Study co-author Sergey Gavrilets of the Working Group on Intragenomic Conflict at the National Institute for Mathematical and Biological Synthesis and a professor at the University of Tennessee-Knoxville and colleagues said the study solved the evolutionary riddle of homosexuality.
The finding that "sexually antagonistic" epi-marks, which give a reproductive advantage to one sex while disadvantaging the other, sometimes carry over across generations and cause homosexuality in opposite-sex offspring, Gavrilets said.
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, Gavrilets said.
Epi-marks are usually produced anew each generation, but recent evidence demonstrated they sometimes carry over between generations and thus can contribute to similarity among relatives, resembling the effect of shared genes.
However, these epi-marks aren't always erased, they're passed from father-to-daughter or mother-to-son, which might explain why homosexuality runs in families, but isn't genetic -- otherwise gays and lesbians would have died out as a result of natural selection, the study said.
"Transmission of sexually antagonistic epi-marks between generations is the most plausible evolutionary mechanism of the phenomenon of human homosexuality," Gavrilets said in a statement.
The findings were published in The Quarterly Review of Biology.