There are four types of blood groups in human population namely A, B, AB and O. They are determined by presence or absence of two types of RBC surface antigens / sugar polymer A and B. Individuals with blood group A have antigen A, while-those with group B have antigen B, AB have both the antigens and 'O' Persons do not have any antigen. The type of antigens and their presence or absence is controlled by gene I which has three alleles IA, IB and i. IA produces antigen A, IB antigen B whereas allele i (1o) does not form any antigen and is reessive. IA and lB are dominant over i and show dominant-recessive relationship. When IA and IB both are present together in a person, both express themselves equally and independently and produce the surface antigen A and B and therefore show the phenomenon of co-dominance and such genes are called as codominant Because human beings are diploid individuals, each person therefore may have any two of the three alleles of gene I. This results into six different genotypic combination and four phenotypic expressions as follows :
Table showing genetic basis of blood groupings
Blood group alleles thus show both codominance & dominance relationship.
