The key recognition of codominance is the simultaneous expression of both dominant alleles in the heterozygous genotype, resulting in distinct and separate phenotypes. In codominant inheritance, the dominant alleles do not mask or suppress each other, leading to the expression of both traits. This phenomenon is in contrast to complete dominance, where only the dominant allele is expressed, and incomplete dominance, where the heterozygous genotype exhibits an intermediate phenotype. Codominance is commonly observed in specific genetic systems, such as the ABO blood group system in humans, where the presence of different blood type alleles (A and B) results in distinct phenotypes (blood groups A, B, and AB).