Talk about the case of polyallelism.

When three or more alleles of the same gene are present in a population, we say that the locus has multiple alleles. Any diploid organism has, at most, two homologous gene loci that may be occupied by different alleles of the same gene. The simplest case of multiple alleles occurs when three alternative alleles of one gene exist. This situation is illustrated in the inheritance of the ABO blood groups in humans A, B, AB, and O are all types of present antigens that there present is tested using antibodies of them, O has no antigens. A crossmatch (compatibility test between a donor and a patient) is essential before blood transfusion. Group O is the universal donor; group AB is the universal recipient.