Blood typing is an example of codominance.
The blood type is controlled by a single gene (I gene) which has three alleles A, B and i.
The IA allele gives type A,
IB gives type B, and
i gives type O.
Individuals with IAIA or IAi have type A blood, and individuals with IBIB or IBi have type B. IAIB people have both phenotypes, they express codominance (characteristics of both alleles expressed). ii genotype is for O blood group.