How does human blood type show both simple dominance and codominant inheritance patterns?

Answer/Explanation:

There are three alleles associated with the human ABO blood types. The A allele produces A antigens, the B allele produces B antigens, the O allele doesn't produce any antigens.

A is dominant to O, which is recessive. B is also dominant to O. This is simple dominance

However, A and B are co-dominant, meaning neither is dominant over the other. They are both expressed equally.

If an individual inherited two A alleles (AA) they are blood group A.

If an individual inherited an A allele and an O allele (AO) they are also blood group A.

If an individual inherited two B alleles (BB) they are blood group B.

If an individual inherited a B allele and an O allele (BO) they are also blood group B.

If an individual inherited an A allele and a B allele (AB), they are blood group AB.