Here's why:

* Blood type inheritance: Blood type is determined by the presence or absence of specific antigens (molecules) on the surface of red blood cells.

* ABO blood group: The ABO blood group is determined by three alleles: A, B, and O.

* Codominance: The A and B alleles are codominant, meaning both are expressed equally when present together.

So, when an individual has the AB genotype, they inherit one A allele and one B allele. This results in both A and B antigens being present on their red blood cells, leading to the AB blood type.