The abo blood type is controlled by a single locus, called i, and the human population harbors three alleles: i a, i b , and i o . each individual, of course, only carries two alleles at the locus. these alleles give rise to the blood type phenotypes, a, b, ab, and o. the relationship between genotype and phenotype is complicated. i a is dominant to i o , i b is also dominant to i o , and i a and i b are codominant, which means that the effect of both can be seen in the heterozygote.