NATURE OF THE GENETIC EFFECTS 



369 



chromatid per chromosome and no crossing over), it is in most species, 

 for most translocations, more or less a matter of chance whether they 

 assume the "old combination arrangement," as in Fig. 7-3ai, b], such that 

 the two translocated chromosomes become pulled to one daughter 

 nucleus and the two nontranslocated to the other nucleus, or the "recom- 

 bination arrangement," shown in Fig. 7-3a2, b2, whereby one translocated 

 and one nontranslocated chromosome become pulled to each nucleus. 

 It is only when the points of breakage had been very near the centromere 



TRANSLOCATION OF PARTS OF ARM 



6, EUPLOID SEPARATION 



TRANSLOCATION OF 

 WHOLE ARMS 



60 ANEUPLOID SEPARATION 

 I 



bi EUPLOID SEPARATION ONLY 



Fig. 7-3. Meiotic arrangements in cells heterozygous for eucentric mutual transloca- 

 tions. In columns 1 and 2 the translocation involved the exchange of parts of arms, 

 while in column 3 virtually whole arms were exchanged. Clear areas represent 

 centromeres. Diagrams simplified by omission of crossing over and of split into 

 chromatids. 



in both chromosomes, giving what are called whole-arm translocations, 

 that the recombinational arrangement (that giving aneuploids) is of very 

 infrequent occurrence, since here the two nonhomologous pairs of centro- 

 meres are so close together that the orientations of their members with 

 respect to the plane of separation are strongly correlated with one 

 another (Fig. 7-3a3, bs). 



Now the first arrangement, that giving the "old combinations," 

 results both in apparently normal offspring which, however, like the 

 parent, again contain the translocation, and in entirely normal offspring. 

 But the second or recombinational arrangement results in aneuploid 

 zygotes which, being deficient with regard to one translocated chromo- 

 some region and at the same time having an extra representation ("dupli- 

 cation") of another region, usually die of their genie imbalance during an 



