The Frequency of Lethals in Crossover and Noncrossover Second Chromosomes of Drosophila Melanogaster.

T has long been known in different species that some individuals carry lethals I even though they were descended from lethal-free ancestors. These lethals usually have originated by mutation at various loci. There exists, however, a second possible source for new lethals. Since individuals in a population are heterozygous for a large array of loci, recombination, it has been argued, might give rise to ill-adapted assortments of pre-existing alleles some of which might give lethal effects. Evidence for the production of such “synthetic” lethals has been presented by WRIGHT and DOBZHANSKY (1946) ; MISRO (1949) ; and WALLACE, KING, MADDEN, KAUFMANN and MCGUNNIGLE (1953). The extensive work of the last named group of investigators concerned recombination among second chromosomes of Drosophila melanogaster. They found a considerable number of lethals among the offspring of females which had been made heterozygous for different lethalfree second chromosomes, in contrast to a small number of lethals from corresponding heterozygous males. In the females the presence of crossing over was considered to have been responsible for the origin of synthetic lethals while the absence of crossing over in the males did not lead to their occurrence. Recent work by HILDRETH (1955,1956) on the X chromosome and on the third chromosome of Drosophila melanogaster shows none of the marked deviations in the frequency of lethals from recombinant as compared to that from nonrecombinant chromosomes. Further, HILDRETH was able to localize at single loci the lethals which did occur in recombinant X and third chromosomes indicating that they were not due to lethal complexes synthesized by recombination. In none of the former studies had there been a direct check on crossing over. The present experiment was SQ designed that specific crossover and noncrossover chromasomes could be selected and then tested for the presence of lethals. All data refer to the second chromosome of D. melanogaster.