NEW THEORY: The Gay Trait Is Passed Down From Parent To Child

Gay Marriage

[credit provider=”AP Photo/Elaine Thompson”]

Some people believe gay people make a choice to be gay, while others believe they were born that way.No “gay gene” has been found to explain homosexuality, but a recent analysis published by The Quarterly Review of Biology suggests a new theory: The trait could be due to changes in the regulation of genes in the womb, and not the genes themselves, and could be passed down from parent to child.

Studies clearly show that homosexuality runs in families, with an increased rate among siblings and the maternal uncles of gay men, so naturally many people think there must be some “gay gene” or combination of genes that controls the behaviour. 

But, if there were a “gay gene” identical twins should be more likely to have the same sexual identities, but that doesn’t seem to be the case. Some theories have stated that being gay is all psychological, and others posit possible evolutionary advantages of the behaviour. 

The problem? Evolutionarily, a gay gene would be a dead end, since gay people are less likely to have children of their own. At the same time, about 8 per cent of people are gay, so the trait is somehow getting passed on. The big question is how. 

William Rice, from the University of California in Santa Barbara, and his colleagues think they’ve answered that question with their new theory.  

Starting in the womb

In biology classes we are normally taught that if an embryo has an XX chromosome it will develop into a girl, and if it has an XY chromosome it will develop into a boy. Genes on the Y chromosome trigger the formation of testes which release testosterone and kick start male development.

This is too simplified, though, since female embryos are also exposed to testosterone in the womb. The researchers think that the development of sex differences could be regulated through epigenetics, the name for modifications to the genome that cause changes to how and when genes are expressed.

Researchers think that these modifications, which researchers call “epi-marks” are temporary switches that may protect sex-related genes in females from being over exposed to testosterone, and their absence would make male embryos more sensitive to testosterone. The researchers think these hormones could control whether a foetus develops as “masculine” or “feminine” and potentially impact sexual preferences.

A question of heritability

These epigenetic modifications can change over time, and are erased at conception, so they aren’t usually passed down through the generations. But recent research indicates that some modifications are actually sometimes retained in offspring.

One study mentioned in the review showed that certain epigenetic modifications in male mice feminized their brain and behaviour, and these changes were passed on to their offspring.

So, the theory goes that a woman possesses epigenetic traits that made her react to hormones in a typically “female way” — and if they were to pass to her son they could potentially interfere with his normal male development by making his genes react to hormones the way a female’s genes would. The same goes for a man passing on his epigenetic traits to his daughter, which would expose her to testosterone in a “male way.”

So, the researcher’s theory goes that these epigenetic modifications would be useful to the parent, and only infrequently make their way to the second generation and cause “abnormal” fetal development — so they would still be useful evolutionarily.

Environmental factors could influence epigenetics, and frequently do, so they also play a role in when or how often these epi-marks get passed on.

Just a theory

“We’ve found a story that looks really good,” study researcher Rice told the US News And World Report. “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.”

Rice’s theory isn’t based on actual experiments, just a review of previous studies, others are quick to point out. We also have no idea what genes these epigenetic modifications would be modifying, how they would be added or removed, or what would make them more or less likely to be passed from parent to offspring.

“The study provides a very interesting, but as yet untested, genetic mechanism for the evolutionary maintenance of human homosexuality,” Nathan Bailey, a researcher at the University of St. Andrews in Scotland who was not involved in the research, told Popular Science. “We are going to have to wait until more evidence is in, but I do think it would be exciting to know whether epi-marks contribute to the expression of sexual orientation in humans.”

Others were bolder:

Leonid Kruglyak, a researcher from Princeton University tweeted “Since it keeps showing up: Rice et al. “study” is only a model; offers no evidence against genetics or for epigenetics.”