INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1201 



Simultaneously, the frequency of nondisjunction of the sections in which 

 crossing over is prevented is increased, and that of the other sections 

 becomes rare. 



In a series of translocations involving transfers of sections of the 

 second chromosome of Drosophila to the F-chromosome the effect of 

 inversions was found to depend upon the relative position of the inversion 

 in respect to the breakage point. If the inversion is homologous to the 

 part of the second chromosome attached to the Y, the frequency of cross- 

 ing over in the part of the second chromosome remaining free is increased. 

 On the other hand, if the inversion is homologous to the part of the second 

 chromosome remaining free, crossing over in the part attached to the Y 

 increases strikingly. The frequency of nondisjunction was found to be 

 inversely proportional to that of crossing over. These facts cannot be 

 reasonably interpreted otherwise than as corroborating the hypothesis 

 of competitive pairing (Dobzhansky, 32). 



Another corollary of the competitive pairing hypothesis is that the 

 presence of duplicating fragments of chromosomes should have an effect 

 on crossing over in the chromosomes to which the fragments are homo- 

 logous. Individuals may be obtained which have the normal diploid set 

 of chromosomes plus a duplication for a section of one of the chromosomes. 

 Since the duplication should tend to pair with either of its two normal 

 partial homologues, the pairing of the latter with each other should be 

 disturbed, and the frequency of crossing over in them should be decreased. 

 Thus, a reduction of the frequency of crossing over is expected to take 

 place in chromosomes which are not themselves involved in any chromo- 

 some aberration. Rhoades (113) studied crossing over in the normal 

 second chromosomes in the presence of a duplication for a section of the 

 same chromosome. A reduction of the frequency of crossing over was 

 found. Dobzhansky (in press) found the same to be true for a series of 

 duplications for sections of the X-chromosome of Drosophila melano- 

 gaster, the strength of the reduction being a function of the length of the 

 duplicating sections. 



Beadle (8) and Offermann, Stone, and MuUer (86), however, dis- 

 covered the existence of a factor other than competitive pairing that 

 accounts for a part of the reduction of crossing over observed in trans- 

 locations. In individuals homozygous for translocations the conditions of 

 the chromosome pairing are, as far as competition of chromosomes is con- 

 cerned, similar to the conditions found in normal individuals. Neverthe- 

 less, the frequency of crossing over is found to be very different from 

 normal in certain intervals of the chromosomes. The only explanation 

 of this fact is that a change in the relation of the chromosome segments 

 in respect to the spindle attachment may alter the freqviency of crossing 

 over in that segment. As shown before, the cytological maps of the 

 Drosophila chromosomes indicate that crossing over in all chromosomes 



