Here's why:

* Codominance means that both alleles (versions of a gene) are expressed equally in the phenotype (observable traits).

* In the ABO blood group system, there are three alleles: A, B, and O.

* Individuals with blood type AB have both the A and B alleles. These alleles are codominant, meaning that both A and B antigens are expressed on the surface of red blood cells.

* This results in a unique blood type distinct from either A or B alone.

Let me know if you'd like to learn more about blood type genetics!