CHROMOSOME ABERRATIONS IN ANIMALS 653 



were associated with gross chromosomal aberrations. An increase would 

 be expected if the lethals were dependent for their expression on the 

 production of rearrangements involving two independent breaks (Fano, 

 in Kaufmann et al., 1947). This evidence, and that afforded by the 

 dose-frequency analyses of Lea, Catcheside, and Herskowitz, indicate 

 that the lethals associated with induced chromosomal rearrangements in 

 Drosophila do not represent a special class caused by a position effect. 

 Muller (1950a) has objected to this interpretation, with the suggestion 

 that near-infrared pretreatment may actually increase the frequency 

 of position-effect lethals associated with gross chromosomal exchanges, 

 but that such increase is counterbalanced by a decrease in the frequency 

 of lethals associated with small rearrangements. This explanation 

 involves the assumption, for which there is no experimental evidence, 

 that small rearrangements, unlike the gross ones, are not increased in fre- 

 quency by pretreatment of spermatozoa with near-infrared radiation. 



Another aspect of the phenomenon of chromosomal exchange involves 

 the breakage and recombination of homologous chromosomes, designated 

 by the term "crossing over." In females of Drosophila, crossing over 

 occurs regularly in the course of meiosis, presumably during prophase in 

 the oocyte. In males crossing over occurs spontaneously with very low 

 frequency, but can be induced by X rays, as shown by Friesen (1933, 

 1934), Patterson and Suche (1934), and others. The frequency of cross- 

 ing over in the female may also be increased by irradiation (see, for 

 example, Whittinghill, 1938). Naturally occurring and induced crossing 

 over have many similarities; exchange takes place at identical loci while 

 the chromosomes are in the four-strand stage, and the recombinants are 

 usually not lethal when homozygous (references in Shapiro, 1945). 

 Many cases of mosaic formation in X-rayed larvae are assumed to result 

 from crossing over that is induced in somatic tissues (Stern, 1936). In 

 these cells the presence of chiasmalike configurations, which may afford 

 the structural basis conducive to crossing over, has been demonstrated by 

 Kaufmann (1934). Similar configurations have been observed by 

 Cooper (1949) in spermatogonial cells. 



The question thus arises whether irradiation promotes crossing over by 

 facilitating a method of exchange comparable to that normally operating 

 in the course of meiosis in the female, or whether the breakage-recombina- 

 tion process resembles that involved in exchange between nonhomologous 

 regions. A related problem concerns the stage of gametogenesis during 

 which induced crossing over occurs. Solution of these problems has 

 generally been assumed to require analysis of the progeny of individual 

 irradiated flies so as to detect the possible occurrence of "clusters" of 

 crossover types. The production of such clusters has been interpreted as 

 indicating that crossing over is induced in gonial stages (e.g., Whitting- 

 hill, 1938, 1950; Hinton and Whittinghill, 1950). Some parallel lines of 



