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Spectators watch the 50th annual Pride Parade in Chicago, Illinois, U.S., June 30, 2019. Photo by REUTERS/Kamil Krzaczynski

There is no ‘gay gene.’ There is no ‘straight gene.’ Sexuality is just complex, study confirms

There is no single gene responsible for a person being gay or a lesbian.

That’s the first thing you need to know about the largest genetic investigation of sexuality ever, which was published Thursday in Science. The study of nearly a half million people closes the door on the debate around the existence of a so-called “gay gene.”

In its stead, the report finds that human DNA cannot predict who is gay or heterosexual. Sexuality cannot be pinned down by biology, psychology or life experiences, this study and others show, because human sexual attraction is decided by all these factors.

“This is not a first study exploring the genetics of same-sex behavior, but the previous studies were small and underpowered,” Andrea Ganna, the study’s co-author and genetics research fellow at the Broad Institute and Mass General Hospital, said in a press briefing on Wednesday. “Just to give you a sense of the scale of [our] data, this is approximately 100 times bigger than any previous study on this topic.”

The study shows that genes play a small and limited role in determining sexuality. Genetic heritability — all of the information stored in our genes and passed between generations — can only explain 8 to 25 percent of why people have same-sex relations, based on the study’s results.

Moreover, the researchers found that sexuality is polygenic — meaning hundreds or even thousands of genes make tiny contributions to the trait. That pattern is similar to other heritable (but complex) characteristics like height or a proclivity toward trying new things. (Things like red/green colorblindness, freckles and dimples can be traced back to single genes). But polygenic traits can be strongly influenced by the environment, meaning there’s no clear winner in this “nature versus nurture” debate.

It is worth keeping in mind that this study only covers some types of sexuality — gay, lesbian and cis-straight — but doesn’t offer many insights into gender identity. In other words, the team only looked at the “LGB” within the acronym LGBTQIA+.

Of course, ethical concerns arise with any attempt to use biology to explain complex human behavior like sexuality. People like Michael Bailey, a psychologist at Northwestern University who conducted much of the early research into the heritability of sexuality, warned against taking this new genetics study — or any research on sexual behavior — out of context.

For instance, Bailey added, there is no evidence that things like conversion therapy work.

“Obviously, there are environmental causes of sexual orientation. We knew that before this study.” said Bailey, citing the well-defined role that life experiences play in sexual development. “But that doesn’t mean we know how to manipulate sexual orientation mentally.”

What the scientists did

The study set out to investigate a 20-year-old genetics debate in sexuality by combing through two huge collections of DNA profiles: the UK Biobank and 23andMe.

With a reported 9 million users in its database, 23andMe is arguably the most popular, direct-to-consumer DNA testing company on the planet. The UK Biobank was established in 2007 by the medical charity The Wellcome Trust as a resource for research. It contains the DNA sequences of 500,000 middle-aged people, who were 40 to 69 years old when they were recruited between 2006 and 2010.

This study pulled the information for 477,500 people across the UK Biobank and 23andMe who had taken a survey about various life behaviors, including whether they had engaged in a sexual experience with a person of the same sex at any point in their life. About 26,800 individuals — or 5 percent of the subjects — fit this description, which is similar to the percentage reported across society more generally. All of the subjects consented to this research, including those pulled from 23andMe’s archives.

With this genetic trove available, the researchers conducted what’s known as a genome-wide association study, or GWAS. As the PBS NewsHour has reported previously, a GWAS study scans the DNA of hundreds or thousands of individuals, looking for common patterns that correspond with our health or our behaviors.

Think of all of humanity as consisting of 7 billion copies of the same book. All humans contain the same words — or individual genes — that make up how we think and how our organs function.

But the words in our respective genetic books — or their code — look slightly different. Some of my letters might be red, while some of yours are colored blue. They vary, which explains why we don’t look exactly the same nor have the same health.

This may sound counterintuitive, but those variations can also share similarities. The books that make up my family look similar to each other — in this example, they contain other shades of red.

The same applies if two people have the same height or if we’re both bald or if we’re depressed. These genetic patterns look more similar among myriad types of groupings and that’s what GWAS hunts down.

The technique can be used to suss out why certain people (and their particular genetic variations) correlate with health conditions like autism, physical traits like curly hair or colorblindness, behaviors like handedness or emotions like loneliness.

What they found

This GWAS study found that, like with many human behaviors, sexuality doesn’t have a strong genetic backing.

When the team looked for DNA patterns that had strong correlations, they found that no one gene could account for any more than 1 percent of people’s sexuality. The strongest signals came from five random genes.

Two of those genes correlated with same-sex sexuality in males, one of which is known to influence the sense of smell. One gene cropped up for females and two others showed solid patterns in both males and females. But their individual scores never passed this 1-percent mark — meaning they are all minor contributors to same-sex sexual behavior.

When the team looked more broadly across all the genomes — across the thousands of genes that they screened for the nearly 500,000 subjects — the genes similarities they found could only account for 8 to 25 percent of same-sex sexual behavior.

“It’s effectively impossible to predict an individual’s sexual behavior from their genome,” said Ben Neale, a geneticist at Massachusetts General Hospital and the Broad Institute who led the study. “Genetics is less than half of this story for sexual behavior.”

Why this study matters — and what it can’t tell us

Humans have tried to understand human sexuality for centuries — and genetics researchers joined the fray in the early 1990s after a series of studies on twins suggested homosexuality ran in families. These kinds of studies have continued through the years, going as far as pinpointing a gene on the X chromosome — Xq28 — as the culprit.

“As a teenager trying to understand myself and understand my sexuality, I looked at the internet for “the gay gene” and obviously came across Xq28,” said Fah Sathirapongsasuti, a study co-author and senior scientist at 23andMe, which he joked once led him to believe he inherited his gayness from his mother.

His comments speak to the larger narrative about using biology to define complex behaviors — like sexuality — when science is always evolving and takes time to find anything close to definitive.

Those early studies stumbled upon a concrete pattern: Sexuality can run in families and thus must have a genetic component. But back then, the scientists had no way of comprehensively exploring this issue. Genome sequencing took decades to slowly mature into what it is today, and twins alone cannot represent the genetic complexity of our species.

“We worried a lot about volunteer bias,” said Bailey, whose research includes a widely publicized study on Xq28 and gay brothers from 2018.

Those projects — known as linkage studies — were designed to find single major genes that appeared to have a big effect on sexuality, said Dr. Alan Sanders, an associate director for psychiatric genetics at NorthShore University HealthSystem Research Institute. Sanders collaborated with Bailey on those earlier studies and said their work had always admitted that there was no single “gay gene.”

“The field has moved on more to genome wide association studies,” said Sanders, who is also a co-author of the research published Thursday in Science. “Genome wide association studies are better at mapping genes with small effects, which are at play here.”

And even this new study has a big limitation, one that has been inherent to major genomic studies for the last two decades: GWAS studies are too white.

“There’s many politically correct ways of saying this, but basically the study is mostly a Caucasian sample of European ancestry. So, it does not include peoples from Latin America, Asia and Africa,” said Dr. Eric Vilain, director of the Center for Genetic Medicine Research at Children’s National Health System. “The second limitation is that the way they lumped together what they call ‘nonheterosexuals’”

The researchers had members of the same-sex community review the study’s design and language, and they admit that their terminology and definitions for gay, lesbian and heterosexual do not reflect the full nature of the sexuality continuum.

They did attempt to examine some elements of this continuum by conducting GWAS analysis on three smaller DNA databases wherein the participants had been surveyed using the Kinsey Scale. The Kinsey Scale is a somewhat infamous test for determining the strength of a person’s feelings toward members of the same- and opposite-sexes. In other words, it tries to judge if a person leans gay, straight or bisexual.

The team found genetics cannot explain people’s scores on the Kinsey Scale.

“We discovered that the Kinsey Scale … is really an oversimplification of the diversity of sexual behavior in humans,” Neale said. Bailey disagrees, arguing that people’s feelings of sexual interest and arousal — and therefore, their readouts on the Kinsey Scale — may be too complicated to validate through genetics.

He did agree with Neale that the debate is now closed on whether any single gene is responsible for sexual orientation.

“[Our study] underscores an important role for the environment in shaping human sexual behavior and perhaps most importantly there is no single gay gene but rather the contribution of many small genetic effects scattered across the genome,” Neale said.

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