There is evidence of a genetic component
Originally Posted by Blackdog
J. Michael Bailey and Richard Pillard also studied the gayness between MZ twins, DZ twins, and non-related adopted brothers. They examined how many of the sample population examined were gay and how many were straight. They found that 52% of MZ twins were both self-identified homosexuals, 22% of DZ twins were so, and only 5% of non-related adopted brothers were so. This evidence, repeated and found to be true a second time, showed to the biological camp that the more closely genetically linked a pair is, the more likely they both are to exhibit gay or straight tendencies. Later experimenters found similar evidence in females. One such scientist is Dean Hamer. Hamer examined the possibility of homosexuality being an X-linked trait. He examined the family trees of openly gay men, and thought he saw a maternal link, leading him to investigate his theory of X-linkage. He took 40 DNA samples from homosexual men, and genetically examined them. He found that there was a 'remarkable concordance' for 5 genetic markers on section of the X-Chromosome called Xq28 .
Hamer hypothesized upon examining the family trees of the same men that on each subject's mother's side, there were markedly larger numbers of homosexual men, all stemming through the maternal lineages. This observation, along with his startling discovery on Xq28, led his findings to be dubbed the "gay gene study". The statistical probability of the 5 genetic markers on Xq28 to have matched randomly was calculated to be 1/100,000 , lending even more support to his findings.
Conclusive? No, but there is evidence