Codominance is a genetic phenomenon where both alleles in a pair contribute equally and visibly to the organism’s phenotype. The key to recognizing codominance lies in observing traits that are expressed simultaneously. A classic example can be found in the ABO blood group system in humans. In this system, individuals with IA and IB alleles both express the A and B antigens on the surface of their red blood cells, leading to blood type AB.
To identify codominance, look for instances where neither allele masks the other, resulting in a distinct phenotype that showcases both traits. For example, in some plants, a cross between red and white flowered plants can produce offspring with flowers that are both red and white—this is another clear demonstration of codominance. By noting such characteristics, one can distinguish codominance from other inheritance patterns like complete dominance or incomplete dominance.