150 Applied Biophysics 



Chromatid breaks produce a series of analogous chromatid 

 structural changes (simple chromatid breaks are shown in figure 

 4b* and a chromatid interchange in figure 3c),* some of which 

 are defective, leading to death, and others of which are fully 

 functional and viable. In general, a functional nucleus must have 

 a full complement of genes, and each chromosome must be rod 

 shaped and have just one centromere. This is not strictly true, 

 since very small deficiencies (absences of one or a few genes) 

 may be viable. Thus, a proportion of the recessive lethals induced 

 in Drosopliila sperm are actually small deficiencies, as is disclosed 

 by examination of the giant salivary-gland chromosomes.-® 



The yield of persistent chromosome breaks and chromatid 

 breaks is linearly proportional to dose in the case of 

 X -ray s, -•'••-"■ '^-^ e.g., neutrons, ^^'- ^"^"^ and alpha rays ( Kotval and 

 Gray, unpublished ) . The yield is also independent of the radia- 

 tion intensity.^- -"' Therefore, simple breaks are products of 

 single radiation hits. 



The vields of interchanges and other two-break al)errations 

 produced by X-rays diminish with increase of the time over 

 which the irradiation is spread, i.e., with decreasing intensity. 

 These two-break aberrations also increase more rapidly than 

 the first power of the dose. With high intensities, the yields are 

 practically proportional to the square of the dose; at lower 

 intensities, the power of the dose is lowered.-^* A square law 

 is also found if the dose is varied by varying the intensity at a 

 constant exposure time. These facts are readily explicable if 

 the two breaks are produced by separate ionizing particles. 

 However, the effects may be distorted by restitution of breaks, 

 unless the irradiation is completed in a short time or the irradia- 

 tion extends over the same time at all doses. The data also may 

 be employed to show ^'' that the mean life of an original break 

 in a Tradcscantia chromosome is about 4 minutes at 20° C. At 

 lower temperatures, its life is probably longer. 



\\ ith neutrons, the yield of interchanges is independent of the 

 time over which a given dose is spread, i.e.. of the intensity, 

 suggesting that a single ionizing particle usually causes both the 

 breaks in the neutron-induced interchanges.^^ In agreement 



