The mode of inheritance of dieldrin-resistance in house-flies, Musca domestica L., was investigated by crossing experiments, using a selected, inbred resistant colony (R) and a normally susceptible one (N). Levels of susceptibility in both colonies and in various hybrids derived from them were determined by treating batches of flies with different concentrations of dieldrin by topical application.
Reciprocal crosses gave closely similar results, which were pooled. Results obtained with males and females were analogous, but were treated separately. The F1 hybrids were intermediate in resistance and the F2 generation segregated into susceptible, hybrid and resistant individuals in the ratio 1:2:1. The F1 generation back-crossed to either parental strain gave 1:1 segregation, corresponding to NN + NR or NR + RR. After killing the more dieldrin-susceptible forms in each case, the remaining males were back-crossed again. The second back-cross to normal gave a further 1:1 segregation of susceptibles and hybrids. The second back-cross to resistant now gave all homozygous resistant forms.
These results strongly indicate monofactorial inheritance.
Two highly dieldrin-resistant colonies of the sheep blowfly Lucilia cuprina (Wied.) had been colonised, one from Australia and one from South Africa. These were crossed and the progeny maintained to the F2 generation. Tests with dieldrin on the F1 and F2 generations gave results resembling those obtained with the parents. Since there was no appearance of susceptible or hybrid forms in the F2, it is concluded that the genes in the two colonies occur on the same chromosome and are either allelic (possibly identical) or very closely linked.