Here's why:

* Multiple alleles: There are more than two possible alleles for the gene that determines blood type. In the ABO blood group system, there are three alleles: I^A, I^B, and i.

* Codominance: The I^A and I^B alleles are codominant, meaning that both alleles are expressed in the phenotype when present together. This is why individuals with the genotype I^A I^B have blood type AB, as both A and B antigens are present on their red blood cells.

* Recessive allele: The i allele is recessive, meaning it is only expressed in the phenotype when two copies of the i allele are present (genotype ii). This results in blood type O.

Therefore, blood groups demonstrate the principles of multiple allelic inheritance, codominance, and recessive inheritance.