Explanation

A non-sense mutation is the one which stops polypeptide synthesis due to the formation of termination or non-sense codon. In lac operon, sequence of structural genes is Z, Y and A, which respectively code for β-galactosidase, lactose permease and transacetylase. If the gene Y has non-sense mutation, gene expression will stop at it, resulting in non-expression of both gene Y and successive gene A. Thus, only β-galactosidase enzyme will be produced2.