What is Blood type a combination of alleles?

Blood type is determined by the presence of specific antigens on the surface of red blood cells, which are encoded by alleles inherited from parents. The ABO blood group system has three main alleles: A, B, and O. The A and B alleles are co-dominant, meaning if both are present, both will be expressed, resulting in blood type AB. The O allele is recessive, so an individual must inherit two O alleles to have blood type O; otherwise, the presence of A or B will determine the blood type.