NatGeo explains how one's sexual orientation is determined during embryonic development. They look at how it is possible for one identical twin to be gay and the other to be straight. They address a new branch of Genetics called, Epigenetics. This area of Genetics explores and explains how one's DNA sequence is NOT the only factor in one's phenotypic (actual) outcome. This episode explains how both twins could have the gene for a disorder, yet only one twin actually has the disorder. The same science explains sexual orientation differences in twins. I have only included the homosexuality explanations.