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How much of human behavior is genetic? This controversial question has recently been raised with regard to homosexuality, because of twin studies and reported structural differences in the brains of homosexuals (see Journal Watch accession number 910917008).
A team at the National Institutes of Health took family histories from 114 gay men and noted that the men tended to have gay relatives on the mother's side of the family, but not on the father's side. This tendency, confirmed in interviews with relatives, suggested a trait passed on by mothers exclusively, which in turn suggested a gene on the X chromosome. Using genetic mapping, the team found that a set of five genetic markers at the tip of the long arm of the X chromosome were identic…