Homosexuality is passed down through the generations from mother to son and from father to daughter, a new study claims.
From an evolutionary standpoint, homosexuality is a trait that would not be expected to develop and persist in the face of Darwinian natural selection.
It is nevertheless common for men and women in most cultures and 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.
Now in a study published yesterday in the Quarterly Review of Biology, researchers from the U.S. National Institute of Mathematical and Biological Synthesis suggest homosexuality has an epigenetic link, not a genetic one.
Epigenetics explains how gene expression is regulated by temporary switches, called epi-marks. These constitute an extra layer of information attached to our genes’ backbones that determine their effect on our development.
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 carryover between generations and thus can contribute to similarity among relatives, resembling the effect of shared genes.
William Rice, an an evolutionary biologist at the University of California Santa Barbara and lead author of the study, says epi-marks can determine the development of homosexuality in the offspring of heterosexual parents.
‘There is compelling evidence that epi-marks contribute to both the similarity and dissimilarity of family members, and can therefore feasibly contribute to the observed familial inheritance of homosexuality and its low concordance between [identical] twins,’ he told U.S. News.
Dr Rice and his team integrated evolutionary theory with recent advances in the molecular regulation of gene expression and androgen-dependent sexual development to produce a biological and mathematical model that delineates the role of epigenetics in homosexuality.
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, the researchers say.
Sex-specific epi-marks stop girl foetuses from being masculinised when they experience atypically high 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 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.
The study solves the evolutionary riddle of homosexuality, finding that ‘sexually antagonistic’ epi-marks, which normally protect parents from natural variation in sex hormone levels during foetal development, sometimes carry over across generations and cause homosexuality in opposite-sex offspring.
The mathematical modeling demonstrates that genes 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.
‘Transmission of sexually antagonistic epi-marks between generations is the most plausible evolutionary mechanism of the phenomenon of human homosexuality,’ said study co-author Sergey Gavrilets, a professor at the University of Tennessee-Knoxville.