December 2008 National Geographic – Explains the Biology of Homosexuality – Epigenetics
How can 1 identical twin be gay and the other twin be straight if they both have identical DNA? Scientists believe that the answer is Epigenetics.
Although no gay gene has been found, that does not mean there is not a biological basis for homosexuality. This video explains how epigenetic markers that are found on genes, affect the manner in which a gene behaves. Therefore although the genes themselves are identical in twins, the epigenetic markers on the genes can make them respond differently.