a1 Institute of Genetics, University of Stockholm, Drottninggatan 116, 113 86 Stockholm, Sweden
In a population of Drosophila melanogaster started from an inbred wild-type strain the recessive second chromosome lethal studied had shown overdominance which after many generations was lost. In the present study the persistence of this lethal was tested in three series each of five populations. The genetic backgrounds of the different series of populations were obtained from (a) the inbred strain, (b) the above original population after the overdominance had been lost, and (c) a population started from the same inbred strain and where another lethal had shown overdominance which subsequently had been lost. The lethal was overdominant in the (a) background but detrimental to the heterozygous carriers on the other backgrounds. The detrimental effect of the lethal was stronger in the (b) background than in the (c) background. The varying behaviour of the lethal is possibly due to different adapted background genotypes and/or different degrees of heterozygosity of the gene pools.
(Received February 02 1970)