Dominant H gene (located on chromosome 19) encodes for an enzyme which converts a carbohydrate precursor substance in red cells into H substance (H antigen). A and B genes encode for specific transferase enzymes which convert H substance into A and B red cell antigens. O gene encodes for an inactive transferase enzyme which results in no conversion of the H substance in group O red cells.
Persons who do not inherit H gene (very rare hh genotype) are unable to produce H substance and therefore even when A and B genes are inherited, A and B antigen
cannot be formed. This rare group is referred to as Oh, or Bombay group (originally recognized in Marathi-speaking
people of India). Antibodies anti-A and anti-B are present in the blood of Oh Bombay persons.
Secretors: Up to 80% or more of people inherit the secretor gene Se and secrete water soluble H, A, and B antigens in
their saliva, plasma, and other body fluids in addition to expressing the antigens on their red cells. Knowing whether a
person is a secretor is not important in routine blood transfusion practice.
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