What determines whether an allele is recessive or dominant? Through reading Wikipedia I have encountered one theory: gene A has two alleles which actually determine something important, like Parkinson's disease. In addition, gene M has two alleles which don't really do anything on their own, but they determine which allele of gene A is recessive and which is dominant (this mechanic is easy to model: you have two copies of each gene, making four total, and you need any two of the four to be flipped to "Parkinson"). The evolution fairy then makes one version of gene M universal in the population. This seems iffier to me: gene M and gene A act symmetrically, so why would one become universal and the other not? But maybe gene M is responsible for the dominance or recessiveness (?) of multiple genes, so there's greater selection pressure because it's relevant more often. After this, the dominance or recessiveness of gene A becomes essentially fixed, and we talk about one allele being dominant and the other recessive. However, the other thing that the (very short) Wikipedia article on evolution of dominance says is that this hypothesis has been discredited, and is no longer generally believed. It's unclear to me why not to believe it, and what the alternative is, but there you have it.