Scientists from Sweden and the U.S. have come up with a mathematical model that they say explains how homosexuality is passed on to children from their parents – and it doesn't involve a "gay gene."


The key, they say, is in epigenetics, which is when factors outside of a person's DNA sequence can affect how the instructions in their genetic code are implemented.


Researchers from Uppsala University, the University of Tennessee-Knoxville and the University of California, Santa Barbara, hypothesized that "epi-marks" that regulate the expression of certain genes could influence how a developing fetus' brain is exposed to the sex hormone testosterone, resulting in same-sex desire for both gay and bisexual people. A study outlining their model was published Tuesday in The Quarterly Review of Biology.


Previous work has indicated some degree of heritability is implicated in same-sex desire, but studies of identical twins and molecular experiments have not identified strong candidates for a gene or genes that could be responsible, the authors say.


Other studies have indicated a connection between testosterone exposure in the womb during key developmental phases and sexual orientation. Genital structures and certain areas of the brain develop at different stages of gestation, so a male embryo could be exposed to normally high levels of testosterone and have masculine genitals, but then for some reason experience a dip in testosterone as his brain matures and develop same-sex attraction.


According to the epigenetic model, a person can inherit sex-specific "epi-marks" from an opposite sex parent – daughters from fathers, sons from mothers – that would result in atypically high testosterone exposure in a female fetus and atypically low testosterone sensitivity in a developing baby boy at these key developmental points. These theoretical epi-marks would be very specific, affecting areas of the brain related to sexual orientation while leaving the genitals and other parts of the brain unaffected.


“Transmission of sexually antagonistic epi-marks between generations is the most plausible evolutionary mechanism of the phenomenon of human homosexuality,” study co-author and University of Tennessee-Knoxville researcher Sergey Gavrilets, said in a statement Tuesday.


Epi-marks are also not always passed from parent to child, which could explain how identical twin pairs could differ in sexual orientation despite having the same genes. However, their model is not totally incompatible with some genetic contribution to homosexuality, the authors say.


The paper provides a theoretical framework that can be tested in real life. If their model is correct, then large-scale epigenetic profiles should find telltale differences between gay and straight individuals relating to sex hormone signaling, the authors say.


“We've found a story that looks really good,” author William Rice told US News & World Report on Tuesday. “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. ... If it's a bad idea, we can throw it away in short order.”


The prevalence of homosexual behavior throughout nature -- dolphins do it, as do rams, fruit flies and penguins, just to name a few -- suggests there is a natural mechanism that leads to homosexuality (if not more than one mechanism). But scientific studies of homosexuality can sometimes venture into a treacherous thicket of assumptions.


After all, the modern social concept of homosexuality as an exclusive orientation hasn't always been the norm -- throughout human history, men and women that experienced same-sex attraction have also taken opposite-sex partners. Irish writer Oscar Wilde, who was put on trial for his affairs with men, was also married to a woman and sired children. Ancient Greeks had established social codes governing pederasty, but both parties usually married women as well.


So, one of the more common assumptions in evolutionary studies of homosexuality -- that same-sex attraction is incompatible with having children and, by extension, natural selection -- may not be set in stone.


Another obstacle to scientists that try to peek at the roots of homosexuality is that their studies are often caught up in controversy.


"Most mainstream biologists have shied away from studying it because of the social stigma," Rice told US News & World Report. "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."


SOURCE: Rice et al. “Homosexuality as a consequence of epigenetically canalized sexual development.” Quarterly Review of Biology published online 11 December 2012.