Master Blood Types with Picmonic for Medicine

The second allele that codes for blood type is called Allele "B". Allele B is a dominant allele.

Allele "O" is the third blood type allele. It is recessive.

Genotypes AA and AO refer to individuals having either two "A" alleles for blood type, or one "A" allele and one "O" allele. Both of those genotypes results in having Type A blood. The AO genotype results in Type A blood because A is dominant over O.

Genotypes BB and BO refer to individuals having either two "B" alleles for blood type, or one "B" allele and one "O" allele. Both of those genotypes results in having Type B blood. The BO genotype results in Type B blood because B is dominant over O.

If someone's genotype for blood has both an A and a B allele, they have the AB blood type. This is because both alleles are dominant and neither dominates over the other. So they are said to be "co-dominant." Both dominant phenotypes are expressed, so the person has AB blood. Type AB blood with a negative rhesus factor (AB^–) is the rarest blood type.

As with most recessive characteristics, the only way to have Type O blood is to have the genotype OO. This means that an individual must have two "O" alleles to have type O blood. Type O blood with a positive rhesus factor (O⁺) is the most common blood type.