Dominance and Number of Genes

D = 1 Complete dominance 0 D 1 Incomplete dominance -1 D 0 Incomplete recessivity D = -1 Complete recessivity D = 0 Intermediate 

A single gene may be responsible for the resistance, and in these cases the level of resistance can be quite high. Well-known examples are the 2000-fold resistance to organophosphates in spider mites, spinosad resistance in western flower thrips, DDT resistance in houseflies, and dieldrin resistance in several Diptera. In other cases, multiple genes are clearly involved in the resistance. Examples are carbaryl resistance in houseflies (Georghiou, 1972), carbaryl resistance in fall army worms (Yu et al., 2003), and malathion resistance in oriental houseflies (Yeoh et al., 1981). 

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