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 codominance. 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 alleles written as superscript letters above the letter "I". It is written this way to distinguish the alleles as codominant alleles. The "O" allele is expressed simply as a lower case "i" in textbooks to indicte it is recessive. For simplicity, I am just showing the alleles.)