1198 BIOLOGICAL EFFECTS OF RADIATION 



somal aberrations in general, and of chromosome rearrangements in 

 particular, opened the possibility of an experimental attack on the 

 problems of the mechanism of meiosis. Chromosomal aberrations 

 produce characteristic disturbances of the different stages of meiosis. 

 A comparative genetic, and, where possible, also a comparative cyto- 

 logical investigation of individuals carrying chromosomal aberrations 

 and of those having normal chromosomes reveals a series of facts which 

 are probably very important for a causal analysis of the processes 

 involved. 



Crossing over between the normal and rearranged chromosomes is 

 characteristically affected in individuals heterozygous for translocations 

 or inversions. The frequency of crossing over is, at least in Drosophila, 

 almost invariably reduced in such individuals as compared with normal 

 ones, although the extent of reduction is extremely variable, ranging from 

 a complete suppression to an approximate normality of crossing over. 

 This reduction of crossing over is subject to a series of general rides, the 

 existence of which suggests that the whole phenomenon is due to the 

 action of comparatively few equally general causes. 



Inverted section heterozygotes show a reduction of crossing over not 

 only within the limits of the inversion (where only doubles are observed) 

 but also on both sides of it (Sturtevant 131, 133). The degree of the 

 reduction becomes progressively smaller the farther an interval is 

 removed from the ends of the inversion. In V-shaped autosomes of 

 Drosophila inversions affect crossing over only in the limb in which they 

 lie and not in the opposite limb. In individuals homozygous for inverted 

 sections the reduction of crossing over disappears (Sturtevant, 133). 



In heterozygous translocations a reduction of crossing over is observed 

 in all the chromosomes involved, in donors as well as in the recipients. 

 The relatively greatest reduction is found in the intervals in which the 

 loci of breakages or attachments are located, and in the adjacent intervals. 

 The farther an interval is removed from these loci the more nearly 

 normal is the frequency of crossing over therein. If the breakage is 

 located in one of the limbs of a V-shaped chromosome, crossing over is 

 affected in that limb only, not in the other limb. If, however, a breakage 

 takes place at or very close to the spindle attachment, crossing over is 

 affected to a certain extent in both limbs (Dobzhansky and Sturteyant, 

 36; Dobzhansky 34). 



The degree of reduction in the frequency of crossing over is a function 

 of the relative lengths of the transferred or exchanged sections of the 

 chromosomes. Crossing over is frequently almost totally suppressed 

 in small sections of chromosomes attached to other much longer chromo- 

 somes, while in the latter it is normal or nearly so. The longer the 

 attached section, and the shorter the chromosome (or the fragment) to 

 which it is attached, the more nearly normal is crossing over in the 



