A STUDY OF THE GENETICS OF DIELDRIN-RESISTANCE IN THE HOUSEFLY (MUSCA DOMESTICA L.).

Reciprocal mass crosses and back-crosses were performed between two homogeneous strains of the housefly (Musca domestica L.), representing the extremes in susceptibility and resistance to dieldrin. The heterozygotes were found to be intermediate between susceptible and resistant parents, and showed no evidence of sex linkage or cytoplasmic effects. The F(2) generation segregated in an approximate ratio of 1:2:1 into susceptible, heterozygote and resistant phenotypes, while the back-cross to the susceptible parent yielded 49.8:50.2 susceptible: resistant males, and 48.8:51.2 susceptible: resistant females. Elimination of susceptible forms in the back-cross progeny by use of a discriminating dosage and interbreeding the survivors produced offspring segregating into 26% susceptible, 50.1% heterozygote and 23.9% resistant, in excellent statistical agreement with a ratio of 1:2:1 expected in simple Mendelian inheritance. It is concluded that resistance to dieldrin in the housefly strain studied is due primarily to a major single pair of alleles or to a number of closely linked alleles so that they are inherited as a single unit.