INDUCED CHROMOSOMAL ABERRATIONS IN ANIMALS 1199 



attached section, and the stronger is the reduction of crossing over in 

 the recipient. The reduction of crossing over in the donor is strongest 

 if a section of a certain intermediate length is transferred to another 

 chromosome; donors which have lost very short or very long sections 

 may show approximately normal crossing over (Dobzhansky, 28, 32, 34). 

 The occurrence of crossing over between homologous chromosomes 

 or their fragments increases the probability of their normal disjunction, 

 that is, of their passing to the opposite poles of the spindle at the reduction 

 division. Chromosomes which have not crossed over are likely to 

 undergo nondisjunction, i.e., to be distributed at random in respect to 

 their homologues at the reduction division (Bridges, 12; Anderson, 6; 

 Dobzhansky, 34). Since in translocations the frequency of crossing 

 over is a function of the relative lengths of the fragments of the donor 

 and the recipient chromosomes (see above), there exists a functional 

 connection between the latter variables and the mode of the disjunction 

 of the chromosomes involved in a translocation (Dobzhansky, 30, 34). 



All the regularities mentioned before can be brought into a self- 

 consistent system if one assumes that : (a) the pairing of the homologous 

 loci of the chromosomes at meiosis is due to a mutual attraction exhibited 

 by these loci (Dobzhansky, 28), (6) the frequency of crossing over at a 

 given point is a function of the distance between that point and the 

 spindle attachment (Beadle, 8; Offerman, Stone and Muller, 86), and 

 (c) the occurrence of crossing over is related to the presence of chiasmata 

 between the chromosomes, the presence of chiasmata regulating the 

 position of the chromosomes on the spindle at the reduction division, 

 and, consequently, the disjunction of the chromosomes (Darlington, 21a). 

 In individuals having normal chromosomes every chromosome has one 

 and only one complete homologue, i.e., there exist only two chromosomes 

 in the nucleus which carry the same genes arranged in the same order. 

 The mutual attraction of the loci contained in these chromosomes brings 

 them together at meiosis, the chromosomes pair, undergo crossing over, 

 and disjoin normally at the reduction division. Chromosome rearrange- 

 ments disturb the normal pairing owing to the competition between the 

 different loci and parts of the chromosomes. In heterozygous transloca- 

 tions some chromosomes consist of parts which are homologous to two 

 (or more) different chromosomes in the same nucleus. Such chromosomes 

 (considered as wholes) are attracted simultaneously toward more than 

 one partial homologue. Competition between the different sections each 

 of which tends to pair with its own homologue leads to a failure, or at 

 least to a delay, of pairing of some sections. This obviously prevents, 

 or reduces the probability of, crossing over taking place in the sections 

 whose pairing was delayed and thereby disturbs their disjunction. 



The most acute conflict of the attraction forces should ensue in the 

 vicinity of the points where the chromosomes change their homologies. 



