P blood group locus
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Polymorphism
- different combinations or absence of the P blood group antigens define 5 different phenotypes: P1, P2, P1(k), P2(k), & p
- genetic variation in A4GALT determines the p phenotype, which is rare & does not express any antigens; it is also known as null phenotype; p individuals have antibodies against P, P1 & Pk antigens in their sera; these antibodies are clinically important because they can cause severe transfusion reactions & miscarriage