In codominance, neither allele in the heterozygote crowds out the phenotype of the other. The individual expresses both phenotypes. The alleles which code for the A and B blood type antigens are codominant. Both alleles are fully expressed when they occur together.

The distinction between incomplete dominance and codominance is the kind of thing MCAT writers love. In incomplete dominance, only the dominant allele is expressed. The recessive allele is not expressed, but there is not enough active gene product to create the same dominant phenotype seen with the homozygous dominant individual. In codominance, the phenotypes of both alleles are expressed.