Chromosome linkage studies
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. 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.
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.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, a rigorous replication of the maternal loading was reported on samples in Italy in England. One study by Rice
et al. in 1999
failed to replicate the Xq28 linkage results. 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. A recent study of 894 heterosexual and 694 homosexual men found
no evidence of sex linkage.