Genetic Effects of Radiations 149 



rejoining in the original way so that no permanent effect can 

 be seen.^'^' -^ This restitution is a matter of inference from 

 intensity experiments to be mentioned later. A further proportion 

 of breaks undergo reunion in new ways. Thus, two breaks, 

 one each in two different chromosomes in the same nucleus, 

 would produce four fragments Ai, Aq, Bi, Bq. Two of them 

 ( Ai and Bi) have centromeres and two (Aq and Bq) are without 

 these bodies. Reunion in a new way to produce interchanges 

 could be symmetrical, producing two new viable chromosomes 

 Ai-Bo and Bi-Ao, each with one centromere; or could be 

 asymmetrical, producing two defective chromosomes, one 

 (Ai-Bi) having two centromeres and the other (Aq-Bo) having 

 none. Similarly, two breaks within one chromosome could pro- 

 duce symmetrical changes (inversions, cf. figure 5)* or defective 

 (ring or deficient rod) asymmetrical changes. The defective 

 chromosomes are not permanently functional, since a chromo- 

 some without a centromere is inert on the spindle (figure 3/?),* 

 while in one with two centromeres there is a complete lack of 

 coordination of the two kinetic bodies. The inertness leads to 

 loss of parts of chromosomes from the daughter nuclei and, if 

 this entails the loss of vital genes, the nuclei die. The non- 

 coordination of two centromeres leads to chromosome bridges at 

 anaphase, and ultimately to breakdown and death of the cells. 

 Causes of this type are responsible for those dominant lethals, 

 referred to earlier, that are dependent upon two or more hits. 



A final proportion of the original breaks neither restitute nor 

 undergo reunion in new ways, but instead remain open as 

 chromosome breaks, the chromosome being present as two frag- 

 ments, one centric and the other ^acentric. In some cases the 

 pairs of sister chromatid ends may undergo sister union (figure 

 4a),* and in other cases not. Where sister union occurs in the 

 centric fragment, a bridge would be formed at anaphase (figure 

 3a), leading ultimately to cell death. Single chromosome breaks, 

 exhibiting sister union, account for the major proportion of 

 dominant lethals, namely for those proportional in frequency to 

 the first power of the radiation dose.^^- ^^' ^^ 



* Figs. 3 to 5 are on p. 104 and 105. 



