Scientists have come up with a new theory on what could contribute towards making people gay and why homosexuality has not disappeared due to evolution.

While it has been widely believed that homosexuality could, at least in part, be a genetic trait, the latest research suggests that it is indeed passed on by parents, but not through their genes.

According to American scientists, epigenetics – how gene expression is regulated by temporary switches, called epi-marks – appears to be a critical factor contributing to homosexuality.

Published on Tuesday in The Quarterly Review of Biology, the study claims that sex-specific epi-marks, which normally do not pass between generations and are usually “erased,” can lead to homosexuality when they escape erasure and are transmitted from father to daughter or mother to son.

From an evolutionary standpoint, homosexuality is a trait that would not be expected to develop and persist in the face of Darwinian natural selection, said the researchers. Homosexuality is nevertheless common for men and women in most cultures. 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.

In the current study, researchers from the Working Group on Intragenomic Conflict at the National Institute for Mathematical and Biological Synthesis (NIMBioS) in Tennessee have developed a model that outlines the role of epigenetics in homosexuality.

Epi-marks constitute an extra layer of information attached to our genes’ backbones that regulates their expression. While genes hold the instructions, epi-marks direct how those instructions are carried out – when, where and how much a gene is expressed during development.

Epi-marks are usually produced anew each generation, but recent evidence demonstrates that they sometimes carry over between generations, resembling the effect of shared genes.

Sex-specific epi-marks produced in early foetal development protect each sex from the substantial natural variation in testosterone that occurs during later foetal development. Sex-specific epi-marks stop girl foetuses from being masculinised when they experience higher than usual testosterone, and vice versa for boy foetuses.

Different epi-marks protect different sex-specific traits from being masculinised or feminised – some affect the genitals, others sexual identity, and yet others may affect sexual partner preference.

However, when these epi-marks are transmitted across generations from fathers to daughters or mothers to sons, they may cause reversed effects, such as the feminisation of some traits in sons, such as sexual preference, and similarly a partial masculinisation of daughters, said the scientists.

The study’s co-author Sergey Gavrilets, a professor at the University of Tennessee-Knoxville, believes that the study solves the “evolutionary riddle of homosexuality” and that the “transmission of sexually antagonistic epi-marks between generations is the most plausible evolutionary mechanism of the phenomenon of human homosexuality”.

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