By Amina Khan

Los Angeles Times

A study that analyzed the DNA of nearly half a million people has found that while genetic differences play a significant role in sexual preference, there is no single gene responsible.

The findings, described Thursday in the journal Science, looked at sexual behavior and not sexual identity. Still, the results debunk the idea of a so-called singular “gay gene,” call into question such sexual orientation frameworks as the Kinsey scale — and hint at the complex factors that influence human sexuality, including society and the environment.

“The findings themselves reinforce this idea that diversity of sexual behavior across humanity is really a natural part of our overall diversity as a species,” said Benjamin Neale, a geneticist at the Broad Institute of the Massachusetts Institute of Technology and Harvard and one of the study’s senior authors. “That’s a really meaningful and important result.”

While estimates of same-sex experiences vary, a 2016 CDC study found that 6.2% of men and 17.4% of women between 18 to 44 in the U.S. reported at least one same-sex experience in their lifetimes. A smaller portion, 1.3% of women and 1.9% of men, identified as lesbian or gay, and 5.5% of women and 2.0% of men said they were bisexual — underscoring the difference between sexual behavior and sexual orientation identity.

Scientists have long probed the nature of same-sex behavior, finding evidence in twin studies that genetics plays a role. But such research has typically involved small numbers of people, and it hasn’t used modern methods of genomic analysis, scientists said.

“I had seen some quite poor studies of small samples and false claims and things, so I was glad that finally this topic was examined in a very scientific way with a large sample,” said Melinda Mills, a social and molecular geneticist at the University of Oxford who was not involved in the work.

Neale and an international team of researchers performed what’s known as a genome-wide association study. Using this method, scientists use statistical methods to search for connections between SNPs — single nucleotide polymorphisms, or individual differences in a single building block in the genetic code — and a particular trait.

Finding clear and verifiable patterns in that genetic data requires a huge sample, and the scientists knew where to find it.

They pulled 408,995 individual records from the UK Biobank, as well as 68,527 records from the U.S.-based personal genomics company 23andMe. This gave them an overall sample size of 477,522 people, 26,827 of whom reported same-sex sexual behavior.

The researchers found two significant spots in the genome that were linked to same-sex behavior across people of both sexes. And when they analyzed male and female genomes separately, they found three more — two specifically for men and one specifically for women — bringing the total number of significant genetic markers up to five.

Nonetheless, when taken altogether, these five locations on the genome could account for much less than 1% of same-sex sexual behavior on a population level, the researchers said.

Using a different analytical technique, the scientists found that when taking into account all of the subtle influences of many, many markers across the genome that they did not specifically identify, genetics could potentially account for up to 8% to 25% of the population’s same-sex behavior. That’s because, in all likelihood, a huge and currently unknown number of genetic markers likely play infinitesimally tiny roles in shaping behavior, Neale said.

Another analysis in the paper, which did not focus on DNA but on familial relationships between 106,979 pairs of individuals, suggested that a slightly larger share of the variation in same-sex behavior, 32.4%, could be attributed to genetics. That number may take into account other complex genetic effects beyond SNPs, though it might also be influenced by some assumptions baked into the framework, the scientists said.

Among the five significant SNPs they found, the ones specific to men were linked to smell receptor genes, sensitivity to certain scents, and regulation of the sex hormones such as testosterone.

That finding “makes a certain amount of sense,” Neale said, “but again, we don’t have much more to say beyond that sort of high-level description.”