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Sexual Orientation

Biological And Genetic Linkage Studies

Biological studies looking at the hypothalamus have found differences between homosexual and heterosexual men and women. Some researchers found differences in parts of the hypothalamus, while others did not. What these findings mean is not clear because they were inconsistent.

In 1995 William Turner examined the ratio of males to females among relatives of the mothers of male homosexuals. He reported that the sex ratio was not normal in maternal relatives. The normal ratio, for relatives of heterosexual males, was an even split: 50 percent male relatives, 50 percent female relatives. The number of male relatives of homosexual males, on the other hand, was significantly lower than the number of female relatives. Also, 65 percent of the mothers of homosexuals had no live-born brothers, or else they had only one live-born brother. On the paternal side, however, the number of male and female relatives of male homosexuals was same as that found for heterosexuals, and the sex ratio of relatives on both the maternal and paternal side for female homosexuals was the same as for heterosexuals. These findings would support genetic factors on the X chromosome, which males inherit from their mothers, as a factor that may cause fetal or neonatal loss of males.

Molecular studies found a linkage between male homosexuality and the X chromosome. Dean Hamer and colleagues in 1993 and Nan Hu and colleagues in 1995 conducted DNA linkage analyses in U.S. families with two homosexual brothers. There was significant linkage at Xq28 for 64 percent of homosexual male siblings but not for homosexual females (Xq28 is band 28 of the long arm of the X chromosome). George Rice and colleagues in 1999 examined four alleles at Xq28 in fifty-two Canadian male homosexual siblings but did not find any such linkage. This could represent genetic variation, diagnostic differences, and/or different methods of data analysis.

In summary, family, biological, and molecular data support multiple genetic and environmental bases for sexual orientation, and evidence exists for childhood gender nonconformity.

Harry Wright

and Ruth Abramson


Bailey, J. Michael., M. P. Dunne, and N. G. Martin. "Genetic and Environmental Influences on Sexual Orientation and Its Correlates in an Australian Twin Sample." Journal of Personal and Social Psychology 78 (2000): 524-536.

Friedman, R. C., and J. I. Downey. "Homosexuality." New England Journal of Medicine 331 (1994): 923-930.

Hamer, Dean H., et al. "A Linkage between DNA Markers on the X Chromosome and Male Sexual Orientation." Science 261 (1993): 321-327.

Kendler, Kennneth S., et al. "Sexual Orientation in a U.S. National Sample of Twin and Nontwin Sibling Pairs." American Journal of Psychiatry 157 (2000): 1843-1846.

Additional topics

Medicine EncyclopediaGenetics in Medicine - Part 4Sexual Orientation - Twin And Family Studies, Biological And Genetic Linkage Studies