What is null allele?

Explanation

In ABO blood group, each allele `(I^(A) " and " I^(B))` produces a transferase enzyme.`I^(A)` allele produces N-acetyl galactose transferase and can add N-acetyl galactosamine (NAG) and `I^(B)` allele 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 "null" allele and hence cannot add NAG or galactose to the precursor.