Explain the reason the Type AB and Type O blood phenotypes always have the same genotype, whereas the Type A and Type B blood phenotypes have two possible allelic
combinations.

Because A is co-dominant to B and O is recessive, so the genotype is the same as the phenotype. But A can have 2 genotypes because A can mask a recessive allele, which makes the phenotype A. B can also mask a recessive allele, which makes the phenotype B.

Examples

AO (genotype) = A (phenotype)

BO (genotype) = B (phenotype)