362 RADIATION BIOLOGY 



4. CONSEQUENCES OF THE PRODUCTION 

 OF A SINGLE CHROMOSOME BREAK 



Much more varied and more important in their consequences for cells, 

 tissues, individuals, and populations, as well as more heuristic, than 

 the effects of radiation on the distribution of whole chromosomes or on 

 crossing over, are its effects in producing structural changes of chromo- 

 somes, i.e., permanent changes in the linear arrangement of their genes, 

 and in the distribution of genes among different chromosomes. Clear 

 evidence that ionizing radiation produces an abundance of structural 

 changes of varied types, which become reproduced at mitosis and meiosis 

 so as to be inherited by subsequent generations of cells and individuals, 

 was first obtained in Drosophila (Muller, 1927, 1928a, b, d; Muller and 

 Altenburg, 1928). These findings were very soon extended to organisms 

 of the most varied kinds, including monocotyledons, dicotyledons, and 

 mammals, by various investigators, among whom should be named 

 especially Goodspeed, Stadler, McClintock, Levitsky, Sax, and Snell. 

 Ultraviolet radiation as well has been found to produce structural 

 changes, but the relative incidence of different types is not the same as 

 with ionizing radiation, and the frequency of structural changes induced 

 by a given dose of ultraviolet is, according to most investigators, much 

 lower than that from ionizing radiation given in such a dose as to match 

 the ultraviolet in the production of gene mutations (see Chap. 8). These 

 peculiarities of ultraviolet will be further discussed in the second volume 

 of this series. The following pages are concerned mainly with the results 

 of breakage induced by ionizing radiations. 



Structural changes of the same types as are produced by radiation also 

 arise "spontaneously," i.e., in untreated material, although with far 

 lower frequency. In fact, "spontaneous" examples of most of the types 

 had already been recognized and had to some extent had their conforma- 

 tions determined, largely in Drosophila (especially by Bridges, Sturtevant, 

 Mohr, Muller, Stern, and Altenburg) and in Datura and a few other 

 plants (especially by Belling and Blakeslee), before the flood of cases con- 

 tributed by radiation genetics had become available. However, analyses 

 of the cases produced by ionizing radiation, and of the conditions of their 

 production, added much to the understanding both of the pattern of 

 effects produced, and of their mechanism of origination, and from these 

 analyses of the radiation cases a general theory of the process of struc- 

 tural change of chromosomes, whether resulting from radiation or other 

 causes, gradually took shape. It would be beyond our scope here to give 

 an account of the intricate series of steps whereby this theory has been 

 established, but they are to some extent discussed on pp. 363 to 388 and 

 in Chap. 8. The work has utilized some of the advanced techniques of 

 both experimental breeding (involving linkage maps) and cytological 



