There is this misconception that red-haired people are going to die out. This person on the Internet ("howstuffworks") also connected it to a marketing campaign of a hair dyer company. But I'm interested in a specific citation that he gives in that text, because it's cited on Wikipedia (so I want to check it):

While redheads may decline, the potential for red isn't going away. ["Red Alert." National Geographic. September 2007. Page 14.]

I don't have that National Geographic magazine, but I wonder: Why should the population of redheads decline? If it doesn't have a selection disadvantage, the frequency of the alleles causing red hair (which are recessive) should stay equal and if further assuming random mating in the human population, also the frequency of people with red hair, should stay equal, shouldn't it? So is the howstuffworks author wrong?

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    $\begingroup$ I don't think random mating is a realistic assumption... $\endgroup$ – barbecue Jun 16 at 22:00
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    $\begingroup$ The howstuffworks article notes that National Geographic did not endorse or provide any evidence for a claim that "redheads may decline". To me, that wording just means they don't know one way or another (may [or may not]). All these sorts of silly claims may be related to the eugenics movement of a hundred years ago, which thought that "undesirable" recessive genes could be eliminated from populations by preventing homozygotes from breeding. Of course, they didn't realize that the majority of recessive alleles in a population are typically in asymptomatic heterozygotes. $\endgroup$ – Armand Jun 20 at 19:00

Let's consider a possible scenario, with a simplified assumption that red hair is a simple Mendelian recessive trait.

Let's say we have a population that is 10% red hair. Everyone with red hair lives on one island, and they only breed among each other. They all have red hair, so we can infer they are all homozygous recessive (rr) for that trait. Everyone without red hair lives on the other island. Red hair never appears in this population, so we can infer they are all homozygous dominant for the non-red hair trait (RR).

Now, we develop inter-island transportation, and breeding becomes randomly assorted among the whole population. The allele frequencies stay the same: 10% r and 90% R. There is no selection based on the red hair trait. However, from Hardy-Weinberg we would calculate that only q^2 = 0.1^2 = 1% are homozygous recessive. 2 * p * q = 2 * 0.9 * 0.1 = 18% are heterozygous carriers for red hair, but they don't express the phenotype.

Just by changing the mating scheme, you can go from 10% with red hair to 1% without any change in allele frequencies.

It seems reasonable to infer that something like this is happening in the real world. People with red hair are much more common in certain places in the world. When people tend to mate only with people in their local communities, the likelihood of homozygous recessive individuals is higher than when people immigrate and mate with partners around the world. That doesn't mean there is any selection for the red hair allele, just a consequence of the mating scheme. That's the takeaway from the National Geographic quote you found on Wikipedia:

"while redheads may decline, the potential for red isn't going away"

In this statement, "redheads" are the homozygous recessive folks that express the red hair phenotype. "Potential for red" is a colloquial way of referring to the allele frequency, "r".

Of course you could also get actual changes in allele frequency for reasons that have nothing to do with fitness of the red hair phenotype. If you think about population growth on a regional basis, if there is more population growth in countries with lower frequency of a red hair allele, then the frequency of red hair in the human population as a whole would decrease. It seems likely this is occurring today since regions with higher frequency of red hair (northern Europe/UK/Ireland, also the US) have lower population growth than those with lower frequency (Asia, Africa, South America). There's no hint that this would result in a disappearance of the trait, though.

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    $\begingroup$ In the end a clear "yes/no" answer is kinda missing, I read the piece and the conclusion is not really clear to me. $\endgroup$ – PascalVKooten Jun 17 at 18:02
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    $\begingroup$ @PascalVKooten I read and answered the biology question in the title and explained the National Geographic quote. If by yes/no you're meaning the question about whether the "howstuffworks author" is right or wrong, I don't really have any interest in addressing that especially because it's a long piece with a lot of different statements, most of them not about biology. $\endgroup$ – Bryan Krause Jun 17 at 18:41
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    $\begingroup$ @PascalVKooten - "While redheads may decline, the potential for red isn't going away." According to this answer, that's a correct statement. And in the future there will be no redheads is almost undoubtedly false. $\endgroup$ – Mazura Jun 18 at 1:13
  • $\begingroup$ Thanks Bryan for this extensive answer. I will search for evidence for the changed mating patterns that you mention in the coming days and include it in this answer. $\endgroup$ – akraf Jun 21 at 9:08

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