Blood types are controlled by multiple alleles. There are actually three different alleles; A, B, and O that determine a person's blood type. (Although there are three alleles possible, remember that each person only has two genes for every trait.) Of the three alleles, A and B show incomplete dominance. This means that a person possessing both A and B alleles as their genotype, has AB blood because both alleles are expressed in the phenotype. The O allele however, is recessive to both the A and B allele. Therefore, a person possessing both A and O alleles will have a blood type of A. Likewise, a person with both B and O alleles will be blood type B because B is dominant, and therefore, masks the O allele. The following table indicates all of the possible allele combinations and genotypes. Move your cursor over the phenotypes to discover the possible blood groups produced by each genotype. Note: The "A" and "B" alleles are written as capitalized letters to indicate that they are dominant alleles. However the "o" allele is written as a lower-cased letter to indicate it is recessive. (In textbooks you will see the A, B and o alleles written as superscript letters above the letter "i". It is written this way to distinguish it as incomplete dominant alleles as opposed to codominant alleles. For simplicity, I am just showing the alleles.) Genotype Phenotype AA Ao BB Bo AB oo