“Is a dominant version of a trait always more common than the recessive one?”
Does the dominant allele for a given trait show up as the prevalent phenotype in the population at large? For example, the presence of dimples is controlled by a dominant allele, but the majority of people do not have dimples. Why is this?
Editor’s note (3/15/21): At the time this article was written, dimples were described as simple traits caused by one (or two) genes. We now know that the inheritance of dimples are not as predictable as described below. Instead, having dimples is an example of a complex trait, which is affected by many different genes plus environmental factors. The inheritance of this trait can be attributed to multifactorial inheritance, where traits are determined by a combination of many genes and environmental factors.
As you've noticed, dominant does not mean common. Dominant just means that it'll win out over recessive.
How common a trait is has nothing to do with this. It depends on how many copies of that gene version are in a population.
So having dimples is rare because the dimple version of the gene is rare. And as long as there is no advantage or disadvantage to having dimples, then the relative amounts of people with dimples won't change over time in a stable population.
Let's go a bit deeper to see what I mean. As you've said, dimples are dominant over not having dimples. In genetics, the dimple version of a gene is D and the non-dimpled version is d.
Remember, we have two copies of most of our genes, one from mom and one from dad. You will have dimples if both copies are D (DD) or if only one is D (Dd). That is the definition of dominant.
And of course, if both copies are d (dd), then you don't have dimples. In our world, most people do not have dimples. There are a few DD and Dd types around, but most everyone is dd.
And it'll stay this way even though dimples are dominant over not having dimples. Why? Because D is rare. And if there is no advantage, then there is no reason for it to increase in the population.
Dimples are dominant yet rare
A common way to look at this is to think about all of the gene versions as marbles in a bag. We throw 998 blue marbles that represent d (no dimples) and 2 brown marbles that represent D (dimples) into a sack and mix them up. We then pull out two.
Odds are you're going to get two blue marbles. Occasionally you'll pull out a brown but almost always you'll get two blues. Which means almost always the kids won't have dimples.
After you take out the marbles, you return them to the sack and start again. As long as there is no increase in the number of brown marbles, the results will be the same over and over.
Now I was never particularly satisfied with this example. It makes sense and a lot of people find it useful but what I need is an example that really deals with gene versions and not marbles.
So let's say we have 11 dd people and 1 Dd person. The Dd person has 4 kids with one of the dd folks and each dd couple also has 4 kids.
As you can see below, using regular old Mendelian genetics, we'll have 20 dd people from our 5 dd couples and 2 Dd and 2 dd from our mixed couple. This is 2 people with dimples and 22 people without. The same ratio!
Now these folks all pair up randomly and have 4 kids each. Since Dd is very rare, odds are they'll find a dd for a mate. If they have 4 kids each, then we have 44 dd and 4 Dd. Again the same 11:1 ratio.
Even though D is dominant, it doesn't increase in the population.
In other words, there is no change in the percentage of D's in the population. Eventually, assuming no advantage for or against dimples, the ratio of people with and without dimples will settle down and stay constant. This is because the ratio of D's to d's stays the same.
So, dominant does not mean common because dominant does not mean frequent. O blood type predominates in most of the world even though it is recessive to A or B. Same thing with light colored eyes in Scandinavia.
A gene version can dominate a recessive one but be rare like Huntington's Disease. Gene versions only become more common by virtue of their selective advantage or by luck. Not by being dominant.