Homosexuality Passes from Fathers to Daughter or Mother to Son, Says New Study.

A generally overlooked gene expression "epigenetics," is a critical factor in the development of sexual orientation of the foetus, not the genes, suggests new research published in the Quarterly Review of Biology.

The research with the help of a biological and mathematical model delineates the role of epigenetics in developing same-sex desire, contrary to previous perception that a gene or genes could be responsible for homosexuality that runs through the family.

Epigenetics refers to the factors outside a person's genetic DNA which is regulated by temporary switches, called epi-marks.

The study finds that "sexually antagonistic" epi-marks, sometimes carried over generations, are transmitted through the opposite gender, such as father to daughter or mother to son during foetal development.

Researchers from the Working Group on Intragenomic Conflict at the National Institute for Mathematical and Biological Synthesis (NIMBioS) integrated the evolutionary theory with the latest advancements in the study on gene expression and androgen-dependent sexual development to create the model for the study.

"Transmission of sexually antagonistic epi-marks between generations is the most plausible evolutionary mechanism of the phenomenon of human homosexuality," said study co-author and University of Tennessee-Knoxville researcher Sergey Gavrilets.

Epi-marks are the extra layer of information attached to genes' backbones which direct the genes on how to use the particular traits attached to them in the phase of development.

Sex-specific epi-marks protect the foetus from the natural variation of testosterone levels in later foetal development and help in preventing the foetus from acquiring the opposite gender traits, girl foetuses from being masculinised or boy fetuses from being feminised.

However, when it was carried over through generations, they may cause reverse effects such as feminisation of some traits in sons or masculinisation of some traits in daughters.

Epi-marks are not always passed from parent to child, as demonstrated in the case of some identical twin pairs who have different sexual orientations despite having the same genes, say the authors.

The researchers say their model still needs to be tested in real-life but maintain that the epigenetic link to homosexuality makes more sense than any other explanation.

"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. ... If it's a bad idea, we can throw it away in short order," lead author William Rice told US News &World.

Though homosexuality is common for both men and women in most cultures, from an evolutionary point of view, it is a trait that would not be expected to develop and persist in the face of Darwinian natural selection.