In humans, AB blood group shows codominance. Here, allele $\mathrm{I}^{\mathrm{A}}$ for A-type blood is codominant with allele $\mathrm{I}^{\mathrm{B}}$ for B-type blood. In heterozygotes, $\mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{B}}$ expresses the characteristics of both A and B antigens, i.e. $\mathrm{I}^{\mathrm{A}} \mathrm{I}^{\mathrm{B}}$ possess RBCs that carry both A and B antigens at their surface.