Fig. 3. Synergistic lethality between weak alleles of zen and sog. (A) At 25°C, sog^P129D and zen1 are both homozygous viable. Occasional sog^P129D; zen1 double mutant flies arise from a sog^P129D/FM7; zen1/TM3 stock maintained at 18°C. When these flies are transferred to 25°C, they produce dead embryos with a partially ventralized phenotype (as seen in B).