Scientists may have finally solved the puzzle of what makes a person gay, and how it is passed from parents to their children.
A group of scientists suggested Tuesday that homosexuals get that trait from their opposite-sex parents: A lesbian will almost always get the trait from her father, while a gay man will get the trait from his mother.
The hereditary link of homosexuality has long been established, but scientists knew it was not a strictly genetic link, because there are many pairs of identical twins who have differing sexualities. Scientists from the National Institute for Mathematical and Biological Synthesis say homosexuality seems to have an epigenetic, not a genetic link.
Long thought to have some sort of hereditary link, a group of scientists suggested Tuesday that homosexuality is linked to epi-marks — extra layers of information that control how certain genes are expressed. These epi-marks are usually, but not always, "erased" between generations. In homosexuals, these epi-marks aren't erased — they're passed from father-to-daughter or mother-to-son, explains William Rice, an evolutionary biologist at the University of California Santa Barbara and lead author of the study.
"There is compelling evidence that epi-marks contribute to both the similarity and dissimilarity of family members, and can therefore feasibly contribute to the observed familial inheritance of homosexuality and its low concordance between [identical] twins," Rice notes.
Rice and his team created a mathematical model that explains why homosexuality is passed through epi-marks, not genetics. Evolutionarily speaking, if homosexuality was solely a genetic trait, scientists would expect the trait to eventually disappear because homosexuals wouldn't be expected to reproduce. But because these epi-marks provide an evolutionary advantage for the parents of homosexuals: They protect fathers of homosexuals from underexposure to testosterone and mothers of homosexuals from overexposure to testosterone while they are in gestation.
"These epi-marks protect fathers and mothers from excess or underexposure to testosterone — when they carry over to opposite-sex offspring, it can cause the masculinization of females or the feminization of males," Rice says, which can lead to a child becoming gay. Rice notes that these markers are "highly variable" and that only strong epi-marks will result in a homosexual offspring.
Though scientists have long suspected some sort of genetic link, Rice says studies attempting to explain why people are gay have been few and far between.
"Most mainstream biologists have shied away from studying it because of the social stigma," he says. "It's been swept under the rug, people are still stuck on this idea that it's unnatural. Well there are many examples of homosexuality in nature, it's very common." Homosexual behavior has been observed in black swans, penguins, sheep, and other animals, he says.
Rice's model still needs to be tested on real-life parent-offspring pairs, but he says this epigenetic link makes more sense than any other explanation, and that his team has mapped out a way for other scientists to test their work.
"We've found a story that looks really good," he says. "There's more verification needed, but we point out how we can easily do epigenetic profiles genome-wide. We predict where the epi-marks occur, we just need other studies to look at it empirically. This can be tested and proven within six months. It's easy to test. If it's a bad idea, we can throw it away in short order."
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Jason Koebler is a science and technology reporter for U.S. News & World Report. You can follow him on Twitter or reach him at firstname.lastname@example.org.