According to a newly released hypothesis, homosexuality might not lie in DNA itself. Instead, as an embryo develops, sex-related genes are turned on and off in response to fluctuating levels of hormones in the womb, produced by both mother and child. This benefits the unborn child, however if these epigenetic changes persist once the child is born, and has children of its own, some of these offspring may be homosexual.

The scientists published their findings in the journal The Quarterly Review of Biology. Evolutionary geneticists propose that this is the reason why homosexuality didn’t fade away due to evolutionary pressure. Current research estimates that 8% of the population is gay, and it has been known that homosexuality can run in families. If one of a set of identical twins is gay, there’s a 20% chance that the other will be, too.

Homosexuality isn’t just tied to the human species. There are many species exhibiting homosexual traits, including fish and birds, and geneticists have not been able to find a gene that is responsible for sexual orientation.

Testosterone doesn’t explain everything. Female fetuses are exposed to small amounts of testosterone from their adrenal glands, the placenta and the mother’s endocrine system. At many key points of gestation, male and female fetuses are exposed to similar amounts of testosterone. Levels of the hormone can be higher than normal in females and lower than expected in males without any effects on genital or brain structures.

The authors propose that the differences in sensitivity to sex hormones result in epigenetic changes, which don’t affect the structure of a gene, but can be activated by chemically altering a gene’s promoter region. Epigenetic changes could enhance or blunt testosterone’s activity as needed.

Epigenetic changes involve alterations in the proteins that bind together long strands of DNA and can be handed down to offspring. The authors propose that homosexuality may be a carry-over from one’s parents’ own prenatal genes to resist excess testosterone, and this could alter the gene activation in areas of the child’s brain involved in sexual attraction and preference. This could explain why homosexuality persists throughout evolution, state the authors.

Going from changes in gene expression to why someone is attracted to a person of the same sex is probably a question for which science may never find the answer, states Marlene Zuk, an evolutionary biologist at the University of Minnesota, Twin Cities.