Non Mendelian Genetics

Human beings have four blood groups - A, B, AB and O. The blood groups are determined by two types of antigens present in the surface coating of red blood cells - A and B. The antigens occur on oligosaccharide rich head region of a glycophorin. Blood group A persons have antigen A, group B have antigen B, AB have both antigens while blood group O persons do not carry any antigen in the coating of their erythrocytes. ABO blood group system in human beings is an example of dominance, co-dominance and multiple alleles. Blood group A and B show dominance, co-dominance is shown by blood group AB. In AB blood group, alleles for blood group and blood group are codominant, so characterised by both antigens. Multiple allelism is shown by ABO blood group as blood group is controlled by three alleles, only two of which are present an an individual.