Inheritance in the human blood group shows multiple alleles, dominance as well as codominance. a. The Rh(D) group is also dominant and will be expressed if inherited from either parent. b. Within the ABO Blood Group system, the A and B genes are co-dominant, i.e. these will be expressed whenever the gene is present. The O gene is silent and only expressed when neither A nor B is present. c. Three alleles are responsible for the different types of blood groups in humans which shows multiple allelism.