THE GENETICS OF SEXUAL ORIENTATION

January 6, 2005

By Jeff S. Hatfield, PhD, Laurel, Maryland

Some interesting -- and little noted -- genetic research has recently been published that bolsters the argument for a genetic basis of sexual orientation. And a fascinating connection is being made between sexual orientation, a person’s scalp hair, brain laterality, and left handedness -- another trait that carried social stigma in the not so far distant past.

Dr. Amar Klar, Chief of Developmental Genetics Section, Gene Regulation and Chromosome Biology Laboratory of the National Cancer Institute at the National Institute of Health (NIH) published a paper in the journal Genetics (published by the Genetics Society of America) in September of 2003 concerning the genetics of handedness. Klar was widely acclaimed in scientific journals like Nature and American Scientist as having proposed a model which solved the seemingly complicated problem of explaining how a person becomes right handed, left handed, or ambidextrous. It seems that there has been a great deal of literature on the subject, with many theories as to whether handedness was caused by environmental factors or genetics ("nurture" vs. "nature"). Klar presented a rather simple genetic model (explained below), tested it with data on left handers, and the model held up to the predictions.

This same gene, as Klar’s 2003 paper explains, also determines the direction a person’s hair whorls at the back of the head. If you look closely at the back of the heads of the people seated in front of you on the metro or in a theater, you will notice that human hair tends to whorl either clockwise or counterclockwise. This is of course much easier to observe in people with short hair. Klar’s research, collected by observing people in shopping malls in Maryland, shows that about 92% of the general population has clockwise hairwhorl; that is, the hair spins in a clockwise pattern at the back of the head. In the other 8% of the population, however, the hair spins in a counterclockwise pattern at the back of the head.

This same gene, it turns out, may determine most, if not all, sexual orientation in our species, as well as handedness and hairwhorl pattern. So how does hair orientation relate to sexual orientation? Klar conducted more research at a well-known gay area of the beach at Rehoboth, Delaware, in 2003 and again in 2004. It should be mentioned that Klar paid for this research himself and it was not funded by the Federal Government like his previous studies. Klar tabulated the hairwhorl patterns in a sample of 272 men whom he suspected were mostly gay or bisexual, based on what he described as stereotypical behaviors that gay men do at the beach (presumably noting the male couples or groups of men). Admitting the possibility of some error in his assumptions, he calls this an "enriched" sample of gays, because it may contain some heterosexuals but is probably mostly gay or bisexual. He found that 30% of these men had counterclockwise hairwhorl. This may seem insignificant, but statistically, it’s a hugely significant result when compared to the 8% figure found in the general population.

In December of 2004, Klar published these results in the Journal of Genetics which is published by the Indian Academy of Sciences (the paper is available on the Web at http://www.ias.ac.in/jgenet/Vol83No3/251.pdf). So what mechanism is at work here? Why would gay or bisexual men be more likely to have counterclockwise hairwhorl? (And the same is predicted for lesbians and transgendered individuals as well, although data haven’t been collected on these groups yet.)

It’s actually a simple genetic model, albeit with an unusual mode of expression. What this hairwhorl gene is thought to do (Klar calls it RGHT1), is it directs the placement of asymmetrical brain structures in the early stages of a developing fetus. Klar postulates that the gene has 2 forms ("alleles") with 60% dominant (R) and 40% recessive (r) in the general population. Thus, 36% of people (which you get by multiplying 60% x 60%) will be homozygous dominant (R/R), 16% (40% x 40%) will be homozygous recessive (r/r), and 48% (60% x 40 % + 40% x 60%) will be heterozygous (R/r). Most people (84%) will be homozygous dominant or heterozygous, thus they carry the dominant form of the gene, and what ever the gene product from this gene does, it directs their hair follicles to whorl in a clockwise way at the back of the head in the developing fetus, and it also directs their brain architecture such that they will become right handed and their speech will be processed on the left side of their brain ("left brained"). My guess is that these people are all heterosexual (except for minor pathway discussed below), although this has not been tested yet.

What about the homozygous recessives? (16% of the general population.) Well, their brains basically flip a coin at each juncture in how they get wired up in the developing fetus, because they don’t have this gene product, what ever it is, to direct them to become right handed, left brained, and have clockwise hairwhorl. Half will have clockwise hairwhorl, and half will have counterclockwise hairwhorl, and independently, half will be right handed and half will not be right handed (i.e., left handed and ambidextrous), and half will have their speech processed on the right side of the brain and half will have their speech processed on the left side of their brain. Thus, in these individuals in what Klar calls the "random recessive" pathway, one can expect to have 25% of them be right handed and left brained, 25% right handed and right brained, 25% left handed and right brained, and 25% left handed and left brained. My guess is that many of the right handed, right-brained fetuses will go on to become gays, lesbians, bisexuals and transgendered individuals, as will many of the left handed, left-brained individuals, although again, this remains to be tested.

These predictions are consistent with studies that have shown gays and lesbians to be more likely to be left handed than the general population, as well as more likely to be right brained. These data are also consistent with studies of monozygotic ("identical") twins that have shown that, if one member of a twin is left handed, there is a 50% chance that the other twin will be right handed, and other studies of monozygotic twins that have shown that, if one member of a monozygotic twin is gay, bisexual or lesbian, then there is a 50% chance that the other twin will be heterosexual.

There may also be another minor pathway, whereby a homozygous recessive mother, if she bears a heterozygous son, then later heterozygous sons born by her will be more likely to be gay or bisexual. Again, this is consistent with studies that have shown gay men are slightly more likely to have an older brother than the general population. Similar to the way Rh factor is already known to work in humans, the homozygous recessive mother produces antibodies to the gene product from the first heterozygous son, and these antibodies knock out the gene product in later heterozygous sons, causing them to enter the random recessive pathway of brain development, even though they themselves carry the dominant form of the gene. Why sons and not daughters? That remains to be determined, but it does explain why there are slightly more left handed males than females, and also probably why there are more gay males than lesbians. Interstingly, RhD, the main gene determining Rh factor in humans, has an unknown function but expresses itself very early in fetal life, and for Caucasians, 85% of the population is dominant, and 15% is homozygous recessive.

Why should such a mechanism exist for producing individuals with brain architecture different from the norm, as well as different handedness and sexual orientation? One of the most interesting aspects of all this is that the recessive allele has a frequency of 40%. If the "gay" gene had had a frequency of, say, 5% or less, one might argue that it’s just a deleterious mutation that somehow got fixed at a small percentage in our species, and that eventually evolution will weed it out. Perhaps we are just in that stage of our evolution when this is happening. However, it is extremely unlikely that an allele could reach a level of 40% in our species just by chance, if it didn’t have some beneficial effect. Evolution isn’t weeding it out, it is selecting for it, or it couldn’t have possibly gotten to such a high level.

Another possibility is that the recessive form of the gene is actually our ancestral pattern, and the dominant form is on the increase, because it produces more heterosexuals who go on to reproduce. Eventually then, after a long period, evolution will fix the dominant form of the gene at 100% and everyone in the distant future will be right handed, left brained, and heterosexual. This seems like an unlikely scenario, but it should be easy to test by looking for the gene in our close relatives (like chimpanzees). Do chimps have a hairwhorl pattern? Does it all whorl in one way or does it vary as in humans?

My best guess is that this gene’s function in our species is to allocate a small percentage (about 20%) of the population into these pathways of alternative brain architectures, because having some individuals who think differently from the norm is somehow advantageous to society. We already know that right-brained individuals (as well as left-handed individuals) are more likely to be artistic, to be holistic thinkers, to become leaders and philosophers, and somehow evolution must be selecting for these traits. However, evolution doesn’t want too many of these individuals around because many of them are reproducing at a lower level (or not at all) compared to the right handed, left-brained heterosexuals, so this gene exists to fine tune the percentages to get just the right amount of alternative brain patterns in our species.

It should also be pointed out that half or more of the random recessives will be heterosexual, so knowing that someone has counterclockwise hairwhorl or is left handed or ambidextrous, does not necessarily mean they are gay or bisexual. In fact, 50% or less are probably gay or bisexual. Thus, it should not be assumed that someone with counterclockwise hairwhorl is gay or bisexual.

So, what else remains to be done? As Klar mentions in his most recent paper, studies need to be done on self-reporting gays and lesbians, to get better estimates of the percentage of counterclockwise hairwhorl among these groups, as well as in other races and populations. More importantly, the actual gene, though hypothesized to exist, has yet to be found. The NIH currently has a clinical trial looking for the gene, and given how much we know about it already, it shouldn’t be too long before they find it. Once the main gene is found, then other genes which might modify some of these behaviors will also undoubtedly be located, so we should eventually have a much better understanding of how sexual orientation is determined, in addition to serious, debilitating conditions like schizophrenia and bipolar disease, which are known to be more common among left handers. In fact, this is why Klar began this avenue of research initially, to try to figure out the genetics of schizophrenia and bipolar disease, but he may have gotten a lot more than he originally hoped for.

Also, once the main gene is found, then it will be possible to do the evolutionary studies to measure the beneficial effect of the recessive form of the gene (that is, the increase in genetic fitness to heterozygous individuals who carry the recessive allele). It may also be possible to estimate when homosexuality first appeared in our species, by looking at mutations in the dominant and recessive forms of the gene, if such mutations exist. It is even possible that homosexuality could have evolved more than once in our species, and this will become apparent, once large scale screenings are performed, looking for different alleles of the gene.

Finally, there are some serious ethical issues that need to be discussed once the gene is found, if not before. What if genetic tests become available, and some people decide to abort their homozygous recessive fetuses, for fear they will turn out to be gay, lesbian, bisexual, or transgendered? Even if that doesn’t happen, what if it becomes possible to have therapies that will cause a developing fetus to become right handed, left brained, and heterosexual, in order to prevent it from becoming gay or bisexual? What will be the effect upon our species if this practice becomes prevalent? Will many great artists and philosophers not be born in the future, for fear of producing gays, lesbians, bisexuals, or transgendered individuals? I don’t have any answers to these difficult questions, but they will undoubtedly be debated by our society at some point soon.

Author’s note: Jeff Hatfield is currently a research ecologist with the U.S. Geological Survey in Laurel, Maryland, and previously, he studied biostatistics, ecology and evolutionary genetics for his graduate degrees (see http://www.pwrc.usgs.gov/hatfield.htm). He is right handed, gay, and has counterclockwise hairwhorl. This article was written on his personal time, however, and was not funded by the Federal Government.

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