Blonde Equilibrium

I was originally planning to hold off on starting a series of posts on the genetics of evolution until next week, but John Wilkins recently found a myth that's just too good to ignore.

According to this legend, a World Health Organization study conducted a few years ago revealed that the natural blonde is scheduled to become extinct by the year 2202. The reason given for the imminent demise of the unperoxided flaxen haired is that blonde hair is caused by a recessive gene, and too few people now carry that gene. Since two people who carry the gene must meet (and do certain other things) in order to produce a blonde, the number of blondes is decreasing.

The myth is in fact just that - a myth. It was a wildly successful hoax carried out by someone pretending to be on the WHO staff. They were able to get a large number of media outlets to bite, and the story received international attention when it was first released. Despite a World Health Organization 'clarification', it would appear that some media outlets still haven't quite figured out what's going on.

Still...

The looming extinction of the natural blonde is a legend that's probably going to stick around for a while, for a couple of reasons. For starters, there's a certain ironic appeal to having the blonde meet the fate of the dodo. More importantly, though, the scenario sounds plausible enough. There do seem to be more dark haired people around than there used to, and if natural blonde's aren't meeting and mating as much as they used to, then why shouldn't we expect the blonde to go extinct?

That's a classic population genetics question if there ever was one, and it's one that I'll be looking at in this post (and, hopefully, in a few follow-ups). There are a few different ways to look at the question. We'll start with the easiest, but there is a little bit of vocabulary to get out of the way first.

It's just two terms, but they are important words in genetics. The first is "locus" (plural: "loci"). You can think of a locus as a single gene. (To be a little more specific, a locus is a specific location at a chromosome.) The second is "allele". An allele is one possible version of a gene. Since humans have two copies of each chromosome (one from each parent), we can have either two identical alleles or two different alleles at any locus.

Now back to the blondes.

The question of whether or not the blonde hair allele will disappear from the human species can actually be treated as the simplest situation in all of genetics: it's called a one-locus, two-allele model. This means that the trait (in this case, hair color) is controlled by a single gene, and there are only two variations of the gene in the population. In the case of human hair color, it's not really that simple, but because blonde hair is a recessive trait, we can get away with pretending that it is. We'll just call the dominant allele "not blonde" (and we'll represent it with 'B') and the recessive allele "blonde" ('b'). It's simpler to do it that way, and the math comes out the same.

Let's assume that 1% of humans are natural blondes. That means that they carry two copies of the 'b' allele. If we assume that people don't choose who they marry based on natural hair color, we can use this to figure out how common the 'b' allele is in the population. The equation for that is easy. If 'b' is the probability of picking a random allele from the populaiton and finding that it is a 'b', then the percentage of people with two 'b' alleles will equal b*b. We know that b*b = .01 (or 1%), so b must equal 0.1.

To put it another way, if one percent of the people are blonde, then ten percent of the alleles are the blonde allele. (Which means that 90%, or 0.9) of the alleles are the 'B' allele.

That's the case in the first generation, but what happens in the next one? That's the key question. If the next generation has a smaller percentage of blonde alleles, then blondes may be on the way out. So how do we find out what's going to happen next?

It turns out that there is a famous genetic principle that provides the answer to that pressing question. It's called Hardy-Weinberg equilibrium, and it says that, given certain assumptions (which we will discuss later), then the allele frequencies won't change. 10% of the alleles in the next generation will still be 'b' alleles. There's a famous equation that goes with that, but it's got exponents and other annoying mathy things, so I'll skip it for now. If you don't want to trust me, you can always google it.

The assumptions are where things get a little tricky. For Hardy-Weinberg to work, a number of conditions must be met. The population must be very large, there can't be migration into or out of the population, there can't be mutation, and there can't be any form of selection going on. It's clear that there will be few cases where this is true, but there are lots of cases where it comes close.

The blonde gene, if we consider the entire human race, is one of them. The mutation rate for the gene is going to be so low we can ignore it, the population is very large, and if we are talking about the whole planet, there's not a lot of migration to worry about. Selection is a possibility, of course, but peroxide's taken that one out of the equation.

Since the Hardy-Weinberg conditions are met on a global level, we can safely say that the frequency of the "blonde allele" isn't going to change in the species as a whole. That means that blondes won't ever go entirely extinct.

That takes care of the gene that causes blondeness. But what about the number of blondes? Will that change? Actually, it might - in fact, it's likely to drop. That's becuase of ethnic differences and our increasingly international society, and it's what I'll talk about when I write the next post in the series.