GENETIC AND CYTOLOGICAL EFFECTS 255 



therefore, which are reuUzed only at a later stage in cell division would 

 not be detected. The absence of ultraviolet-induced translocations in 

 the pollen tube chromosomes of Tradcscantia, as contrasted to their 

 occurrence in the Fi populations of maize and Gasteria, may well be due 

 to the formation of these aberrations at later stages of division or during 

 the process of fertilization. 



GENETIC EFFECTS OF ULTRAVIOLET RADIATION 



Guyenot's (1914) early attempt to induce mutations with ultraviolet 

 radiation was unsuccessful. Adequate techniques for the quantitative 

 screening of mutations were not a\'ailable at the time, and genetic 

 knowledge was too scanty to provide a background against which such 

 studies could be properly evaluated. After Muller's discovery (1927) 

 of the significance of X rays as a mutagenic agent, the question of the 

 spectral limits of genetic effectiveness arose. Genetic studies with 

 ultraviolet radiation by Altenburg (1928) on Drosophila and Stubbe 

 (1930) on Antirrhinum gave no positive indication that this radiation 

 could induce mutation. As the techniques of irradiation were improved, 

 however, it became apparent that under favorable conditions of exposure 

 mutations could be induced in both plants and animals. Early indi- 

 cations of effectiveness were reported by Altenburg (1930, 1931), Geigy 

 (1931), and Promptov (1932) in Drosophila. Largely as a result of the 

 development of the polar cap technique of exposure by Geigy, unequivocal 

 confirmation of the mutagenic action of ultraviolet in Drosophila was 

 provided by Altenburg (1933, 1934). Noethling and Stubbe (1934) also 

 demonstrated in Antirrhinum that exposure of the pollen grains to ultra- 

 violet could significantly increase the mutation freciuency. 



Experiments with Drosophila. A summary of Altenburg's Drosophila 

 data is given in Table 7-1. From these figures it is clear that a significant 

 increase in the frequency of recessive lethals can be obtained by exposure 

 to ultraviolet. As might be expected on the basis of penetration, eggs in 

 the polar cap stage are readily affected. The replicated lethals appearing 

 in 5 per cent or more of the Fi females from a single male represent mutations 

 occurring in pole cells at the time of treatment, and their frequency is 

 clearly increased by irradiation. The distinct increase in "isolated" 

 lethals (i.e., lethals that appear singly) is assumed to be due, at least in 

 part, to the inclusion of some eggs beyond the polar cap stage at the time 

 of treatment. 



The results of Geigy (1931) and Promptov (1932), on the induction of 

 recessive lethals in eggs of Drosophila, are in essential agreement with 

 those reported by Altenburg (1934). Both Altenburg and Promptov 

 noted an increased incidence of visil)le mutations, in frequencies con- 

 siderably lower than for se.x-linked lethals. 



