630 RADIATION BIOLOGY 



Many aspects of such studies have been examined in extensive detail 

 in a series of reviews, texts, and symposia since the publication of Dug- 

 gar's "Biological Effects of Radiation" in 1936 [e.g., Timofeeff-Res- 

 sovsky, 1937; Timofeeff-Ressovsky and Zimmer, 1939; Bauer, 1939c; 

 Delbriick, 1940; Muller, 1940, 1950b; Fano and Demerec, 1944; Catche- 

 side, 1945, 1946, 1948; Lavedan, 1945; Gray, 1946; Spear, 1946; Lea, 

 1946; Giese, 1947; Buzatti-Traverso and Cavalli, 1948; Fano, Caspari, 

 and Demerec, 1950; Sparrow, 1951; and the numerous contributions to 

 the 1941 Cold Spring Harbor Symposium on Genes and Chromosomes, 

 the 1946 London Conference on Certain Aspects of the Action of Radia- 

 tions on Living Cells (published in 1947), the 1948 Brookhaven Confer- 

 ence on Biological Applications of Nuclear Physics, the FIAT Review of 

 German Science from 1939 to 1946 (published in 1948), and the 1948 Oak 

 Ridge Symposium on Radiation Genetics (published in 1950)]. 



The present review will deal with chromosomal aberrations induced in 

 animal cells by ionizing radiations. Separation of animal from plant 

 materials, even for the sake of description, imposes arbitrary limitations 

 that are not always advantageous; accordingly, pertinent botanical 

 literature will be cited when it seems desirable. Designation of chromo- 

 some aberrations and gene mutations as sharply delimited classes is also 

 to a large extent arbitrary. Small chromosomal aberrations cannot 

 always be distinguished from the so-called "point mutations." More- 

 over, radiation studies have clearly shown that the reaction system 

 responsible for the characteristic phenotypic expression of a gene may 

 be profoundly altered by a realignment of parts of chromosomes in the 

 process of structural rearrangement. It is thus apparent that no sharp 

 line can be drawn between gene mutations and chromosome aberrations; 

 but this article will be limited to a consideration of the types of gross 

 chromosomal alterations that can be detected either by direct cytological 

 examination or by breeding tests. Even with attention focused on this 

 restricted segment of a large body of information, it will not be possible to 

 review all the available experimental evidence, and the discussion will be 

 concerned primarily with the types of induced aberrations, the methods 

 used in their diagnosis, and their significance with respect to evaluation 

 of the processes of chromosome breakage and recombination. The 

 effects of radiations in retarding mitosis and in modifying the normal 

 distribution of chromosomes on the spindle are considered in Chap. 11 

 by Carlson. 



2. NATURE OF THE INDUCED REARRANGEMENTS 



Various types of cells from many different species of plants and animals 

 have been irradiated in order to obtain information about the process of 

 structural rearrangement, but the majority of available data have been 



