In codominance, how do the alleles interact in the phenotype?

In codominance, both alleles contribute to the phenotype in a way that they are both fully expressed simultaneously. This means that neither allele is dominant over the other, leading to an organism displaying characteristics of both alleles without blending. A common example of codominance is seen in the ABO blood group system in humans, where individuals with type AB blood have both A and B antigens on the surface of their red blood cells, showcasing the traits imparted by both alleles distinctly. This characteristic of codominance allows for the expression of traits from both alleles in a straightforward manner, rather than one overshadowing the other or blending together.