system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is polydactyly so uncommon when it is a genetically dominant trait?

Shouldn't dominant traits become more popular over time?
gpt
Dominant traits don't in general become more common over time. 

This is somewhat counter-intuitive. You generally have two copies of each gene, one from each of your parents. The gene that expresses itself in you is called dominant. However the other gene is still there and you will have a 50% chance of giving it to your child. Dominance or recessiveness doesn't, in itself, affect the prevalence of that gene in the population.

The usefulness of a genes expression is what causes its prevalence to change over time.