652 RADIATION BIOLOGY 



complex cyclic rearrangements. In the most complex rearrangement so 

 far reported (induced in a spermatozoon of D. melano g aster) , at least 32 

 breaks were involved, and the broken ends had reassociated to form sev- 

 eral independent exchanges (Kaufmann, 1943). Because of difficulties of 

 diagnosing breaks in the proximal heterochromatic regions of the chromo- 

 somes, it was not possible to determine exactty all the details of recombina- 

 tion, but on the simplest possible explanation there were seven independ- 

 ent exchanges: four with two breaks, one with four breaks, and two with 

 ten breaks each. In one of the latter a cyclic rearrangement occurred 

 among the ends of chromosomes broken at the ten points indicated by the 

 map numbers 87B/51E/81/49F/70C/33E/42C/56B/64C/76A. (Using 

 this system of notation a simple reciprocal translocation would be repre- 

 sented by two breaks, such as 3C/24D.) 



Chromosomal aberrations in Drosophila, even though they reveal no 

 perceptible loss of chromosomal material when examined in salivary-gland 

 preparations, are for the most part inviable when homozygous (Schultz, 

 1936, has reviewed the supporting evidence in considerable detail). 

 This suggests that the production of a rearrangement alters the genetic 

 constitution of the chromosomes involved. Analysis of radiation- 

 induced lethal mutations has shown that a considerable fraction of them 

 are associated with chromosomal rearrangements. The frequency varies 

 with the dose (Oliver, 1932; Demerec, 1937; Herskowitz, 1946); it is 

 about 35 per cent at 3000 r. (The association of a high proportion of 

 chemically induced lethals with chromosomal aberrations has also been 

 established by Slizynska and Slizynski, 1947.) Demerec (1937) tested a 

 series of 26 X-ray-induced lethals associated with chromosomal rearrange- 

 ments, and found that in 24 the locus of the lethal coincided with one of 

 the breakage points. This correlation suggests either that a submicro- 

 scopic change (in the nature of a gene mutation or inactivation) has 

 occurred adjacent to a break, as a consequence of the passage of an ion- 

 izing particle through the chromosome at that position, or that functional 

 modification has followed displacement of a gene from its normal rela- 

 tions with adjacent regions of the chromosome (position effect). Evi- 

 dence in favor of the former of these alternatives has been obtained in 

 Drosophila by analysis of the dose-frequency relations determined experi- 

 mentally in studies of lethal mutations and chromosomal aberrations 

 (Lea and Catcheside, 1945; Lea, 1946; Herskowitz, 1946), and by analysis 

 of the modifying effect of near-infrared radiation on the frequency of 

 X-ray-induced lethals and chromosomal rearrangements (Kaufmann and 

 Gay, 1947). When spermatozoa of D. melanogaster were exposed to near- 

 infrared radiation before treatment with X rays, they showed an increase 

 in frequency of chromosomal rearrangements, as compared with controls 

 that received only the X-ray treatment; but there was no increase in the 

 frequency of sex-linked recessive lethals, although about one-third of them 



