368 



RADIATION BIOLOGY 



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Fig. 7-2. Translocation produced by breaks in two different chromosomes. (1) Two 

 nonhomologous chromosomes are shown, prior to their splitting, one represented in 

 white with a black centromere and the other in black with a white centromere. 

 (2) Two breaks have been induced by radiation, one in each chromosome. (3o) The 

 two centric pieces have united to form a dicentric chromosome, and the two acentric 

 pieces to form an acentric chromosome. (36) The alternative type of rearrange- 

 ment has occurred, giving monocentric translocated chromosomes. (4) The chromo- 

 soines have split to form "daughters" or chromatids, and are becoming condensed 

 and placed in position for cell division, in both the a and h cases. (5) Mitosis is 

 nearly completed. In case 5a the dicentric daughter chromosomes are forming bridges 

 and are not being properly pulled all the way to opposite poles (they may later be 

 broken again by the tension), while the acentric daughter chromosomes fail to be 

 transported at all. In case bb the translocated daughter chromosomes become prop- 

 erly transported, so that each of the two daughter nuclei receives a complete outfit 

 of the original chromosome material that underwent translocation, although in a new 

 arrangement. It should be noted that in case 5a it will not always happen that the 

 two centromeres of a dicentric chromosome are pulled to opposite poles, since they 

 are not symmetrically placed ; but if this does not chance to occur at the first mitosis 

 after the translocation process, it is bound to occur at some subsequent mitosis and 

 so the same kinds of effect as in case 5a will finally be produced, and will involve the 

 loss of two chromosomes and the production of two bridges instead of one. 



complete sets of chromosomes and of genes, that is, "balanced" gene 

 ratios. Yet, when their germ cells begin to mature and to enter the 

 stages of meiosis, their translocated chromosomes, in matching their 

 homologous parts in synapsis with the nontranslocated ones, have to 

 make a kind of cross figure (see Fig. 7-3ai, 2^2). In this (if we may 

 simplify the situation so as to show only one meiotic division, with one 



