the lac operon consists of three structural genes in the order: z, y, and a. The z gene codes for beta-galactosidase, the y gene for permease, and the a gene for transacetylase. A non-sense mutation in the lac Y gene introduces a stop codon, terminating translation at that point. Since the z gene is located upstream (before) the y gene on the polycistronic mRNA, its translation proceeds normally, producing beta-galactosidase. The translation of permease is incomplete, and the downstream transacetylase is typically not expressed due to polarity.