Here's why:

* Codominance: When two different alleles for a trait are both expressed in the phenotype. In blood types, the A and B alleles are codominant, meaning if a person inherits both A and B alleles, they will have AB blood type, expressing both A and B antigens.

* Multiple alleles: There are more than two possible alleles for a single gene. In blood types, there are three alleles: A, B, and O.

Let me know if you would like a more detailed explanation of how blood type inheritance works!