There has been continued interest in the possibility that homosexuality, particularly among men, can be explained scientifically. This has prompted studies in a broad range of disciplines including sociology, psychology and most recently genetics, that have endeavoured to account for the ubiquitous nature of male homosexuality. While the notion of a so-called ‘gay gene’ has aroused much public interest, studies conducted into a possible genetic basis for homosexuality have so far proved inconclusive. However, the latest human genome studies would tend to support the hypotheses that a genetic link cannot, at the very least, be excluded (Mustanski , 2005)....

Familial studies

The interest into a possible genetic component of homosexuality was sparked by studies that investigated whether sexual orientation could be familial in nature. In 1986, almost a decade before genome probing techniques came to prominence, Pillard and Weinrich published the first empirical study in this field. The premise was that homosexuality has a familial basis because it is an “enduring characteristic of the individual” (Pillard & Weinrich, 1986). Researchers assigned over a hundred American men and their siblings a value from zero to six on the Kinsey scale (Kinsey et al., 1948) to quantify the level of “homosexual involvement”, with zero signifying exclusive heterosexuality and six, exclusive homosexuality (Pillard & Weinrich, 1986). This approach represented an improvement on previous studies that had relied solely on the judgments of probands to determine the sexual orientation of their siblings (reviewed in Pillard, Poumadere and Caretta, 1981). 

A significant association was found between homosexuality in probands and homosexuality or bisexuality in their brothers; 20 to 22 percent of the brothers of homosexual probands were also homosexual or bisexual, as determined by values on the Kinsey scales. This percentage was considerably higher than the prevalence of male homosexuality estimated in the general population at the time; approximately four percent (Pillard & Weinrich, 1986). Note that this figure has since been revised, with the value currently estimated to be between two and six percent (Diamond, M., 1993, Mustanski, 2005). The authors suggested that their findings indicated a familial basis for male homosexuality and therefore pointed toward the possibility of a genetic influence on sexual orientation (Pillard & Weinrich, 1986). 

However, Pillard and Weinrich (1986) found no significant association between homosexual probands and the prevalence of bisexuality or homosexuality in their sisters, concluding that male and female homosexuality are significantly different traits. The discovery of such a marked distinction between male and female homosexuality tended to divide the majority of future investigations along gender lines. This distinction was reinforced a decade later by another familial study which claimed that each additional older brother increases the odds of homosexuality in an individual male by 33 percent, but the number of older sisters has no influence (Blanchard and Bogaert, 1996). Current thinking suggests that this phenomenon may be due to the varying effect of Y-linked minor histocompatibility antibodies (‘H-Y antibodies’) on the sexual orientation of the brain in each succeeding male foetus (Blanchard and Klassen, 1997).  

Genes that influence behaviour

The early studies had pointed to a genetic basis for homosexuality (Green and Stoller, 1971; Pillard & Weinrich, 1986; Bailey and Pillard, 1991). However, the question still remained as to what exactly might be inherited? If genes contribute to sexual orientation, what might be the “behavioural nature of this contribution?” (Bailey and Pillard, 1991). One avenue of research examined the link between childhood gender nonconformity (CGN) and the development of homosexuality in later life (Bell in 1981 cited in Bailey and Pillard, 1991). CGN manifests itself during childhood and represents the level of sex-typed behaviours observed, such as taking part in activities or games that are stereotyped to a particular sex. It also takes into account personal feelings about gender identity at childhood. However, in an extension to their 1991 study, Bailey and Pillard found “no evidence that homosexuality associated with CGN is more heritable” (Bailey and Pillard, 1991). More recent empirical evidence examining homosexuality in both men and women (Bailey, Dunne and Martin, 2000; Dawood , 2000) provide somewhat limited support for the original theory that CGN is the heritable component of homosexuality (Bem, 1996).

THE BIG DEBATE

The 1990s saw the arrival of the first genome-probing experiments that could differentiate between the inheritable, genetic influences on sexual orientation and the “environmental, experiential, social, and cultural factors” (Hamer ., 1993a). Meanwhile, social and political interest surrounding this research reached an all-time high and many gay rights charities and campaigners insisted that being gay is not a choice but is predetermined, placing pressure on scientists to find a ‘gay gene’ that would support this view.  

The first signs of a ‘gay gene’

In July 1993, the journal published a report by Dean Hamer and colleagues claiming that homosexuality have a genetic basis. DNA probing techniques revealed a number of genetic markers on the long arm of the X chromosome that were inherited in approximately 64 percent of the sample group of homosexual men. This provided an apparent 99 percent confidence level that “at least one subtype of male sexual orientation is genetically influenced” (Hamer ., 1993a).  

One hundred and fourteen homosexual male probands underwent ‘pedigree analysis’ which involved the evaluation of male family members (i.e. their fathers, brothers, sons, uncles and male cousins) as either homosexual (representing a Kinsey rating of 5 or 6) or not known to be homosexual (heterosexual, bisexual or unclear). The probands’ assessment of their relatives’ sexuality was ascertained to be 100 percent reliable by direct confirmation in interviews with the relatives themselves. The design of the experiment to include not only the probands’ brothers but also the male members of their extended families permits a purely genetic approach to explaining the pattern observed in pedigree analysis. This is because maternal uncles and sons of maternal aunts are rarely brought up in the same household as the proband. Confounding factors such as the rearing environment are thus excluded from the analysis.  

Hamer and colleagues (1993a) determined that the brothers of homosexual probands had a 13.5 percent chance of being homosexual themselves, a figure almost seven times that of the currently accepted two percent base rate in the population (<0.001) (Diamond, M., 1993, Mustanski, 2005). The most compelling finding was that approximately eight percent of the maternally-related male relatives of the probands (namely, maternal uncles and the sons of maternal aunts) were homosexual, significantly higher than non-maternal relatives (<0.05). This suggested a link between homosexuality and the maternal line in the probands’ families. The findings were confirmed with a second round of pedigree analysis using an ‘enriched’ sample of 38 families. These families contained at least two homosexual brothers who did not have homosexual fathers or sons and so exhibited no evidence of paternal transmission of homosexuality. For this ‘sib-pair’ population, levels of homosexuality in maternally-related male cousins and uncles were even more significant compared to both non-maternal relatives (<0.001) and the original ‘random’ population (<0.05). Conversely, the levels of homosexuality among fathers and all other paternally-related classes of relatives were lower than or not significantly different from the background rate (Hamer ., 1993a). 

In order to explain the pattern of maternal transmission revealed by pedigree analysis, DNA linkage analysis was used to isolate genes inherited solely from the proband’s mother. Genes that fit this pattern are situated on the X chromosome and were identified using a series of 22 genetic markers which were later pinpointed by the polymerase chain reaction (PCR) (Hamer et al., 1993a). Five markers that showed significant linkage to homosexual orientation were discovered in a region known as ‘q28’, located on the distal portion of the X chromosome. The 40 homosexual male sib-pairs used in the linkage study shared 82 percent of their alleles in the Xq28 region (Hamer et al., 1993a). This result is significantly greater than the 50 percent allele sharing that would be expected by chance (P= 0.00001) (Hamer, 1999). While seven pairs of homosexual brothers did not coinherit all of the markers in the Xq28 region, the researchers suggested that this might be due to genetic recombination, or sources of variation that were not genetically related (Hamer et al., 1993a). A single genetic locus cannot account for all the variability in a population in the trait that it codes for. It follows that in genome probing studies such as these, the discovery of a minority that does not conform to the observed trend should be expected.  

Moreover, correlation does not always imply causation. A gene that has been suggested to have an impact on the occurrence of male homosexuality may do so indirectly and this appears not to have been recognised by Hamer (1993a). For example, there may be genes present that influence the level of psychological attachment between brothers and that subsequently increase the likelihood of them having the same sexual orientation, but could not be described as ‘gay genes’ (Fausto-Sterling and Balaban, 1993). Using both pedigree and DNA linkage analyses, Hamer and colleagues (1993a) believed they had found a ‘gay gene’. This immediately prompted a number of criticisms on grounds of experimental design (Fausto-Sterling and Balaban, 1993), ethics (Diamond, R., 1993) and the interpretation of linkage data (Risch, Squires-Wheeler and Keats, 1993). The strength of the reaction to the publication was unsurprising given the prevailing attitudes towards homosexuality. This was in some ways anticipated by Hamer . (1993a), who cautioned against the unethical use of their data to determine an individual’s current or future sexual orientation. 

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