Explain multiple alleles with respect to ABO blood group in humans

An allele is a variant or alternate form of gene which are found on same gene loci. Humans are diploid organisms having two alleles for a given gene. However in nature more than two variants are possible. One such example is multiple alleles in human ABO blood group. The blood groups known are A, B, O and AB which are determined by a single gene 'I'. having three alleles. The allelic forms are represented as I^A, I^B and I^o for A, B and O antigen respectively. There are 6 possible genotypic combinations resulting in the four blood groups determined by the presence of the two inherited alleles.Now, I^A, I^B are codominant that is in the presence of each other both antigens are expressed where as I^o is recessive to both (i.e expressed only in homozygous condition).
I^AI^A I^A I^o - results in blood type AI^BI^B I^B I^o - Results in blood type BI^AI^B - Results in blood type ABI^o I^o - results in blood type O