In heretozygous condition, the individual expression of both the alleles in the phenotype is exemplified by

ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles IA, IB and i.  When IA and IB are present together they both express their own types of sugars: this is because of co-dominance. Hence red blood cells have both A and B types of sugars. Therefore AB blood group is always expressed as IAIB.