What is the genotype of someone who is heterozygous for Type B blood?

A person who is heterozygous for Type B blood has the genotype I^B i, where I^B represents the allele for Type B blood and i represents the allele for Type O blood. This means they carry one allele for Type B and one for Type O. As a result, they express the Type B blood phenotype due to the presence of the dominant I^B allele.