The page below is a sample from the LabCE course Introduction to the ABO Blood Group System. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Inherited Antigens

"A subgroup" antigens are inherited, as are other ABO antigens with A1 being dominant over A2. Individuals who are phenotypically A1 may be genotypically A1O, A1A1, or A1A2. A phenotypically A2 individual may be genotypically A2A3. These alleles are passed to offspring in the same manner as other ABO antigens.
Weak variant forms of the B antigen (B3, Bx, and Bel) also exist, but are very rare.