What biology and genetics say about same sex attraction

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  Last updated November 10, 2017 at 4:54 pm

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Homosexual behaviour is a natural biological feature and is common among non-human animals. In at least one species – sheep – individual animals have been known to form lasting preferences for same-sex partners.


Genetics may hold part of the answer, but  few today believe only one gene or set of genes makes determines anyone’s sexual orientation.


In fact, recently, carers discovered that the partner of the world’s oldest animal, a 186-year-old tortoise, was a male, not female as they had previously assumed. It suddenly explained why their continuous mating over the past 25 or more years had failed to ever produce any offspring.


Animals have also been often seen displaying bisexual behaviour – interacting with both opposite and same sex. This suggests a more biological purpose, in which it’s proposed that this may increase an animal’s chance of successful breeding later with the opposite sex. That may sound counter-intuitive, but one University of Frankfurt study, published by Royal Society Biology Letters, found that homosexual behaviour increases male attractiveness to females.


The researchers pointed out that females regularly use social information to choose a mate and male attractiveness increases after he has interacted sexually with a female – a phenomenon known as “mate choice copying”. More unexpectedly, the same appears to be true, at least for the the tropical freshwater fish Poecilia mexicana, with same-sex interaction. The female fish found the males more attractive after the male had a same-sex interaction in the same way they did with opposite sex interaction.


“Hence, direct benefits for males of exhibiting homosexual behaviour may help explain its occurrence and persistence in species in which females rely on mate choice copying as one component of mate quality assessment,” the paper said.


While these studies are useful, it is important to note that most animals can only be classified through sexual orientation rather than sexual preference.


“Sexual preferences require a higher cognitive involvement, something that only primates show,” says Dr Angelo Tedoldi, a neuroscientist at University of Queensland’s Queensland Brain Institute.


“Indeed, primates have been shown to have a more complex sexual behaviour that other animals in the animal kingdom. Bonobos are well known to have sexual intercourse for pleasure other than just for reproduction, both with different or same sex partners.”


Other studies have found evidence that sex hormones influence the development of very different gendered traits between gay and straight people; that there are differences in brain organisation between gay and straight people; and that birth order influences sexual orientation in men.


In 1993, a landmark paper was published in the academic journal, Science, that showed a linkage between DNA markers on the X chromosome and male sexual orientation. This spearheaded discussion on the possibility of a male “gay gene” and heritability of the trait.


For the study, Geneticist Dean Hamer at the National Cancer Institute in the US and his team analysed 114 families of homosexual men. They found increased rates of same-sex orientation in the maternal uncles and male cousins of the subjects, but not in their fathers or paternal relatives. The research revealed that a patch of DNA in the X chromosome – labelled Xq28 – was likely to be shared by brothers who were both gay.


While the media leapt on the catchy concept of the “gay gene”, subsequent studies to replicate this study have met varying levels of success.For more than 20 years Hamer’s research was called into question by subsequent studies. Then, in 2014, a Northwestern University study of 409 pairs of twins found a significant linkage for male sexual orientation to the same stretch of the X chromosome that Hamer had identified – Xq28.


The X-linked gene could explain how the trait persists in the population, even though gay men have fewer offspring, and studies such as Hamer’s and the later Northwestern one suggest there is a heritable component to homosexuality. However, few today believe only one gene or set of genes makes determines anyone’s sexual orientation.


“It’s very rare that a behavioural phenotype, or complex trait comes from a single gene. Most things don’t,” says Dr Hannah Brown, a reproductive epigeneticist at the University of Adelaide. “Many of our traits are from the combination of many genes working, or linking together.”


Is epigenetics the missing piece of the ‘gay gene’ puzzle?


With one overarching “gay gene” proving elusive, a group of researchers suggested that the search for one had been misguided from the start.


They proposed that the answer may instead lie in epigenetics – the study of the chemical changes that can alter how the genetic code is expressed, without changing the genetic code itself.


William Rice, an evolutionary geneticist at the University of California, Santa Barbara; Urban Friberg, an evolutionary biologist at the University of Uppsala and Sergey Gavrilets, a mathematician at the University of Tennessee suggested that the search for a “gay gene” presented their hypothesis in a 2013 paper published in The Quarterly Review of Biology,


Rice and his colleagues proposed a model in which the epigenetic markers which steer sexual development in males could promote homosexual orientation in females, and vice versa. The elusiveness of the “gay gene” along with the tendency of homosexuality to run in families, supported this model, they said.


While there was undeniable heritability, only 20 per cent of identical twins are both gay, Rice noted, and the apparent absence of any “major” homosexual genes “made us suspicious that something besides genes produces heritability that isn’t genetic”.


The model could, they say, explain why not all identical twins share sexual orientation.


While it should be noted that the hypothesis has not yet been tested with real data, many researchers believe something of the kind could be in play.


Thomas Conway, a bioinformatics researcher at the Peter MacCallum Cancer Centre, believes that “multiple underlying mechanisms that contribute to sexuality”.


“It is very likely that while there may be a modest number of ‘gateway’ genes, there will be a number of multi-gene interactions combined with developmental factors, including maternal hormone levels at crucial times during gestation, for example, that affect the development of a person’s sexuality and gender identity.


“Height is a complex trait and we can measure it unambiguously, and it’s one dimensional.


“The last time I looked, a big GWAS study on height found that some 200 genes were involved, and surprisingly nutrition, childhood illness and the like play a relatively small role. So there is no height gene, even though height is very strongly genetically determined, with some environmental modulation.


“No one argues that being tall is a choice.”


In all of this, there’s an obvious omission of women and other genders, and other sexual orientations including but not exclusive to lesbians and bisexuals.


These oversights have been made consistently, even in an Australian study in 2000 that reported on the heritability of homosexuality in 25,000 pairs of twins. It used the Kinsey scale, which is a seven-point scale for measuring sexual orientation (zero for exclusively heterosexual to six for exclusively homosexual). Controversially, it classified anyone with a Kinsey score of at least two as homosexual.


“Realistically, I understand the temptation to look at something like sexuality as a spectrum and deal with this in a scientific study by taking each end,” says Frentz.


“However, by doing this, we run the risk of missing key genetic links, and with most recent studies suggesting a genetic basis to sexuality being linked to epigenetic markers, it may be that this would have been more readily picked up.”


“This misses out huge chunks of the LGBTQI+ community.”


And that remains a problem along with the scarcity of funding for studies into sexuality. Private enterprise may take up some of that slack. Commercial genetics company 23andMe are trying the study the genetics of sexual orientation, in the “largest genome-wide association study of sexual orientation ever done”, as announced back in 2012. However, search results reveal that nothing has arisen in recent years from this study.


Science of Sexuality series:


How neuroscience interprets sexual orientation


What role do hormones play in determining sexual attraction?


An evolutionary view of the ‘gay gene


Psychology: Freud has a lot to answer for




About the Author

Kelly Wong
Contributing editor for News + Events and the online producer at Australia's Science Channel. I have a background in immunology, food blogging, volunteering, and social media. I'm passionate about creating communities on social media and getting them excited about science. I enjoy good food and I am on an eternal mission to find the best ice cream. Find me on Twitter @kellyyyllek