HEAT-INDUCED EXCHANGE IN THE FOURTH CHROhIOSOME

TURTEVANT estimated that the total number of spontaneous fourth-chromosome crossovers detected from diploid females up to 1951 was less than five. The linkage map of the chromosome that he published at that time was based on studies of diplo-IV triploids, where an appreciable amount of exchange occurs (MORGAN, STURTEVANT and MORGAN 1945). The present paper describes a method for inducing recombination in the fourth chromosome of diploid females. The procedure involves heat treatment of the first oocytes formed in the immature mother. Judging from the preliminary data presented below, the sensitive period for induction of crossing over in the fourth chromosome is approximately the same as that previously reported for increasing crossing over in other segments of the genome (GRELL 1967,1970). It is at least partially coextensive with the time of DNA synthesis in the oocyte.