What defines codominance in genetics?

Codominance in genetics is characterized by the expression of both alleles in a heterozygous genotype, where neither allele is masked by the other. This results in a phenotype that simultaneously displays the characteristics associated with both alleles, such as in AB blood type in humans, where both A and B alleles are fully expressed.

In contrast to dominant and recessive relationships, codominance reveals a situation where neither allele is dominant over the other, leading to a unique phenotype that is different from that of individuals possessing either allele alone. Thus, when both alleles are present, they contribute equally and visibly to the organism's appearance or traits, confirming their dominance in a different sense compared to complete dominance scenarios.