Minor viability mutants in Drosophila.

Drosophila experiments have demonstrated that spontaneous mutants causing minor decreases in viability occur much more often than those causing drastic effects. They also have a much greater relative effect in heterozygotes, enough that the absolute heterozygous viability decrease is roughly the same for mild as for lethal mutants. On the other hand, with EMS treatment and especially with radiation, the mild mutants are less frequent relative to lethals.--I suggest that the failure to detect heterozygous effects on fitness components from multi-generation radiation experiments on mice due to the relative infrequency of mild effects and the relatively small heterozygous effect of drastic mutants. Chemicals that produce effects more like spontaneous mutations might produce quite different results in such experiments.