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Genomewide analysis

The first ever genomewide scan to search specifically for genes that might be associated with male sexual orientation was published in January 2005. This reignited the debate about the origins of sexual orientation. The study revealed a number of loci on autosomal (non-sex-linked) chromosomes (Mustanski The Human Genome Project made it possible for the first time to assign potential functions to these genes and so added a new perspective to the issue of a genetic basis to male homosexuality. The strongest level of sharing between the homosexual male siblings was for the region 7q36 to which the so called ‘sonic hedgehog’ gene had been mapped (Mustanski , 2005). This gene plays a central role in early embryonic development, including hemisphere separation and asymmetry of the left and right sides (Mustanski , 2005). This was subsequently linked to the development of sexual orientation because brain asymmetry has been shown to influence non-right-handedness, a feature that is 39 percent more prevalent in homosexuals (Lalumière, Blanchard and Zucker, 2000). So not only do genes located at the 7q36 position appear more frequently in homosexual men, they also have a function that is relevant to the development of sexual orientation.  

A region was discovered on chromosome ten called q26 that was also shared significantly between homosexual male siblings (Mustanski et al., 2005). Like the much quoted Xq28 locus, 10q26 was found to correlate with sexual orientation only when inherited from the mother. This was possibly due to genomic imprinting whereby inheritance from either the mother or the father determines the expression of a gene (Bocklandt and Hamer, 2003 cited in Mustanski et al., 2005). However, analysis of the Xq28 region specifically, did not provide evidence of linkage (Mustanski et al., 2005) even though half the sample comprised the very same families used in the previous investigations (Hamer et al., 1993a; Hu et al., 1995). The discrepancy was attributed to a smaller number of markers in the Xq28 region, decreasing the resolution of analysis (Mustanski et al., 2005).

So does a ‘gay gene’ exist?

It still remains unclear whether male homosexuality has a genetic basis. The results of genome probing studies have yet to be replicated, and future research should make additional use of the Human Genome Project to assign functions to any candidate genes. Future studies should also examine patterns of genetic influence amongst female homosexuals to determine whether the traits of male and female homosexuality really are distinct. This would lead to a broader understanding of homosexuality as a phenotype. While harder to achieve, the study of twins separated at birth would reveal whether genes, environment or an interaction between the two, yield the most influence over the development of sexual orientation.

 

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