Are Homosexuals Born Gay? Part Two

The Gay Gene?

Darwinian evolution postulates that those genetic mutations that favour survival and reproduction will be selected and passed on, whereas those that compromise survival and reproduction will be eliminated. Obviously, a gene or series of genes that produce non-reproducing individuals, such as those who express pure homosexual behaviour, will be rapidly eliminated from any population. So, using evolutionary logic, it would be expected that any “gay gene”, even if it did exist, would be quickly and efficiently removed from a population. However, this has not stopped an army of scientists looking for it over two decades.

The initial study that made headlines around the world, leading to the widespread belief in a “gay gene” was a 1993 examination of family pedigrees revealing that gay men had more homosexual male relatives through maternal linages than through paternal lineages, suggesting a linkage to the X chromosome. Researcher Dean Hamer, found such an association at region Xq28. He hypothesised that if male sexual orientation was influenced by a gene on Xq28, then gay brothers should share more than 50% of their alleles (gene pairs) at this region, whereas their heterosexual brothers should share less than 50% of their alleles. His analysis of 40 pairs of gay brothers and found that they shared 82% of their alleles in the Xq28 region, which was much greater than the 50% allele sharing that would be expected by chance. However, a follow-up study by the same research group, using 32 pairs of gay brothers and found only 67% allele sharing, which was much closer to the 50% expected by chance. Attempts by Rice et al. to repeat the Hamer study resulted in only 46% allele sharing, insignificantly different from chance, contradicting the Hamer results.At the same time, an unpublished study by Alan Sanders (University of Chicago) corroborated the Rice results.

Ultimately, no gene or gene product from the Xq28 region was ever identified that affected sexual orientation. When Jonathan Marks, an evolutionary biologist, asked Hamer what percentage of homosexuality he thought his results explained, his answer was that he thought it explained 5% of male homosexuality.

Since the year 2000, DNA sequencing has been perfected and become cheap enough to do detailed studies into homosexuality. Since the turn of the century there have been several attempts already to scan the entire genome for genetic links to homosexual orientation. The accuracy of the new technology should, once and for all, settle the issue of a genetic component to homosexuality. The largest genome-wide scan was conducted by E. M. Drabant et al, in a project called 23 and Me  (Genome Wide Association Study of Sexual Orientation in a Large, Web-based Cohort. Abstract presented at the American Society of Human Genetics annual meeting in San Francisco 2012.) Some 7887 unrelated men and 5570 unrelated women of European ancestry were analysed by a genome-wide association study. Although unpublished, the data was presented at the American Society of Human Genetics annual meeting in San Francisco, showing that there were no loci associated with sexual orientation, including Xq28 on the X chromosome. The preliminary studies on possible genetic causes of homosexual orientation therefore tend to rule out any dramatic genetic component to sexual orientation.