Abstract
1. Seventy-six loci of recessive lethal genes on the second chromosome, isolated from natural populations of Drosophila melanogaster, were determined by using three dominant marker genes (Sp, Bl, L).
2. The distribution of 76 lethal loci was not random along the genetic map; 19 loci being in the left (from 0 to 40 unit), 38 loci in the middle (40-70) and 19 loci in the right (70-108) region.
3. Other eighteen lethal chromosomes seemed to have multilocus-lethals (two or three lethal genes on a chromosome).
4. Twenty-six loci of the spontaneous lethal genes, which had not been through the force of natural selection, were also determined by the same method. Their distribution on the chromosome was apparently at random; 8 loci being in the left, 7 loci in the middle, and 11 loci in the right region.
5. The apparent non-random distribution of the natural lethal genes is difficult to understand in view of the location of most persistent lethal genes near the centromere; it is more likely that loci liable to natural mutation are distributed non-randomly on the second chromosome.
6. Six second-chromosome inversions were found to be associated with the natural lethals. No inversion could be detected in association with the spontaneous lethals. The role of inversions in natural populations are discussed from the view-point of maintenance and occurrence of lethal genes.
