Genetic clue to male homosexuality emerges

Science News, July 17, 1993 by Bruce Bower

Researchers say they have taken a major stride toward identifying a gene that may importantly influence the development of some cases of male homosexuality. The new evidence, published in the July 16 SCIENCE, suggests that a gene lying within a small stretch of the X chromosome, inherited by men from their mothers, contributes to the sexual orientation of a subset of homosexual men.

"We haven't identified the gene yet, and any theory of how it works is speculative," asserts Dean H. Hamer, a geneticist at the National Cancer Institute in Bethesda, Md. who directed the study,

However, a gene wedged into a tiny segment of DNA -- containing perhaps as few as several hundred genes -- probably performs functions linked directly to sexual orientation, Hamer proposes.

Other investigators have proposed that an individual's sexual orientation depends on the interplay of culture, family, hormonal influences, and inherited personality traits.

"If Hamer's data are replicated, this will be the only linkage of a gene to a high-level function performed by the [healthy] human brain," says psychiatrist Elliot S. Getshort, chief of the clinical neurogenetics branch at the National Institute of Mental Health, also in Bethesda, Md.

The failure of different scientific teams to confirm reports of genes linked to schizophrenia (SN: 11/12/88, p.308) and to manic depression (SN: 3/28/87, p. 199) illustrates the importance of independent replication, Gershon notes.

Hamer's group recruited 114 men who met a strict definition of homosexuality. Participants described themselves as gay, felt sexually attracted to other men, fantasized mainly about men, and engaged in sex always or mainly with men.

Hamer and his co-workers estimate that 2 percent of all men and women meet this strict definition of homosexuality, although generally accepted figures for the prevalence of male homosexuality range from 4 to 10 percent.

Participants rated the sexual orientation of their fathers, sons, brothers, uncles, and male cousins. Interviews of 99 of those relatives confirmed nearly all of the ratings.

Only brothers, maternal uncles, and maternal male cousins displayed a markedly higher rate of homosexuality than the general population. Maternal transmission of homosexuality appeared even stronger in a study of 38 families, each of which contained two homosexual brothers and no more than one lesbian.

Hamer's team viewed these findings as an indication that one form of male homosexuality derives partly from a gene on the X chromosome. They employed 22 "marker" enzymes to make cuts at precise points along the X chromosomes of 40 pairs of homosexual brothers (including all those from the family study) and available members of their immediate families.

Thirty-three pairs of brothers displayed the same cluster of five markers bunched into a small region on the X chromosome, suggesting that these families possessed a maternally transmitted gene that predisposed them to homosexuality, the scientists assert.

Genes may play a role in at least some cases of homosexuality, but the seven pairs of brothers who did not both inherit the crucial bit of X chromosome also provide an opening to studying how the environment influences sexual orientation, Gershon points out.

Hamer's team now plans to use DNA markers with more pairs of homosexual siblings in hopes of isolating the key gene so its chemical functions can be deciphered. Gershon says the gene may affect other, nonsexual behaviors in both men and women.

Confirmation of the finding and isolation of the gene may clarify the evolutionary significance of genes that influence homosexuality, adds Richard C. Pillard, a psychiatrist at Boston University School of Medicine who has studied homosexual twins (SN: 8/22/92, p. 117).