The I^A allele produces an enzyme that adds the sugar AcGalNH to the H
   substance to produce the A antigen.
   The  I^B  allele  produces  an  enzyme  that adds a galactose sugar to
   produce the B antigen.
   The  O  phenotype  results from an absence of either sugar, and may be
   due to a mutation in the FUT1 locus. 2