Human blood consists of blood groups A, B, and O. However, there is one more blood group, AB, which contains alleles A and B in the same genotype. What is this phenomenon called

It is codominant inheritance because, if the placement of the A and B molecules on each cell is controlled by the proteins that are coded by different versions of the same gene, then IA and IB are codominant but both are dominant to I o. If a person receives an IA allele and a IB allele, their blood type is type AB, in which characteristics of both A and B antigens are expressed.