Epigenetics — how gene expression is regulated by temporary switches called epi-marks — appears to be critical in determining sexuality, U.S. researchers say. 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. The researchers published their findings in The Quarterly Review of Biology.