Can someone explain replica plating in detail regarding xgal and xgal + iptg? I am studying for an exam and am very confused. I have in my notes something about mutations turn xgal +iptg blue when its supposed to be white and wild type should be xgal (white) and xgal and iptg (blue). I am just confused. what does this have to do with lac- and media with lac+ and tryptophan?