96 Applied Biophysics 



before any observations on mutation production by radiation 

 diverted attention in this direction. These early observations 

 were somewhat restricted and rarely extended to the offspring 

 of irradiated organisms. The effects of radiation were judged 

 by abnormalities in development after irradiating sperm or ova, 

 or by alterations in the chromosome configuration of dividing 

 cells. It was later found that radiation may cause an abnormal 

 distribution of hereditary material without change in its com- 

 position. Then, as cytological technique advanced, it was realized 

 that the alterations in the chromosomes themselves were of at 

 least two kinds : ( 1 ) changes in the linear arrangement of the 

 chromosome threads, resulting from single or double breakage 

 and recombination in new alignments, with or without loss of 

 chromosome fragments; and (2) changes in the composition of 

 the unit hereditary particles or genes, without disturbance of 

 their position on the chromosome thread (gene mutation). 



Chromosome abnormality offers a verv convenient method for 

 making a quantitative measure of radiation effect. The scoring 

 of abnormalities is tedious, but can be made with fair accuracy. 

 Some breaks in the chromosome thread rejoin immediately, but 

 for the rest, the injury, once made, is permanent, so that the 

 result is not complicated by gradual recovery processes. A great 

 variety of structural change is seen after suitable radiation dos- 

 age, and this may be classified according to whether one or more 

 chromosomes have been involved and how the broken ends have 

 reconnected.^^' ''^ The material is almost ideal for statistical pur- 

 poses, because the chromosomes act as targets which mark the 

 hits by breaks in continuity of the thread which can be seen 

 and counted. The tangle in which the broken threads in some 

 cases become involved may cause the breaking up of the cell, 

 or the production of nonviable daughter cells owing to the un- 

 equal distribution of the hereditary material. In this respect, 

 chromosome abnormalities are more detrimental than gene 

 mutations (which may not exert their effects for several genera- 

 tions) since they cause marked infertility in the first-generation 

 offspring. 



Structural changes in chromosomes are most easily investi- 



