A person having genotype  $I^A{I}^B$ would show the blood group as AB. This is because of 

ABO system of blood group in humans is controlled by I  gene having three alleles $I^A, I^B$ and i. Alleles $I^A \mathrm{and} I^B$ are dominant over allele i. But when $I^A \mathrm{and} I^B$ are present together in an individual, they show co-dominance and as a result, the phenotype is AB blood group.