The long-running argument about whether homosexuality is inherited may take a new turn thanks to a recent study by a University of Tennessee researcher.
Evolutionary biology and mathematics professor Sergey Gavrilets at the University of Tennessee co-authored the latest research on the link between homosexuality and heredity. The study was published Tuesday in The Quarterly Review of Biology.
Gavrilets said there is lots of data to suggest homosexuality runs in families. Therefore, scientists have spent years searching for a so-called gay gene.
"In spite of enormous effort people have put toward finding gay genes, nobody has been able to identify them," said Gavrilets.
Gavrilets said years ago he ran mathematical models in an attempt to determine if homosexuality can be the result of an inherited gene. He said the theory proved to be unlikely because gay populations would reproduce less and shrink over time.
"Instead of a specific gene, my colleagues developed a theory that homosexuality could be inherited due to certain epigenetic triggers," said Gavrilets.
To make an analogy, think of the genome and genes as computer hardware. Epigenetics is akin to the chemical software that tells the hardware how to behave. Gavrilets' research says homosexuals and heterosexuals have the same genetic hardware, but parents can pass on epi-marks that are normally erased between generations and cause the fetus to receive male hormones differently.
"Our study proposes that homosexuality is influenced by fetus sensitivity to androgen and testosterone, the different types of male hormones that cause sexual development," said Gavrilets. "It is more like a power switch that is left on and passed down from one parent to offspring of the opposite sex."
Gavrilets said normally the epi-marks that tell a fetus how to receive androgen are erased during fertilization. The switch is flipped to be more or less sensitive once the gender of the fetus is determined by X and Y chromosomes.
"Our theory is that some families can have very strong canalization that can be beneficial in one sex but detrimental if passed to the opposite sex. These strong 'switches' can make a female very feminine or a man very masculine. But sometimes the switches can be passed down to the opposite sex instead of being erased. So if a mother passes her epi-marks to a son, then the male will be less sensitive to testosterone and more feminine. Or if a daughter inherits her father's epi-marks then it can cause the female to become masculinized and affect sexual behavior."
Gavrilets said the research models "easily show" this theory is mathematically possible when compared to homosexual populations that exist in nearly all cultures. He said this is also the first proposal he is aware of that explains homosexuality in both genders.
"These models that we develop will work for both gays and lesbians. I don't think there's any other theory that works for both. The existing research focuses more on gay men," said Gavrilets.
The researcher gives another analogy to explain how genetic material can be identical but expressed differently.
"Just think of a book. All of the pages are genes. You cover a page of text with something else and you're not able to read it. You are not able to express those genes. So the genes are still there but they are not expressed."
Gavrilets said the next step in the research is for biological researchers to conduct empirical tests on epi-marks.
"I am a theoretical researcher. I am not actually manipulating DNA or genes. I develop mathematical models that decide if this framework is possible. Our theory and hypothesis can still be wrong or right, that's how science works."
If the theory is proven, Gavrilets said the epi-marks are unlikely to be the only thing determining sexual orientation.
"This is a very controversial topic and there are likely to be other factors such as culture," said Gavrilets. "But the transmission of sexually antagonistic epi-marks is the most plausible way to explain how one generation can affect another generation's homosexuality in humans."