In the new study, scientists analyzed DNA from pairs of brothers, both of whom were gay, in nearly three dozen families with a history of homosexuality on the mother's side. Focusing on the female X chromosome that men inherit from their mother (they also get a male Y from their father), the researchers found that two-thirds of the gay siblings shared a distinctive pattern along a segment of their X chromosome. Scientists say the possibility is remote that this genetic pattern would appear by chance.
The findings, which were published in the journal Nature Genetics, suggest that at least one gene on the X chromosome--and possibly more--influences whether a man becomes homosexual. The same does not appear to hold true for women. Researchers examined the DNA of 36 pairs of lesbian sisters but found no common genetic pattern along the X chromosome.
The next step for the researchers is to locate the precise gene or genes involved and attempt to determine their biochemical effects. Will finding such "gay genes" rule out the idea that social and psychological influences can have a significant effect on a man's sexual preference? "Absolutely not," declares molecular biologist Dean Hamer of the National Cancer Institute, who headed both the 1993 investigation and the new one. "From twin studies, we already know that half or more of the variability in sexual orientation is not inherited. Our studies try to pinpoint the genetic factors, not to negate the psychosocial factors."