Are people born gay or is it a choice? A new study of gay brothers, the largest to date, adds more scientific evidence that there’s a genetic basis for homosexuality.
A genetic analysis of over 409 pairs of gay brothers found that two areas of the human genome, a portion of the X chromosome and a portion of chromosome 8, were associated with the men’s sexual orientation. The findings gel with a smaller study conducted in 1993 that implicated the same area of the X chromosome.
Before proceeding, it’s important to be clear that this study did not discover a “gay gene.” The regions they identified contain many different genes, so scientists still have a lot of searching to do before finding the specific genes that underlie sexual orientation. With that said, here’s how scientists established a broad genetic link.
Over several years, the study’s lead author Alan Sanders, of the NorthShore Research Institute in Illinois, collected blood and saliva samples from 409 pairs of gay brothers, including sets of non-identical twins. Then, researchers went through each man’s samples looking for unique genetic markers shared by all men in the study.
The 818 men varied in hair color, height, intelligence and other physical attributes. So each man had unique genetic markers matching their unique traits. The one thing they did have in common was that they were all gay. Therefore, if the same genetic variants are found in the same spots in each man, there’s reason to believe these places have something to do with sexual orientation.
The two most frequently shared genetic markers were from the Xq28 region on the X chromosome and the 8q12 region on the 8 chromosome. This commonality suggests there’s a genetic link for male homosexuality. They published their findings Monday in the journal Psychological Medicine.
Not Quite Conclusive
One of the primary weaknesses of the study, as pointed out by Science’s Kelly Servick, is that researchers used a type of analysis, genetic linkage, that’s been phased out by more precise techniques.
Genetic linkage studies only identify relationships between broad regions that could contain hundreds of different genes. Today, the linkage technique has been replaced by genome-wide association studies, which identify specific genes associated with traits being studied.
According to the Associated Press, other researchers have questioned the data as well:
Neil Risch, a genetics expert at the University of California, San Francisco, said the data are statistically too weak to demonstrate any genetic link. Risch was involved in a smaller study that found no link between male homosexuality and chromosome X.
Sanders told the New Scientist that he’s already moving forward with the next phase of the study: comparing the genetic markers in gay men to straight men. If the differences are clear, they could narrow the field to fewer genes and also shore up the strength of the associations they’re pointing to.
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