author(s)
Zunnu Raen Akhtar, Ahmad Ali Anjum, Zain Saeed and Jawad Khalid

abstract
In this review, illustrated by examples, there were rare instances where dominant resistant alleles were found. Reason of resistance in fields may include different concentration of Bt protein used in different regions, this low concentration causes less binding of toxins to insect midgut. Mostly resistance development was observed under laboratory conditions against Cry proteins which are inserted in transgenic cotton. Under laboratory conditions, cadherin was found to be responsible for development of resistance in bollworm. Resistance developed in the boll worms was related to recessive alleles. Different strategies are used to delay the resistance. Development of pyramided cotton, stacking genes, refuge strategies can be helpful to show enhanced effects against resistance developed target pests. Such strategies can only delay resistance development for short time but cannot completely eliminate the resistance development. Further research is needed to know the reason for development of resistance in insects against expressed Bt proteins in transgenic cotton.