Here's why:

* Polygenic traits are traits that are determined by multiple genes. Blood type, specifically the ABO blood group, is determined by three different alleles: A, B, and O.

* Multiple alleles: The existence of more than two alleles for a single trait is known as multiple allelism. This is a key aspect of polygenic inheritance.

* Codominance: The A and B alleles are codominant, meaning that both are expressed if present. This leads to the AB blood type.

* Recessive: The O allele is recessive, meaning that it is only expressed if two copies are present (OO genotype).

Therefore, the combination of multiple alleles and codominance makes blood type a classic example of a polygenic characteristic.