* Codominance means that both alleles in a heterozygous genotype are expressed equally.

* Blood type is determined by the presence or absence of certain antigens (proteins) on the surface of red blood cells.

* The ABO blood group system has three alleles: A, B, and O.

* Alleles A and B are codominant. This means that if an individual inherits both the A and B alleles, they will express both antigens on their red blood cells, resulting in blood type AB.

Here's a breakdown:

* Allele A: Produces the A antigen.

* Allele B: Produces the B antigen.

* Allele O: Does not produce any antigens.

Possible genotypes and their corresponding phenotypes:

* AA or AO: Blood type A (expresses A antigen)

* BB or BO: Blood type B (expresses B antigen)

* AB: Blood type AB (expresses both A and B antigens)

* OO: Blood type O (expresses neither A nor B antigen)

In summary, codominance in the ABO blood group system allows both the A and B antigens to be expressed simultaneously in individuals with the AB genotype, resulting in blood type AB.