GENETIC AND CYTOLOGICAL EFFECTS 257 



tested, 40 or more translocations of type II-III alone would have resulted 

 from this X-ray dose. I'hese data were in agreement with an earlier 

 trial in which no translocations were found from treatments which induced 

 sex-linked lethals in such numbers as to indicate an expectancy of at least 

 25 detectable translocations, on the basis of the relation found with 

 X rays. The discriminatory action of the ultraviolet was thus shown by 

 the absence of detectable translocations in cultures in which at least 65 

 were to be expected if the relation of mutation and gross chromosomal 

 rearrangement were the same with ultraviolet as with X rays. 



2. Similar evidence relating to the occurrence of minute rearrangements 

 was obtained by the use of a special technique, with w^hich minute rear- 

 rangements are induced by X rays in relatively large numbers and are 

 recognizable by mutants at specific loci. In ultraviolet-treated cultures 

 yielding sex-linked lethals at a rate corresponding to an X-ray dose of 

 1000 r, no mutants at these loci were found. In an X-ray experiment by 

 MuUer and Makki (Mackenzie and MuUer, 1940), these had occurred at 

 significant freciuencies following dosage of 1000 r. MuUer and Mackenzie 

 concluded provisionally that the ultraviolet does not produce minute 

 rearrangements, or at least that it is far less efficacious in this respect than 

 X rays. 



3. Wave lengths above 320 m^ were found to be ineffective in inducing 



mutations. 



4. The frequency of mutations was higher when mating of the irradi- 

 ated males followed immediately after treatment. No mutations were 

 transmitted 5 days after irradiation although the supply of mature sperm 

 would not have been exhausted for some days thereafter. Those irradi- 

 ated flies which bred but* died early had higher frequencies of mutations 

 than those which continued to breed over longer periods of time. A 

 correlation was established therefore between the frequency of induced 

 mutations and the amount of physiological damage as determined by the 

 duration of the fecund period. Both phenomena are undoubtedly 

 affected by the degree of penetration of ultraviolet, a factor which varies 

 widely among similarly exposed individuals. 



5. Dose fractionation was without effect on the frequency of mutations 

 or the degree of sterility. 



The occurrence of minute deletions among the mutants induced by 

 ultraviolet was cytologically demonstrated by Slizynski (1942). Among 

 21 of the sex-linked lethals produced in the experiments of Mackenzie and 

 Muller, 5 Avere found to be cytologically detectable deficiencies, 1 involv- 

 ing the loss of 1 band, 3 the loss of 2 bands, and 1 the loss of 14 bands. All 

 were interstitial deficiencies. 



Results of studies by McQuate (1950) support the hypothesis of 

 Mackenzie and Muller that terminal deficiencies, if produced, are not 

 recovered in Drosophila populations derived from mature spermatozoa 



