Do I look gay in these genes?

Zara Jackson-Martin discusses the science of sexuality.

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Originating from a rural area steeped in Catholicism meant that coming to terms with being homosexual wasn’t exactly a walk in the park. However amidst conflicting social influences and the challenge of coming-out, science has been my life raft. Understanding and exploring the biological mechanisms behind sexual orientation has helped to reassure me that despite being homosexual I’m just as ‘normal’ as the next person and that my sexuality is so much more than a ‘lifestyle choice’.

Given the sheer scope of research on sexual orientation, I’ve chosen to stick to a few key areas of interest in the relevant literature. Beginning with a theory known as the Fraternal Birth Order Effect. Several dozen studies, including a British meta-analysis of data from over 16,000 participants, have found that older brothers are associated with homosexual orientation in males.

This may seem like a rather outlandish claim but we’re talking about quite a substantial increase in probability; specifically an increase in the chance of being homosexual of approximately 33% with each subsequent elder brother. If you have no elder brothers the probability of you being gay (as given in the meta-analysis) is low at 2%, however this jumps to 2.66% with your first elder brother and 3.54% with your second.

In order to make sense of this correlation it is necessary to understand the Maternal Immune Hypothesis. In 1996, Canadian researcher Anthony Bogaert hypothesized that the Fraternal Birth Order Effect was a reflection of the progressive immunisation of some mothers to male-specific antigens. Our immune system is designed to attack pathogenic cells that do not exhibit our own genetic code. However, fetuses also exhibit foreign DNA. Luckily mechanisms exist to prevent the maternal immune system attacking her fetus meaning the only immune cells that are able to cross through the placenta from mother to child are antibodies in order to prevent disease.

This is where our problem begins with male H-Y antigens. H-Y antigens are thought to be involved in sex-typical traits in males, and the presence of the H-Y antigen means that with each subsequent male fetus the maternal immune system is able to mount a better immune response through H-Y antibodies. These anti-bodies can cross the placental barrier and affect sexual differentiation in the fetal brain, partially preventing the fetal brain from developing in the male-typical pattern.

Perhaps the best example of this theory in action was conducted in laboratory mice by Singh and Verma who immunized two groups of female mice to H-Y antigen by injecting them with male spleen cells. When the male offspring of these females matured to a sexually active age only ten per cent mated with females, compared to 100 per cent of the male mice in the control group.

A strange side effect of androgen is that it influences the ratio of the length of your 2nd and 4th digits. Males exposed to high levels of prenatal androgens show a smaller ratio of 2nd to 4th digits than females who experience much lower androgen levels on average. However this was not true for all females.

Lesbians prove to be an exception with smaller, more masculine 2D:4D ratios than heterosexual women, whereas gay and heterosexual men did not differ significantly. The likely reason for the lack of difference in men, is that regardless of sexual orientation they are exposed to significant androgen levels during development, whereas females are not usually exposed to androgen except on a very small scale. In other words small changes in androgen make large changes in development. Given that testosterone production begins at around six weeks of gestation, this can be considered the beginning of a critical period in women in the development of sexual orientation.

Now for a look at genetics: homosexuality and indeed sexual behavior in general are notoriously hard to study from a genetic perspective because of confounding social factors. However science has been given one great opportunity to look at both genetic and environmental factors: twins. As monozygotic twins have both identical DNA and prenatal conditions and dizygotic twins have different DNA but the same prenatal conditions, through comparison researchers are able to disentangle the effects of genes from other epigenetic factors.

This is exactly what one Swedish team set out to do in 2010. They took 3826 pairs of twins, and set out to examine sexual orientation. What they found was astounding. By comparison of monozygotic and dizygotic twins their model suggests that up to 35% of same-sex sexual behaviors were related to genetic factors in men and around 18% in women, whilst the remaining factors are most likely epigenetically based.