(Time) A team at the National Cancer Institute’s Laboratory of Biochemistry reported in the journal Science that families of 76 gay men included a much higher proportion of homosexual male relatives than found in the general population. Intriguingly, almost all the disproportion was on the mother’s side of the family. That prompted the researchers to look at the chromosomes that determine gender, known as X and Y. Men get an X from their mother and a Y from their father; women get two X’s, one from each parent. Inasmuch as the family trees suggested that male homosexuality may be inherited from mothers, the scientists zeroed in on the X chromosome.
Sure enough, a separate study of the DNA from 40 pairs of homosexual brothers found that 33 pairs shared five different patches of genetic material grouped around a particular area on the X chromosome. Why is that unusual? Because the genes on a son’s X chromosome are a highly variable combination of the genes on the mother’s two X’s, and thus the sequence of genes varies greatly from one brother to another. Statistically, so much overlap between brothers who also share a sexual orientation is unlikely to be just coincidence. The fact that 33 out of 40 pairs of gay brothers were found to share the same sequences of DNA in a particular part of the chromosome suggests that at least one gene related to homosexuality is located in that region. Homosexuality was the only trait that all 33 pairs shared; the brothers didn’t all share the same eye color or shoe size or any other obvious characteristic. Nor, according to the study’s principal author, Dean Hamer, were they all identifiably effeminate or, for that matter, all macho. They were diverse except for sexual orientation. Says Hamer: “This is by far the strongest evidence to date that there is a genetic component to sexual orientation. We’ve identified a portion of the genome associated with it.”