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‘Epi-marks’ hold answer to homosexuality

Updated: January 15, 2013 11:36AM

Scientists think they may have figured out what makes some people gay.

They believe the answer lies not in the genes themselves, but rather in epi-marks, “temporary switches” that control how and when genes are expressed.

The source of homosexuality has long been pondered. From an evolutionary point, homosexuality cannot be solely genetic because the trait would eventually disappear, since homosexuals can’t reproduce with each other. And despite previous studies having shown that homosexuality runs in families, not being able to find a so-called “gay gene” has raised doubt about it being genetic.

In a study appearing in The Quarterly Review of Biology, William Rice and his colleagues argue that epi-marks solve the evolutionary riddle of homosexuality, finding that “sexually antagonistic” epi-marks, which normally protect parents from natural variation in sex hormone levels during fetal development, sometimes aren’t erased from generation to generation.

As a result, they cause homosexuality in opposite-sex offspring. Rice, an evolutionary biologist at the University of California Santa Barbara and lead author of the study, says epi-marks can contribute to both the similarity and dissimilarity of family members and why homosexuality is rare among identical twins.

The National Institute for Mathematical and Biological Synthesis study was done using a mathematical model with existing research.

Rice stressed that a great deal is still unknown, such as exactly which epi-marks lead to homosexuality.

“We have not figured out how homosexuality occurs, but we’ve figured out a feasible mechanism that can be easily tested,” he said.

The next step will be to compare sperm samples from men who have at least one lesbian daughter to those who don’t.

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