Is homosexuality genetic? It’s a long-running debate. This is an important question, because if this was proven (and accepted by lawyers and lawmakers), then discrimination against gays, such as state bans on gay marriage, would be ruled unconstitutional.

Now researchers say they’ve found a clue that may unlock the mystery. It lies in something called epi-genetics–how gene expression is regulated by temporary switches. They found that the transmission of sex-specific epi-marks may signal homosexuality.

Using mathematical modeling, they discovered that , sex-specific epi-marks, which are "erased" and thus normally do not pass between generations, can lead to homosexuality when they escape erasure and are transmitted from father to daughter or mother to son.

Researcher Sergey Gavrilets says, "Previous studies have shown that homosexuality runs in families, leading most researchers to presume a genetic underpinning of sexual preference. However, no major gene for homosexuality has been found despite numerous studies searching for a genetic connection."

Sex-specific epi-marks produced in early fetal development protect each sex from the substantial natural variation in testosterone that occurs during later fetal development. Different epi-marks protect different sex-specific traits from being masculinized or feminized. The researchers found homosexuality can occur in opposite-sex offspring when the sex-specific epi-marks are carried on to another generation.

"We discovered when these epi-marks are transmitted across generations from fathers to daughters or mothers to sons, they may cause reversed effects, such as the feminization of some traits in sons, such as sexual preference, and similarly a partial masculinization of daughters," says Gavrilets.

The mathematical modeling demonstrates that gene coding for these epi-marks can easily spread in the population because they always increase the fitness of the parent but only rarely escape erasure and reduce fitness in offspring.

Researchers have noticed that gay men are often the oldest in a large family with many sons. They have theorized that a child with a non- or semi-functioning "Y" chromosome may in some way "prepare" the mother’s womb for a boy child, which it might otherwise reject as an "invader," in the same way that our bodies tend to reject organ transplants.

Gavrilets says, "Transmission of sexually antagonistic epi-marks between generations is the most plausible evolutionary mechanism of the phenomenon of human homosexuality."

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