Xq28

Xq28 is a chromosome band and genetic marker situated at the tip of the X chromosome which has been studied since at least 1980. The band contains three distinct regions, totaling about 8 Mbp of genetic information. The marker came to the public eye in 1993 when studies by Dean Hamer and others indicated a link between the Xq28 marker and male sexual orientation.

The 1993 study by Hamer et al. examined 114 families of gay men in the United States and found increased rates of homosexuality among maternal uncles and cousins, but not among paternal relatives. This pattern of inheritance suggested that there might be linked genes on the X chromosome, since males always inherit their copy of the X chromosome from their mothers. Polymorphisms of genetic markers of the X chromosome were analyzed for 40 families to see if a specific marker was shared by a disproportionate amount of brothers who were both gay. The results showed that among gay brothers, the concordance rate for markers from the Xq28 region were significantly greater than expected for random Mendelian segregation, indicating that a link did exist in that small sample. It was concluded that at least one form of male homosexuality is preferentially transmitted through the maternal side and is genetically linked to the Xq28 region.

A follow-up study, Hu et al. (1995), conducted by the Hamer lab in collaboration with two groups of statistical experts in 1995, corroborated the original results for males with homosexual brothers sharing Xq28 at significantly elevated rates. This study also included heterosexual brothers, who showed significantly less than expected sharing of the Xq28 region, as expected for a genetic locus that in one form is associated with same-sex attraction and in another form is associated with opposite-sex attraction. In this study no link to Xq28 was found among homosexual females, indicating a different genetic pathway as for most sex-specific phenotypes.

Hamer's findings were highlighted in scientific journals including Science,Nature and the topic of a mini-symposium in Scientific American.

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