Condition where the heterozygote displays the phenotype associated with both alleles.
Contrast with both complete and incomplete dominance. Codominance is typically illustrated in terms of the ABO Blood group system, i.e., where the alleles underlying types A and B blood are codominant to each other.
Note that with codominance, since the heterozygote possesses a different phenotype from either homozygote, the phenotypic and genotypic ratios coincide. That is, in a monohybrid cross both would be 1:2:1 ratios, with the 2 referring to the heterozygote frequency (rather than 3:1 ratios, for example).
Note also that most phenotypes as measured in molecular terms are codominant. This is because so long as we have some means of distinguishing between two different gene products, such as proteins, then in effect some aspect of both phenotypes are present. Indeed, the ABO Blood system simply is a situation where there is a molecular distinction between the products of different alleles, just one that is detectable by serological means, a relatively old "molecular" technology (and also by physiological means, i.e., upon blood transfusion.