The blood group AB phenotype arises when both A and B alleles are expressed simultaneously without blending, reflecting codominance. Each allele independently produces its respective antigen on red blood cells, so both A and B antigens are present. This differs from incomplete dominance, where heterozygotes have an intermediate phenotype. Blood group inheritance is autosomal, but not recessive or X-linked. Understanding codominance is crucial for accurate blood typing and transfusion compatibility in clinical practice.

Reference: Robbins Basic Pathology, Kumar et al., 10th Edition.