Epigenetics – how gene expression is regulated by temporary switches, called epi-marks – appears to be a critical and overlooked factor contributing to the long-standing puzzle of why homosexuality occurs. According to a study, published online December 11, 2012 in The Quarterly Review of Biology, sex-specific epi-marks, which normally do not pass between generations and are thus "erased," can lead to homosexuality when they escape erasure and are transmitted from father to daughter or mother to son. From an evolutionary standpoint, homosexuality is a trait that would not be expected to develop and persist in the face of Darwinian natural selection. Homosexuality is nevertheless common for men and women in most cultures. Previous studies have shown that homosexuality runs in families, leading most researchers to presume a genetic underpinning of sexual preference. However, no major gene for homosexuality has been found despite numerous studies searching for a genetic connection. In the current study, researchers from the Working Group on Intragenomic Conflict at the National Institute for Mathematical and Biological Synthesis (NIMBioS) integrated evolutionary theory with recent advances in the molecular regulation of gene expression and androgen-dependent sexual development to produce a biological and mathematical model that delineates the role of epigenetics in homosexuality. 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 to what extent a gene is expressed during development.
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