Epigenetics is the study of how genes can be switched “on” or “off” of genes without altering the original DNA sequence. Evidence is now compelling to indicate that epigenetic mechanisms are fundamental to the genesis of homosexuality, which helps explain both its partially heritable nature and the in-utero environmental effects that seem to predispose a male to same-sex orientation. (http://www.glreview.org/article/evolutionary-origins-of-homosexuality/)