The changes in frequency of lethal second chromosomes in four experimental populations, which were derived from a successively irradiated population of
D. melanogaster with a dose of 5.000R of X rays, were examine. The experimental populations A, B, C and D had been subjected to the accumulative radiation exposures of 10, 000, 25, 000, 50, 000 and 75, 000R, respectively.
With the cessation of the irradiation, the frequency of lethal genes decreased in successive generation, being almost equal to or, near the level of original non-irradiated populations.
The analysis of the data indicated that the radiation-induced lethal genes with heavy deleterious effect on the fitness of their heterozygous carriers were rapidly eliminated from the population in the early generations after the cessation of the irradiation, and after that the slightly detrimenal lethals were left in the population. The allelism test among lethal genes also showed that the newly induced lethals were evidently deleterious in the heterozygous condition.
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