Three experiments were performed by the synthetic lines, possessing homozygous combinations of three major chromosomes of the pure M strain and Q strain, the maternal contribution being from the pure M strain or from the Q strain. By the results of monitoring cytotype for 20 generations, the cytotype was seemed to be controlled by the genotype of zygote or strain. Southern blot and in situ hybridization analyses provided evidences that Q and M' strains of Cheju Island population possess complete P and KP elements as well as incomplete P elements (deletion-derivatives), and especially, evidences for possible explanation that X chromosomes possess major factors of cytotype determination and autosomes also possess minor factors. Some possible interaction between cytotype determinants and P elements in Q and M` strains are discussed.