Here's why:

* Codominance: This occurs when two different alleles for a trait are both expressed at the same time. In the case of blood type, the A and B alleles are codominant. If someone inherits both an A and a B allele, they will have blood type AB, expressing both A and B antigens.

* Multiple Alleles: This refers to the existence of more than two alleles for a single gene. For blood type, there are three alleles: A, B, and O. This leads to the various blood types:

* A: AA or AO genotype

* B: BB or BO genotype

* AB: AB genotype

* O: OO genotype

Let me know if you'd like more explanation on any of these concepts!