GENETIC AND CYTOLOGICAL EFFECTS 



277 



et ai, 1948, Table 2) and Neurospora (Hollaender et ai, 1945). The 

 statements of Lea (1946) and Catcheside (1948) to the effect that a linear 

 proportionality holds up to the peak of the ultraviolet mutation curve 

 therefore cannot be accepted without question. The need for more 

 precise measurements of mutation effects at low doses is evident, since 

 the interpretation of data on the comparative effects of different wave 

 lengths must depend in part upon the shape of the dosage curve. 



The striking drop in mutation frequency observed at very high doses of 

 ultraviolet in fungi (Emmons and Hollaender, 1939) has not been found 

 in maize. This apparently is not related to factors of internal filtration, 

 })ut rather to the selective elimination of individuals from the population. 

 A somewhat similar drop in frequency has been reported in Drosophila. 

 The study by Reuss (1935), in which ventral abdominal exposures of 

 adult males w^as first employed, provided the first quantitative data on 

 the effects of increasing increments of ultraviolet on the frequency of 

 recessive lethal, semilethal, and visible mutations in Drosophila. Expo- 

 sures of 15, 22.5, and 30 min were used, and a leveling in the dosage rela- 

 tions was evident, but the data are inconclusive because of the small num- 

 l>ers of flies observed. Those obtained by Sell-Beleites and Catsch 

 (1942), following similar methods of exposure, are more striking. In two 

 different experiments (Table 7-6), the mutation frequency rose to a peak 



Table 7-6. Types and Frequencies of Mutations in the X Chromosome 



OF Drosophila melanogaster Induced in Spermatozoa by Ventral 



Exposure of the Abdomen to Ultraviolet Radiation 



(Sell-Beleites and Catsch, 1942.) 



with increasing increments of ultraviolet and then dropped abruptly as 

 added increments were given. 



Sell-Beleites and Catsch concluded that they were dealing with a 

 greatly disturbed "one-hit" curve, and that the decrease in mutation 

 frequency at high doses results from increasing sterility induced by the 

 penetrating radiation. Many of the irradiated flies were completely 

 sterile or yielded no mutations of any kind. The percentage of sterility 



