NATURE OF THE GENETIC EFFECTS 389 



11. MANNER OF INCIDENCE OF RADIATION-INDUCED 

 AND SPONTANEOUS MUTATIONS OF GENES 



The genetic effect of ionizing radiation that is most important in its 

 long-term consequences for an exposed population is the production of 

 gene mutations, i.e., permanent, heritable changes in individual genes. 

 This is also the genetic effect of radiation that is of greatest theoretical 

 significance. Nevertheless, gene mutations are by no means effects 

 peculiar to radiation, for mutations of sensibly the same types are con- 

 tinually arising "spontaneously" on a widespread scale, without the 

 application of radiation or any other special treatment. Moreover, they 

 can also be influenced greatly in their frequency of occurrence by various 

 conditions and agents other than radiation. Radiation is, however, the 

 first highly effective means that was discovered for producing gene 

 mutations in quantity, as was shown in experiments on X-rayed Drosoph- 

 ila carried out by the present writer in 1926-27 (Muller, 1927, 1928a, b, d) 

 and soon afterward confirmed in the same material by Weinstein (1928). 

 Moreover, it still remains in the first rank of agents having this effect. 

 The effect is produced by ionizing radiation of varied kinds. The first 

 decisive results with ionizing radiations other than X rays were reported 

 by Hanson (1928, et seq.), working in consultation with Muller, for (8 and 

 7 rays; by Ward (1935), working under Altenburg's direction, for a rays; 

 and by Nagai and Locher (1937), working under Altenburg's direction, 

 for neutrons. The effect is also produced by ultraviolet, as was first 

 shown by Altenburg (1930), after earlier negative results in 1928. 



Work on the production of mutations by X rays in higher plant mate- 

 rial was being carried out by Stadler and by Goodspeed and Olson inde- 

 pendently of and simultaneously with the earhest successful experiments 

 along these lines in Drosophila but, on account of the longer time neces- 

 sary for the growth of the plants, the definitive generations were not 

 obtained until 1928 (Goodspeed and Olson, 1928; Stadler, 1928a, b, 

 1930, et seq.). In Stadler's work, recessive "point mutations" (a general 

 term for mutations showing regular Mendelian inheritance and not con- 

 nected with gross chromosome changes) were produced both in maize and 

 in barley. As will be noted in more detail in Sect. 16, however, Stadler 

 has been inclined to interpret most or all of these point mutations pro- 

 duced by X rays in his material as minute deficiencies rather than as true 

 gene mutations, while the mutations which he much later obtained by 

 ultraviolet in maize show more agreement in their characteristics with 

 the spontaneous changes generally regarded as gene mutations. In 

 Drosophila, on the other hand, it was evident practically from the start, 

 from the series of varied alleles, including mutations in the reverse 

 direction, arising at given loci, that gene mutations were being produced 

 by the ionizing radiation. In Goodspeed's experiments the main changes 



