From the laboratory population of D. melanogaster which had been exposed, in every generation, to a dose of 2kR of gamma rays for about 100 generations, three experimental populations were initiated and maintained without further irradiation. Frequency of lethal second chromosomes and their allelic rate were repeatedly examined in these populations.
Frequency of lethal chromosomes was gradually reduced and from the rate of this reduction the heterozygous effect of induced lethal genes was estimated to be about 2%. Unlike the result of our previous experiment, this value was almost constant over all generations. From the result of the allelism tests it was demonstrated that the induced lethal genes were maintained in the irradiation suspended populations, being fairly influenced by the effect of random genetic drift.