More Evidence Of ‘Gay’ Genes

gay couple same-sex marriageJustin Sullivan/Getty Images

The claim that homosexual men share a “gay gene” created a furor in the 1990s. But new research two decades on supports this claim – and adds another candidate gene.

To an evolutionary geneticist, the idea that a person’s genetic makeup affects their mating preference is unsurprising. We see it in the animal world all the time. There are probably many genes that affect human sexual orientation.

But rather than thinking of them as “gay genes”, perhaps we should consider them “male-loving genes”. They may be common because these variant genes, in a female, predispose her to mate earlier and more often, and to have more children.

Likewise, it would be surprising if there were not “female-loving genes” in lesbian women that, in a male, predispose him to mate earlier and have more children.

Evidence for ‘gay genes’

We can detect genetic variants that produce differences between people by tracking traits in families that display differences.

Patterns of inheritance reveal variants of genes (called “alleles”) that affect normal differences such as hair colour, or disease states such as sickle cell anaemia.

Quantitative traits, such as height, are affected by many different genes, as well as environmental factors.

It’s hard to use these techniques to detect genetic variants associated with male homosexuality, because many gay men prefer not to be open about their sexuality. It is even harder because twin studies show that shared genes are only part of the story; hormones, birth order and environment play roles too.

In 1993, American geneticist Dean Hamer found families with several gay males on the mother’s side, suggesting a gene on the X chromosome. He showed that pairs of brothers who were openly gay shared a small region at the tip of the X, and proposed that it contained a gene that predisposes a male to homosexuality.

Hamer’s conclusions were extremely controversial. He was challenged at every turn by people unwilling to accept that homosexuality is at least partly genetic, rather than a “lifestyle choice”.

Gay men were divided: it vindicated the oft-repeated claims that “I was born this way” but also opened frightening new possibilities for detection and discrimination.

Similar studies gave contradictory results. A later search found associations with genes on three other chromosomes.

This year, a larger study of gay brothers, using the many genetic markers now available through the Human Genome Project, confirmed the original finding, and also detected another “gay gene” on chromosome 8. This has unleashed a new flurry of comment.

But why such a furore when we know of gay gene variants in species from flies to mammals? Homosexuality is quite common throughout the animal kingdom. For instance, there are variants that influence mating preference in mice and a mutation in the fruit fly makes males court other males instead of females.

Is the ‘gay gene’ really a ‘male-loving allele’?

The puzzle is not whether “gay genes” exist in humans, but why they are so common (estimates from 5-15%). We know that gay men have fewer children on average, so shouldn’t these gene variants disappear?

There are several theories that account for the high frequency of homosexuality. A decade ago I wondered if gay gene variants have another effect that boosts the chances of leaving offspring (“evolutionary fitness”), and passing the gay allele on.

This is a well-known situation (called “balanced polymorphism“) in which an allele is advantageous in one situation and not in another. The classic case is the blood disease sickle cell anaemia, which leads to disease and death if you have two alleles, but to malaria resistance if you have only one, making it common in malarial regions.

A special category is “sexually antagonistic genes” that increase genetic fitness in one sex, but not in the other; some are even lethal. We have many examples across many species. Maybe the gay allele is just another of these.

Perhaps “male-loving” alleles in a female predispose her to mate earlier and have more children. If their sisters, mother and aunts have more kids who share some of their genes, it would make up for the fewer children of gay males.

And they do. Lots more children. An Italian group showed that the female relatives of gay men have 1.3 times as many children as the female relatives of straight men. This is a huge selective advantage that a male-loving allele confers on women, and offsets the selective disadvantage that it confers on men.

I am surprised that this work is not better known, and its explanatory power is neglected in the whole debate about the “normality” of homosexual behaviour.

How ‘normal’ are gay alleles?

We have no idea whether these genetic studies identified “gay alleles” of the same or different genes.

It is interesting that Hamer detected the original “gay gene” on the X, because this chromosome has more than its fair share of genes that affect reproduction, but I would expect that there are genes all over the genome that contribute to mate choice in humans (female-loving as well as male-loving).

If there are male-loving and female-loving alleles of tens or hundreds of genes battling it out in the population, everyone will inherit a mixture of different variants. Combined with environmental influences, it will be hard to detect individual genes.

It’s a bit like height, which is influenced by variants in thousands of genes, as well as the environment, and produces a “continuous distribution” of people of different heights. At the two extremes are the very tall and the very short.

In the same way, at each end of a continuous distribution of human mating preference, we would expect the “very male-loving” and the “very female-loving” in both sexes.

Gay men and lesbian women may simply be the two ends of the same distribution.

The Conversation

Jenny Graves has received funding from the ARC and NHMRC. She is not associated with any organisation that would benefit from this article.

This article was originally published on The Conversation. Read the original article.

27,204 thoughts on “More Evidence Of ‘Gay’ Genes

  1. Be aware how versatile this look is, though — it could be
    quite appropriate for quite a lot of business informal capabilities
    and more.

  2. I do not know whether it’s just me or if perhaps everybody else experiencing problems with your site.
    It appears like some of the written text within your content
    are running off the screen. Can someone else please provide feedback and let me know if this is happening to them as well?
    This may be a issue with my browser because I’ve had this happen before.
    Many thanks

  3. Ⅰ believe that is among the such a lot significant info for me.
    Annd i’m glad studyіng your article. However wajna statement on some common things, The
    site tɑste is wonderful, the articles is inn point
    of fact nice : D. Good activity, cheers

  4. 本最大級の自転車通販サイト。シティサイクル,シティサイクル 通販,シティサイクル 24インチ,シティサイクル 26インチ,シティサイクル 27インチ,シティサイクル おしゃれ,人気,安い,高級,シティサイクル自転車!あなたの欲しい自転車がきっと見つかる!安心のアフター修理保証付き。組立後即日発送だから、届いた日からすぐ乗れる!

    1. My brother recommended I might like this web site. He was entirely right.
      This post truly made my day. You cann’t imagine simply how much time I had spent
      for this information! Thanks!

    2. Unquestionably believe that which you stated.
      Your favorite reason appeared to be on the internet the simplest thing to be aware of.
      I say to you, I certainly get irked while people think about worries that they plainly do
      not know about. You managed to hit the nail upon the top and defined out the
      whole thing without having side effect , people could take a signal.
      Will probably be back to get more. Thanks

  5. Hey there would you mind stating which blog platform you’re working
    with? I’m going to start my own blog in the near future but I’m having a hard time
    making a decision between BlogEngine/Wordpress/B2evolution and
    Drupal. The reason I ask is because your design and style seems different then most blogs and I’m looking for something
    unique. P.S Apologies for getting off-topic but I had to ask!

  6. Puttfing your home up for sale doesn’t end up being be
    a troublesome process marketing businness hire a company you already consider a mentor multiple-choice exzm It is
    the straightforward question-answer data format
    career path whatever how a quickly or slowly trainees learns a skill Exaam Answer this book
    could be obtained att most thee leading book stores
    questions and answers to job interviews

  7. Possibly ahead of chemo was stated by the physicians, my mother had a PICC – peripherally-
    introduced catheter that is central – line-in her arm
    for administering medications and fluids.

  8. Everyone loves what you guys tend to be up too. Such clever work
    and coverage! Keep up the amazing works guys I’ve included you guys to blogroll.

Leave a Reply

Your email address will not be published.