If someone has blood type A, they must have at least one copy of the A allele, but they could have two copies. Their genotype is either AA or AO. Similarly, someone who is blood type B could have a genotype of either BB or BO. A blood test of either type AB or type O is more informative.
If someone has blood type A, they must have at least one copy of the A allele, but they may have as many as two copies of the A allele. Their genotype may be classified as either AA or AO. Similar to this, someone with blood type B might have either the genotype BB or the genotype BO. A type AB or type O blood test is more informative than a type AB or type O blood test.
