle is this: if people with common characteristics have the same SNP, then there is a likelihood of some relationship.
The researchers divided the subjects into two groups - some admittedly had experience of same-sex sex, others did not - and performed two calculations. In one of them, they tested over a million SNPs to find out whether subjects with a similar set of SNPs displayed similar sexual behavior or not. So scientists have found that from 8% to 25% of variations in sexual behavior is explained by genetics.
In a second study, Gann and colleagues tried to identify specific polymorphisms associated with same-sex sexual behavior - and found five. However, even taken together, they explain less than 1% of sexual behavior.
This suggests that there are a number of genes that influence sexual behavior, many of which are yet to be discovered, says Ganna. A larger sample will help identify missing options.
At the same time, Gann warns that it is impossible to rely on polymorphisms when predicting sexual preferences, because no gene alone determines orientation.
It's Complicated
While researchers have been able to identify some of the polymorphisms involved in same-sex sexual behavior, how the different genetic variants work, they can only guess. As Ganna explained, one of them is close to a gene associated with smell and plays a role in sexual attraction. Another is associated with male pattern baldness, which is caused by the level of sex hormones. This suggests a link to same-sex sexual behavior.
The results show how complex human sexuality is, says Ganna. Another challenge for researchers is how to explain the nuances of such a delicate topic to the general public.
In order to best communicate the research results to the public and to protect themselves from misinterpretations, the researchers worked with advocates of the interests of the LGBT community and experts in the field of scientific communication. For this purpose, they launched a website where the results, with all their reservations, are presented in a delicate language in an accessible form, not overloaded with scientific jargon.
Ewan Birney, geneticist and director of the European Bioinformatics Institute in Cambridge, UK, welcomes the work done. “They, one might say, passed a minefield,” he said.
While some researchers and LGBT advocates may question the wisdom of this kind of research, Birney considers it to be extremely important. There has been a lot of sociological research around same-sex sexual relations, but the topic is incredibly complex, he said. It's time to start the discussion from a biology perspective, says Birney.
Jonathan Lambert