Abstract
Population analysis was conducted for the adaptive superiority of sepia heterozygotes to either homozygote in Drosophila melanogaster. Four populations, which had different genetic backgrounds in the initial generations, were established. In early generations, where the magnitude of linkage disequilibrium (D of Lewontin and Kojima 1960) was large, and heterozygote superiority was seen. The frequencies of sepia homozygotes reached temporary equilibria (ca. 7-14 percent). However, as generation advanced, namely, the magnitude of linkage disequilibrium became small or disappeared, the frequency of sepia homozygotes decreased. However, in three sub-populations out of fifteen, permanent linkage disequilibrium was probably formed. Thus, intrinsic overdominance of the sepia gene was rejected.
Mathematical models were set up for the explanation of the experimental data, and the necessary and sufficient conditions for the pseudo-overdominance in populations were obtained.
