Abstract
SD genes were mostly found in association with a right arm inversion, In (2R)C,'' someSD chromosomes simultaneously carried the same persistent lethal gene which was frequently found and maintained in certain natural populations. The sensitivity of the 2nd chromosome to SD action varied from complete sensitiveness (k = 1.0) to resistance (k = 0.5). The mean k value of many chromosomes isolated from certain populations was 0.77.