Bacterial  cells  that have taken up plasmids containing the ampicilin
   resistance gene can form colonies in a Petri dish containing ampicilin
   and Xgal.

   Cells  with  recombinant polylinker have a defective lacZ gene, making
   them  unable  to  digest Xgal and form white colonies instead of blue.
   Plasmids can carry up to 10 kb of recombinant DNA.