Embargoed Advance Information from Science

Release 6pm EST Thursday, 15 July 1993

Summary of "A Linkage Between DNA Markers on the X Chromosome and Male Sexual Orientation," by Dean Hamer, Stella Hu, Victoria Magnuson, Nan Hu, and Angela Pattatucci at the National Cancer Institute, NIH, in Bethesda MD.

DNA markers link male homosexuality to X chromosome: Researchers at the National Institute of Health have used pedigree analysis and family DNA linkage studies to identify a region on the X chromosome that may be linked to sexual orientation in a selected group of homosexual males. This analysis constitutes the first step toward mapping and isolating a genetic locus which may influence sexual orientation within a certain subset of male homosexuals.

To examine the inheritance pattern of homosexuality, the authors queried 114 homosexual male volunteers about the sexual orientation of their fathers, sons, brothers, uncles, and male cousins. After assessing the reliability of this information, the authors assembled pedigree charts indicating that the men's brothers, maternal uncles, and maternal cousins had a significantly higher probability of being gay than would be expected, given the incidence of homosexuality in the general population (estimated as 2%. At the same time, fathers and paternally related relatives had rates lower than or equal to the general incidence rate.

One explanation for maternal transmission of a trait expressed in males would be linkage to the X chromosome, which every male inherits form his mother. To look for a region that might contain a gene sequence that increases an individual's probability of being a homosexual, the authors used 22 DNA markers to scan the X chromosomes of 40 pairs of homosexual brothers. They found that 64% of the sibling pairs tested had a cluster of five identical markers within a discrete region on the tip of the long arm of the X chromosome. This region, called Xq28, is approximately 4 million base pairs long, and is large enough to contain several hundred genes. Identification of an individual gene that might predispose certain males toward homosexuality will require further linkage analysis to narrow the target chromosomal region or complete sequencing of the region. "Once a specific gene has been identified," say the authors, "we can find out where and when it is expressed, and how it ultimately contributes to development and function in homosexuals and heterosexuals.