1) Genetics play a role in determining the sexual orientation of a person.
2) Here is some evidence:
Chromosome linkage studies of sexual orientation have indicated the presence of multiple contributing genetic factors throughout the genome. In 1993, Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families.[12] Hamer et al. found that the gay men had more gay male uncles and cousins on the maternal side of the family than on the paternal side. Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the X chromosome to test for similar alleles. In another finding, thirty-three of the forty sibling pairs tested were found to have similar alleles in the distal region of Xq28, which was significantly higher than the expected rates of 50% for fraternal brothers. This was popularly dubbed as the 'gay gene' in the media, causing significant controversy. Sanders et al. in 1998 reported on their similar study, in which they found that 13% of uncles of gay brothers on the maternal side were homosexual, compared to 6% on the paternal side.[13]
A later analysis by Hu et al. replicated and refined the earlier findings. This study revealed that 67% of gay brothers in a new saturated sample shared a marker on the X chromosome at Xq28.[14]
Although two other studies (Bailey et al., 1999; McKnight and Malcolm, 2000) failed to find a preponderance of gay relatives in the maternal line of homosexual men,[13] a rigorous replication of the maternal loading was reported on samples in Italy in England.[citation needed]
One study by Rice et al. in 1999 failed to replicate the Xq28 linkage results.[15] Meta-analysis of all available linkage data indicates a significant link to Xq28, but
also indicates that additional genes must be present to account for the full heritability of sexual orientation.[citation needed]
A recent study of 894 heterosexual and 694 homosexual men found no evidence of sex linkage.[16]
Mustanski et al. (2005) performed a full-genome scan (instead of just an X chromosome scan) on individuals and families previously reported on in Hamer et al. (1993) and Hu et al. (1995), as well as additional new subjects.[17] With the larger sample set and complete genome scan, the study found somewhat reduced linkage for Xq28 than reported by Hamer et al. However, they did find other markers with a likelihood score falling just short of significance at 7q36 and likelihood scores approaching significance at 8p12 and 10q26. Interestingly, 10q26 showed highly significant maternal loading, thus further supporting the previous family studies.
Hamer DH, Hu S, Magnuson VL, Hu N, Pattatucci AM (July 1993). "A linkage between DNA markers on the X chromosome and male sexual orientation". Science (journal) 261 (5119): 321–7. doi:10.1126/science.8332896. PMID 8332896.
http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=8332896..
# ^ a b Wilson, G.D., & Rahman, Q. (2005). Born Gay: The Biology of Sex Orientation. London: Peter Owen Publishers.
# ^ Vilain E (2000). "Genetics of sexual development". Annu Rev Sex Res 11: 1–25. PMID 11351829.
# ^ Schwartz, G.; Kim, R.; Kolundzija, A.; Rieger, G.; Sanders, A. (2010). "Biodemographic and physical correlates of sexual orientation in men.". Archives of sexual behavior 39 (1): 93–109. doi:10.1007/s10508-009-9499-1. PMID 19387815.
^ Mustanski BS, Dupree MG, Nievergelt CM, Bocklandt S, Schork NJ, Hamer DH (March 2005). "A genomewide scan of male sexual orientation" (PDF). Hum. Genet. 116 (4): 272–8. doi:10.1007/s00439-004-1241-4. PMID 15645181.
http://mypage.iu.edu/~bmustans/Mustanski_etal_2005.pdf.
# Gay and non-gay people's brains respond differently to two putative sex pheromones (AND, found in male armpit secretions, and EST, found in female urine).[24][57][58]
^ a b Savic I, Berglund H, Lindström P (May 2005). "Brain response to putative pheromones in homosexual men". Proc. Natl. Acad. Sci. U.S.A. 102 (20): 7356–61. doi:10.1073/pnas.0407998102. PMID 15883379.
^ Savic I, Berglund H, Gulyas B, Roland P (2001). "Smelling of odorous sex hormone-like compounds causes sex-differentiated hypothalamic activations in humans". Neuron 31 (4): 661–8. doi:10.1016/S0896-6273(01)00390-7. PMID 11545724.
^ Berglund H, Lindström P, Savic I (2006). "Brain response to putative pheromones in lesbian women". Proc. Natl. Acad. Sci. U.S.A. 103 (21): 8269–74. doi:10.1073/pnas.0600331103. PMID 16705035.
