The inheritance of homosexuality

Read Also

Homosexuality is highly hereditary  -in men (see Turner, 1995, and Byne et Parsons, 1993, for critical reviews).   Monozygotic (identical) twins,  who share 100% of their genes,  are much more often concordant for male homosexuality (about 52% concordance) than are dizygotic (fraternal) twins (about 22% concordance) who share only 50% of their genes,  yet both types of twins share practically the same environment in both cases (same family, same upbringing, same school, same food, etc.). Siblings (brothers born separately) have a concordance rate of about 10%.   The heritability of homosexuality seems to be less strong for the female gender.  One investigation sudied 6 pairs of monozygotic twins (aged 25-48 years; 8 females, 4 males) in which at least 1 member of 5 pairs was homosexual and 1 of the remaining pair was bisexual, from a series of 55 pairs, reared apart from infancy.  All the female pairs were discordant for homosexual behavior. It was concluded that this and other evidence suggest that female homosexuality may be an acquired trait. One male pair was concordant for homosexuality, while the other was not clearly concordant or discordant. However, one study described a set of 45-year-old identical female twins with a history of exclusive homosexuality. No mutuality was reported. Subjects revealed a normal karyotype number and 2 X chromosomes showing some variation in size.  Another study determined if there is a genetic contribution to female sexual orientation by studying lesbian or bisexual adult women with either female cotwins or adoptive or genetically unrelated sisters. 115 probands with female twins and 32 probands with adoptive sisters were interviewed. Of the final subsample, 34 of 71 monozygotic cotwins were either homosexual or bisexual compared with 6 of the 37 dizygotic cotwins and 2 of the 35 adoptive sisters. Twin probands reported 73 nontwin biological sisters about whose sexual orientation they were at least virtually certain. Of these, 10 were thought to be homosexual or bisexual. Heritability remained significant and appreciable.  So, hereditary factors could play a role in lesbianism in some cases,  but certainly to a smaller extent than in male homosexuality.  Furthermore,  the last study suggests that female homosexuality could be higher in twins (mono or dizygotic) in general than in non twins.   This might suggest a uterine hormonal factor:  maybe twins share limited amounts of sex hormones in the womb. Unfortunately, much less is known about the genetics of lesbianism than of male homosexuality.  The molecular biology of hereditary transmission of homosexuality is only in the starting stage.   One research team, led by Dean Hamer (1993), reported that a DNA sequence located on the long arm of the X chromosome (Xq28) seemed to be responsible for male homosexuality in a set of 40 brothers both of whom were homosexual.  The particular sequence observed was present in 67% of these brothers,  a rate much higher than is observed in non-homosexuals.  The same authors have now successfully replicated this finding  -thus making the truth of the finding seem much more plausible.   A priori, the proposed mechanism of transmission (X-linked) seems very plausible for three reasons:  1) it explains how it could be that homosexuality would be as prevalent as it is (10% of the male population) over the ages,  since such a mutation can be transmitted by two heterosexual parents;  2) it explains why men are four times more likely to be homosexual than women;  3) it helps explain why lesbianism and male homosexuality seem to be so different.   However,  much more research of this sort needs to be done.