Problems with Mutation screening?

We used pRE112 suicide plasmids to transfer the DNA fragment in the E.coli chromosome (as the recipient strain)

to apply biparental mating through two screen processes. Firstly we screen the strain sensitive to Sucrose and resist to Cm (the plasmid has X and Cm gene ), secondly we screen the strain which is resistant to sucrose and senetive to Cm, but when we picked out this strain testing with PCR it can amplified two fragments, one was a wild type, and one was a deletion fragment.