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.
More on genetics
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.
Explanation:
