What determine how often a phenotype occurs in a population?
Phenotype is determined by what genes you carry. There are many variations on a gene called alleles. Depending on what alleles you inherit from your parents (use a Punnet Square for that) you get a different phenotype. In a population, mating patterns can also be a big thing. Often times, animals will refuse to mate with creatures that have different appearances or phenotypes than them. Because of this, that particular phenotype is rarely seen.
