Ans. d. Codominance (Ref Robbins 9/e p140; Ganong 23/e p527)Inheritance of ABO blood group is Codominance."Although Mendelian traits are usually described as dominant or recessive, in some cases both of the alleles of a gene pair contribute to the phenotype--a condition called codominance. Histocompatibility and blood group antigens are good examples of codominant inheritance. "--Robbins 9/e p140Codominance is a relationship between two versions of a gene, individuals receive one version of a gene, called an allele, from each parent. If the alleles are different, the dominant allele usually will be expressed, while the effect of the other allele, called recessive, is masked. In Codominance, however, neither allele is recessive nor are the phenotypes of both alleles are expressed, e.g., ABO blood group, where in AB blood group both A and B are dominant."Codominance: When both alleles of a gene pair contribute to the phenotype. E.g., Blood group AB.ABO Blood Group SystemA and B antigens of the ABO blood group system are glycoproteins present on the RBC membrane.H substance is the immediate precursor on which A and B antigens are added.H substance is formed by the addition of fucose to the glycolipid or glycoprotein backbone.The subsequent N-acetyl glucosamine creates the A antigen, while the addition of galactose produces the B antigen.Bombay phenotypeIndividuals with the rare Bombay phenotype (hh) do not express the H antigen (also called the H substance), the antigen, which is present in blood group OQ.As a result of the absence of the H antigen they cannot make either the A antigen or the B antigenQ.These individuals have antibodies not only against A and B antigens but also against the H antigenQ.