If a recombinant DNA is inserted within the coding sequence of ß- galactosidase enzyme of pUC vector then

When we insert foreign gene into  pUC vector at the location of $\mathrm{\beta }$- galactosidase enzyme gene, the galactosidase enzyme gets inactivated. Hence recombinants cannot convert X-gal into 5-bromo-4-chloro-indoxyl. Non recombinants have intact $\mathrm{\beta }$ -  galactosidase gene. Hence they convert X-gal to 5-bromo-4-chloro-indoxyl which forms insoluble blue pigment.