Temporal Changes in the Roles of Lethal and Semilethal Chromosomes within Populations of Drosophila melanogaster

A series of analyses are described which indicate that, in certain laboratory populations of D. melanogaster, the average effects of lethal and semilethal ("drastic")chromosomes on the viability of heterozygous individuals gradually changed; in early generations they lowered the viability of heterozygotes (partial dominance), later they developed an enhancing effect on viability. The newly developed, heterotic effect of drastics was a general rule, however, only in combinations of wildtype chromosomes from the same population; in heterozygous combination with genetically-marked laboratory chromosomes, drastics in one instance were partially dominant, in another they enhanced viability, but in general had little or no effect on the viability of these mutant carriers.