Co-Dominance/Multiple Alleles

Till now we have seen that the F1 generation resembles either of the two parents and not both. However, in co-dominance, the F1 generation resembles both parents. The human blood group is an excellent example of co-dominance. The ABO blood groups are controlled by a single gene I, but it has multiple alleles (IA, IB, and i). RBC with allele 'i' does not possess  any sugar polymer on its surface.

The plasma membrane of the RBCs contains sugar polymers, and the kind of sugars is controlled by the gene. Since humans are diploid, each person contains any two alleles. For example, an offspring will possess a blood group A when both of his/her parents possess IA allele. 

Following are the possible blood type combinations:

These are the possible combinations of blood types.