Codominant expression occurs when two different alleles are inherited at the same genetic location (i.e, one allele from mom and a different one from dad), and the products of BOTH of the alleles are expressed. Codominance is the most common pattern in blood group genetics. For example, in the ABO system, if both A and B alleles are inherited, then both A and B antigens are expressed on the RBC (blood group AB). This differs from dominant/recessive inheritance that everyone learned in high school (for example, the “brown eyes” allele is dominant over the “blue eyes” allele). Codominance has implications for the quantity of antigens on the RBCs, as well (see the discussion of the Dosage Effect for more details.

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