A massive independent genetic survey sought to replicate the findings of a 20 year old controversial study which identified a stretch on the X chromosome as being linked with homosexuality. The latest findings, which took into account the genetic makeup of a staggering 409 pairs of gay siblings, confirm the initial reports and further boost the idea that homosexuality is also influenced by genes, and not environmental cues only. Not everyone is convinced though. Opponents have been quick to criticize the report, citing outdated analysis methods and a lack of more solid arguments. Namely, what everyone is waiting for is a gene or set of genes that define homosexuality, yet where we stand now we only have a region of a chromosome to work with, which can contain hundreds if not thousands of genes.
Is this proof that homosexuality is genetic?
When Dean Hamer published his groundbreaking research in 1993, while working as a molecular biologist at the U.S. National Institutes of Health (NIH) in Bethesda, Maryland, he was quickly met with skepticism and prejudice by the scientific community. His genomic study revealed that a specific stretch of the X chromosome called Xq28 holds a gene or set of genes that predispose a man to being gay. Because of his low study group comprised of only 38 pairs of gay brothers and the fact that attempts to replicate his results have turned out mixed results, many voices in the field have generally dismissed Hamer’s work.
Replicating Hamer’s research isn’t only challenging from a scientific perspective, however. Few scientists venture in this line of work, since they risk scrutiny. One, there’s little funding and, two, researching homosexual genes isn’t politically correct. Imagine if gay genes were positively identified; what would this mean for the gay community and the world at large, for that matter? We’d then have the basis to genetically screen a baby and even a fetus for gay genes. Some would be quick to say – and maybe rightfully so – that people would become discriminated even before they’d been born. It’s a really tense environment, but for Alan Sanders of the NorthShore Research Institute in Evanston, Illinois this made little difference.
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Sanders and colleagues sought to replicate Hamer’s findings using the same methods he used in 1993, but only this time with a much larger sample focus: 409 pairs of gay brothers versus the original 38 pairs. The researchers collected blood and saliva samples from each family member then looked at the locations of genetic markers called single nucleotide polymorphisms (SNPs) – differences of a single letter in the genetic code – and measuring the extent to which each of the SNPs were shared by the men in the study. This took them five years, plus an addition two years until their work was published in Psychological Medicine.
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The only trait common between the 818 men was being gay. Things like hair colour, weight, height or skin colour varied more or less between each individual. Therefore, any SNPs consistently found in the same genetic locations across the group would most likely be associated with sexual orientation. Five SNPs stood out, which could be found in the Xq28 and 8q12 regions on the X chromosome and chromosome 8 respectively.
“It erodes the notion that sexual orientation is a choice,” says study leader Alan Sanders of the NorthShore Research Institute in Evanston, Illinois.
What’s next
This doesn’t mean they’ve found gay genes , though. Instead, they’ve identified what seems like a block of genes that may be linked with homosexuality. This broad estimation is due to the researchers’ use of an outdated technique called genetic linkage. Nowadays, scientists in the field use genome-wide association (GWA) studies to identify the association of a specific gene with a certain trait in the population. Of course, Sanders was interested in replicating Hamer’s results (which he confirmed), and as such he was forced to use a technique popular two decades ago. Next, the researchers plan on making a GWA study, which includes genetic data from the just-published work plus DNA samples from more than 1000 additional gay men. Hopefully, this will help them identify individual genes that may or not be associated with homosexuality.
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“The most pleasing aspect is that the confirmation comes from a team that was in the past somewhat sceptical and critical of the earlier findings,” says Andrea Camperio Ciani of the University of Padua in Italy.
“This study knocks another nail into the coffin of the ‘chosen lifestyle’ theory of homosexuality,” says Simon LeVay, the neuroscientist and writer who, in 1991, claimed to have found that a specific brain region, within the hypothalamus, is smaller in gay men. “Yes, we have a choice in life, to be ourselves or to conform to someone else’s idea of normality, but being straight, bisexual or gay, or none of these, is a central part of who we are, thanks in part to the DNA we were born with.”
“Much hard work now lies ahead to identify the specific genes involved and how they work, as well as to find equivalent genes in women,” he adds.
Hamer, now retired from science and working a documentary film maker, is delighted to see his life’s work back in the spotlight.
“Twenty years is a long time to wait for validation, but now it’s clear the original results were right,” he says. “It’s very nice to see it confirmed.”
Sanders and colleagues stress, however, that even if they find a gay gene, it’s unlikely that these would make too much of a difference. Homosexuality is a highly complex trait, one that can’t be explained by genes or environmental cues alone. It’s a mix, and as such it’s very difficult to pinpoint which of the two is dominant.
