Homosexuality is inherited, not through genes, but through 'epi-markers', a study based around mathematical modelling suggests.
Epi-markers are modifications that alter gene expression, but do not change the DNA sequence. Normally, epi-markers are not passed down from one generation to the next, but sometimes this can happen and can influence a child's characteristics.
In their paper, published in the Quarterly Review of Biology, the authors suggest that when sex-specific epi-markers are passed on they can influence the offspring's sexual orientation.
Professor Sergey Gavrilets, from the University of Tennessee, Knoxville, co-author of the study claims that this 'transmission of sexually antagonistic epi-marks between generations is the most plausible evolutionary mechanism of the phenomenon of human homosexuality'.
From an evolutionary perspective, the prevalence of homosexuality in the general population (estimated at six percent in the UK by the Department of Trade and Industry, as it then was) is perplexing. Twin studies suggest that homosexuality is at least partly heritable but no genes specific for homosexuality have been discovered. Moreover, homosexual adults are less likely to have children and so are less likely to pass on their genes.
This study combined biological and mathematical models to predict that sex-specific epi-markers, rather that any specific 'homosexuality gene', are passed from parents to offspring and influence sexual preference.
These epi-markers are produced early in fetal development to protect the fetus from large natural variations in testosterone exposure in the womb. They can protect girls from becoming masculinised by excessive testosterone, or alternatively protect boys from becoming feminised by insufficient testosterone.
The new model suggests that if a father passes his sex-specific epi-markers to his daughter, rather than her producing her own, she can become more masculinised with this impacting sexual orientation. Similarly if a mother passes her sex-specific epi-markers to her son, he can become feminised, resulting in homosexual preference.
'These epi-marks protect fathers and mothers from excess or underexposure to testosterone', Professor William Rice of the University of California and lead author of this research, told US News. However, 'when they carry over to opposite-sex offspring, it can cause the masculinisation of females or the feminisation of males'.
Professor Rice admits that 'there's more verification needed' on his model but, he says: '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'.