* Blood type A is determined by the presence of the A antigen on red blood cells.

* The A antigen is coded for by the "A" allele.

There are two possible genotypes that result in blood type A:

* AA: Both alleles are "A".

* AO: One allele is "A", and the other is "O". The "O" allele is recessive, meaning it only expresses its trait (lack of antigens) when paired with another "O" allele.

So, someone with blood type A could have either the AA or AO genotype.