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How does codominance produce the AB blood type in the ABO blood group system?

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Four kittens with different fur patterns-three tabby and one black-and-white-lying together in a box, illustrating genetic variation among offspring. — heredity Each offspring is a combination of its two parents, receiving some dominant traits from its mother and others from its father.

In the ABO blood group system, the production of type AB blood is a classic example of codominance within a system of multiple alleles.12

The gene governing ABO blood types consists of three primary alleles: I^A, I^B, and I^O. While the I^O allele is recessive, the I^A and I^B alleles are codominant. Codominance is a genetic phenomenon where two different alleles are expressed to an equal degree, allowing the traits associated with both to be displayed simultaneously.213

When an individual inherits both the I^A and I^B alleles (genotype I^AI^B), neither allele masks the other. Instead, both are expressed, resulting in the presence of both A and B antigens on the surface of the red blood cells. This simultaneous expression of both antigens defines the AB blood type.1245