Blue eyes are a recessive trait in humans. I read an article recently which stated:

People with blue eyes have a single, common ancestor, according to new research.

A team of scientists has tracked down a genetic mutation that leads to blue eyes. The mutation occurred between 6,000 and 10,000 years ago. Before then, there were no blue eyes.

"Originally, we all had brown eyes," said Hans Eiberg from the Department of Cellular and Molecular Medicine at the University of Copenhagen.

Which appears to be a mainstream hypothesis today. My question is, if there is a genetic mutation that occurred in one individual which made their eyes blue, and the rest of the population is brown eyed, how does this make so many individuals blue eyed? If the blue eyes are recessive, the individual's children would mix with the other brown-eyed individuals and their offspring will be brown-eyed. How does anyone treat this theory seriously?

  • $\begingroup$ I think you just stopped at the F1 generation, just take it a step further and you have your answer (I know I am using the terminology miserably)... $\endgroup$ – another 'Homo sapien' Jun 27 '17 at 17:01
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    $\begingroup$ "The blue eyes are recessive in humans" - This is not fully true; there is one recessive blue eyes allele for one gene but eye color is actually polygenic. That said, the answer below covers clearly why recessive alleles don't just disappear which is the more important part of your question. $\endgroup$ – Bryan Krause Jun 27 '17 at 18:39
  • $\begingroup$ This single-mutation theory seems over-simplified. What about people with other eye colors, like gray or green? Or two eyes of different colors, or even two colors in the same eye? (Hetereochromia) $\endgroup$ – jamesqf Jun 28 '17 at 18:30

@another 'Homo sapien' is basically correct, and what they were getting at, is that in just the F1 generation, you're right, any recessive traits are masked. However, if you continue this onward, in subsequent breeding, recessive traits are unmasked. (This is basically why inbreeding is a bad thing).

If we use the traditional Punnet square method for determining genotype:

if A is a Brown-eye dominant allele, but a is a blue-eye recessive allele:


enter image description here

You can see that a single recessive allele is now present in 50% of the offspring of a given pair of parents.

Now, if in F2, we assume that 2 of those recessive offspring were to mate, the recessive gene would be unmasked in 1 quarter of the offspring (statistically speaking), and 50% would be carriers.


enter image description here

Now it gets a little more complicated. Our double recessive F2 member has blue eyes phenotypically, they aren't just a recessive allele carrier.


So, if in F3, our aa mates with another aa, (ignoring spontaneous mutation for simplicity) it should be obvious that all of their offspring in F3 would have blue eyes (and I can't quickly find a Punnett square in Google showing this).

However, if our aa now mates with an AA (homozygous dominant brown), we will get 100% offspring with a heterozygous Aa genotype - in other words, all blue eye carriers, but with brown eyes phenotypically.

If aa mates with a heterozygous individual though (Aa), we have the following:

enter image description here

And thus, 50% blue eye individuals, with 50% brown heterozygous dominant. And so on and so forth, ad infinitum.

Hopefully that explains how a recessive allele can go to fixation despite only occurring once.


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