Just because a trait is dominant does not mean it is common. Each color represents different levels of light eyes. Blue=80%+, teal=50-79%, olive=20-49%, brown=1-19%, black=none. Image courtesy of Dark Tichondrias
One of the first things we’re taught in genetics is that some traits are dominant and others are recessive. And that the dominant traits trump the recessive ones.
So brown eyes trump blue eyes. And red hair is always trumped by other hair colors. And so on.
From this, people often jump to the conclusion that the dominant trait is also the most common one. This isn’t always the case and there is no reason it should be.
Whether or not a trait is common has to do with how many copies of that gene version (or allele) are in the population. It has little or nothing to do with whether the trait is dominant or recessive.
Let’s take eye color as an example. The decision on whether to have brown eyes or not is pretty much controlled by a single gene, OCA2.
We can think of OCA2 as having two versions, brown and not-brown. The brown allele of OCA2 is dominant over the not-brown allele.
Nearly everyone in most of Africa has brown eyes. This isn’t because brown eyes are dominant over blue and green. Instead, it is because there are mostly brown alleles of OCA2 in the African population.
Northern Europe is a different story. In some parts of the continent, over 80% of the population has lighter colored eyes. Here the not-brown allele is more common even though it is recessive.
Now this allele isn’t exclusive, there are still brown-eyed folks in northern Europe. So why don’t their brown eyes dominate over time? Because in populations, dominant isn’t dominant over other people’s recessive gene versions. Your brown eyes can’t affect my kids’ eye color unless we get married.
Let’s do a thought experiment to make this clearer. To simplify things we’ll call brown eyes B and not-brown eyes b.
Without some sort of outside pressure, the ratio of blue to brown eyes stays the same.
Remember, you will have brown eyes if you are BB or Bb and blue or green if you are bb. This is because brown (B) is dominant over blue and green (b).
Imagine we start out with eleven bb people and one Bb person. The Bb person has 4 kids with one of the bb folks and each bb couple also has 4 kids.
Using regular old Mendelian genetics, we’ll have 20 bb people from our 5 bb couples and 2 Bb and 2 bb from our mixed couple. This is 2 people with brown eyes and 22 people with blue or green. The same ratio as we started with. Brown did not become more common.
Now these folks all pair up randomly and have 4 kids each. Since we aren’t going to allow incest, the Bb folks will find a bb for a mate. If they have 4 kids each, then we have 44 bb and 4 Bb. Again the same eleven blue to one brown ratio. Whether an allele is dominant or not does not affect how common a trait is.
Now of course traits can become more common over time. The changes just don’t have anything to do with whether the trait is dominant or not.
If brown eyes gave an advantage, then it would start to become more common. Brown eyes would also become more common if a bunch of Africans moved in or many of the blue-eyed people were killed for some reason (witch burning?). And there are other ways too of getting more brown eyes in Europe. Or more blue eyes in Africa (see South Africa for example).
Dominant does not mean common. Which in some ways is a good thing considering diseases like Huntington’s disease that are dominant.