Q:

What determines how often a phenotype occurs in a population?

A:

Different versions of the same gene, called alleles, determine how often a trait appears in a population. Children inherit one allele from each parent. One allele is always dominant and appears more often, while one is recessive and only appears if the dominant allele is not present.

Each allele can appear in one of three combinations: two dominant alleles, one dominant and one recessive allele, or two recessive alleles. In the first two cases, the phenotype associated with the dominant allele is present in the offspring, since the dominant allele overrides the recessive one at the genetic level. It is only when two recessive alleles are present that a recessive phenotype appears.

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