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There is a widely-held belief that there will no longer be humans with red hair in the future (sometimes cited by 2060 or 2100).

For example, these Facebook groups:

The justifications given include:

  • red hair is a recessive gene (Seattle Times):

    If predictions by the Oxford Hair Foundation come to pass, the number of natural redheads everywhere will continue to dwindle until there are none left by the year 2100.

    The reason, according to scientists at the independent institute in England, which studies all sorts of hair problems, is that just 4 percent of the world’s population carries the red-hair gene. The gene is recessive and therefore diluted when carriers produce children with people who have the dominant brown-hair gene.

  • climate change (The Week):

    Only 1 to 2 percent of people in the world have red hair — and that number could continue to decrease due to climate change, scientists warn.

Is this just a myth, or will ginger hair soon cease to be?

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    I removed the claim that the extinction was due to "genetic admixture", because I couldn't find anyone claiming it. I put other justifications I did find into the question. Note: I had to cherry pick my search results, because most of the results were debunking the claim. Does the Internet really need this question asked again? – Oddthinking May 1 '16 at 22:48
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    Isn't the statement 'the gene is recessive and therefore diluted' debunked by even the most basic knowledge on how inheritance works? Unless it has impact on the ability to reproduce, a recessive trait is passed on to children with the same probability as a dominant trait . – Tor-Einar Jarnbjo May 2 '16 at 10:09
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    If 4 percent of the world’s population carries the red-hair gene, then even in the unlikely event that all of these were single recessive cases (so there were no red haired people in a particular generation) then 2 percent of the relevant alleles would be red-hair genes and with random mixing 0.02*0.02*7 billion i.e. 2.8 million of the next generation would show red hair. But if 2 percent of the world currently actually has red hair despite it being a recessive gene, then 4 percent of the world carrying the gene looks like an underestimate – Henry May 2 '16 at 12:44
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    @Oddthinking, sometimes a claim is too basic to be easily debunked: it's very easy to find explanations of why the sky is blue, but much harder to find support for the statement that it is blue. – Mark May 3 '16 at 1:14
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    From Brit Lab: youtube.com/watch?v=QDMEIrkt7Po – Jan Doggen May 5 '16 at 16:01
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Some phenotypes caused by recessive genes occur in sub-populations that tend to reproduce with other members of the sub-population.

For example, Tay-Sachs disease is a recessive genetic disease caused by having two bad alleles of a gene, and mainly occurs in specific sub-populations who reproduce with each other:

Tay–Sachs disease (also known as GM2 gangliosidosis or hexosaminidase A deficiency) is a rare autosomal recessive genetic disorder.

...

who described in 1887 the cellular changes of Tay–Sachs disease and noted an increased disease prevalence in Ashkenazi Jewish people.

Research in the late 20th century demonstrated that Tay–Sachs disease is caused by a genetic mutation in the HEXA gene on (human) chromosome 15. A large number of HEXA mutations have been discovered, and new ones are still being reported. These mutations reach significant frequencies in specific populations. French Canadians of southeastern Quebec have a carrier frequency similar to that seen in Ashkenazi Jews, but carry a different mutation. Cajuns of southern Louisiana carry the same mutation that is seen most commonly in Ashkenazi Jews. HEXA mutations are rare and are most seen in genetically isolated populations. Tay–Sachs can occur from the inheritance of either two similar, or two unrelated, causative mutations in the HEXA gene.

If individuals with the alleles for red hair continue reproducing at the same rate that others are, then the frequency of the red-hair allele will remain the same.

However, if populations with a high frequency of the red-hair allele decide to stop reproducing predominantly with other members of the same sub-population, then it's possible that the overall frequency of the red hair phenotype may decrease, as the number of individuals homozygous for the red hair allele will decrease, and the number of individuals heterozygous for the allele will increase.

The section Deviations from Hardy–Weinberg equilibrium in the Wikipedia article on the Hardy–Weinberg principle also states that non-random mating, also known as inbreeding, causes an increase in homozygosity.

Interracial marriage is becoming more socially acceptable in the United States (XKCD), and is becoming more common (blog post citing Pew Center research). If the same trend is happening in Scotland and Ireland, which have a high proportion of redheads (do I really need a citation?), then this will lead to a decrease in the number of individuals homozygous for the red head allele, and therefore fewer redheads.

  • I was planning to self-answer this question, but seeing this I'm glad I didn't: +1 for the sheer effort. – wizzwizz4 May 2 '16 at 14:26
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    You explain very well how their numbers might be changing. The question is if they actually are. Will there be less red heads in the future? – fredsbend May 2 '16 at 18:56
  • I don't think a predictive answer is possible. Data on breeding habits of high red-haired populations is likely difficult to find, if not impossible. The answer does provide a plausible mechanism for how it could happen, and demonstrates that the current social situation seems to provide opportunity for that mechanism. – Ethan May 2 '16 at 20:55
  • @fredsbend Under the premise that the existence of the allele is not decreasing in numbers (nothing is selecting against red-haired people/carriers), it is not, as a trait, going extinct. We may by chance see less red-heads if they have more kids with people who have dominating alleles, but they will still be present and can pop up. Hypothetically, the allele (expressed or unexpressed) could be growing in the overall population, and we could see an increase of them later when they're more populous in a broader group. – Harris May 2 '16 at 20:58
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    I agree with @fredsbend, this answer gives a nice introduction to the genetics around this question, but doesn't answer it. If redheads marry with non-redheads, then their offspring will unlikely be red headed. However, do they? I thought this was the whole point of the question -- not "is the red head gene recessive?" but "is this recessive gene disappearing?" – Sklivvz May 2 '16 at 22:59

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