ABSTRACT The genetic determination of resistance to the sterility-producing genetic elements called P factors was studied in a strain characterized as neutral (Q) in the P-M system of hybrid dysgenesis. Sixteen lines were synthesized, representing all possible homozygous combinations of the three major chromosomes and differing maternal cytoplasms of an original resistant (Q) and susceptible (M) strain.—The results provide a detailed genetic analysis of the determination of cytotype (which mediates resistance or susceptibility to P factors) in the absence of the P-M dysgenic interaction. They extend the findings of Engels (1979) by providing specific information on both the location and relative magnitude of effect of cytotype-determining chromosomal factors and their interaction over time with maternally transmitted cytoplasm.—Cytotype was found to be primarily controlled by the genotype, but the maternal cytoplasm, under some circumstances, has an important short-term effect. Major cytotype-determining chromosomal factors are localized to the distal half of the X chromosome. However, there was also evidence for minor factors located on the major autosomes, particul
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