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 Io. 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.