What are they enzymes encoded by the alleles

Explanation

`I^(A)` I allela produces. N-accetyl galactose transferase and can add N-acetyl galactosamine (NAG) and `I^(B)` allela encodes for the enzyme galactose transferase that adds galactose to the precursor (i.e. H substances). In the case of `I^(O)//I^(O)` allele no terminal transferase enzyme is produced and therefore called "hull" allele and hence cannot add NAG or galactose to the precursor.