Differences in base sequences of alleles of a single gene may result in non-functional proteins, including non-functional enzymes.

Bases in DNA can change (mutate) during replication.

The amino acid that the base pair coded will change, this will result in a different protein being made.

This different protein is called an allele, because it is a different form of a gene. (E.g blue or brown eyes.)

Sometimes the changes in the base pairs can make a polypeptide chain that makes a non-functional protein.

If a base is changed, one amino acid will be different which will change the protein a bit.

If a base is added in or taken away, it will change every amino acid in the chain- because it is read as in triplets so the whole sequence will move along one- which is likely to make a non-functional protein.

Enzymes are proteins, they can be made non-functional by changes in bases.