The A, B, and H antigens, like many other blood group antigens, are the expression of genes inherited from the previous generation. If the antigen is demonstrated, the gene controlling it must have been inherited from one or both of the parents.
As previously mentioned, the genes A, B, and O are allelic genes. Assuming the production of H substance, these three genes, in various possible combinations of two, account for the four recognized ABO groups: A, B, AB, and O. Each individual inherits two ABO genes, one from each parent, and these genes determine which ABO antigen will be present on that individual’s red cells. These genes exhibit co-dominance, meaning that if both A and B genes are present, both will be expressed.