A natural question that arises is how can we translate genotypes to phenotypes; given a specific genotype, what phenotype will be displayed? This question is simplified in the case of homozygotes because both alleles correspond to the same phenotype.

 

For example, let's say that the B allele encodes the brown eye phenotype and the b allele encodes the blue eye phenotype. Then, it follows that a BB homozygote will certainly have brown eyes and a bb homozygous individual will definitely have blue eyes. However, what happens in the case of a heterozygous Bb individual?

​

In the case of eye color, a heterozygous Bb individual will have brown eyes. This brown eye phenotype (and therefore the B allele) is said to be dominant, whereas the blue eye phenotype (and the b allele) is recessive. In fact, in genotype notation, uppercase letters denote dominant alleles whereas lowercase letters denote recessive alleles.