278 KADI \'I'I<>N HIOLOCY 



increased witli increasing dosage and resnlted presumably from physio- 

 l()gic;il damage unassociated with the mutation process. Once a maxi- 

 mum mutagenic elTect was obtained in those indivi(hiais in which liic 

 ra(hation had successfully penetrated, added increments of ultraviolet 

 would be likely to contribute more rapidly to sterility than to the induc- 

 tion of additional mutations. A drop in mutation frequency would there- 

 fore be expected since the fertile flies with a high degree of filtration and a 

 low frequency of mutations would contribute a disproportionately greater 

 number of offspring to the next generation. 



Attempts to determine dosage relation in Drosophila by the polar cap 

 techniciue have been reported. Altenburg ct al. (1950) indicated that a 

 linearity of efTect obtains at low doses, but that a leveling of the curve is 

 rapidly achieved, after which large increments of dose are relatively inef- 

 fective in raising the frequency of mutation. Meyer ct al. (1950) also 

 found that a clustering of mutations occurs in cells or chromosomes 

 favorably oriented with respect to the radiation. A study of 1 1 chromo- 

 some II lethals showed that 10 of these chromosomes carried additional 

 mutations. Also, increases in dosage caused death to an increasing num- 

 ber of pole cells, as evidenced by the greater proportion of Fi offspring 

 carrying a particular mutation. 



Perhaps the best evidence oii the relation of ultraviolet effects to dosage, 

 from the standpoint of the avoidance of internal filtration difficulties, is 

 that from Tradcscaniia pollen-tube treatments. Chromatid deficiencies 

 here provide the criterion of effect (Swanson, 1942). The data indicate 

 that the relation is linear. Figure 7-1 illustrates the curve obtained with 

 wave length 254 m/n. 



Wave-length Dependence Studies. The earliest studies of the differen- 

 tial genetic effectiveness of various wave lengths of monochromatic light 

 on the germinal material of higher plants were made by Noethling and 

 Rtubbe (1934, 1936; see also Stubbe and Noethling, 1936). The dry 

 pollen of Antirrhinum majus was treated, and the detection of induced 

 mutations was made by observing the segregations in F2 populations. 

 The control populations had a comparatively high frequency of spon- 

 taneous mutations (1.6 per cent). From a study of Fo offspring from 

 some 3000 pollen grains treated with wave lengths 265, 297, 302, and 

 313 mn, the authors were able to show that each wave length was capable 

 of increasing significantly the frequency of mutations, the highest rate 

 obtained being approximately four times the control frequency. 



Several doses were compared at wave lengths 265 and 297 m^u. At 

 wave length 265 m/x, doses of 2 X 10'^ ergs/mm' failed to raise the muta- 

 tion frequency significantly, and the rise with increasing doses was slow. 

 At doses of 2.7 X 10' ergs/mm^ the frequency w^as twice that of the 

 controls, while doses of 14.6 X 10' and 50.3 X 10' ergs/mm''^ increased 

 it onlv to three times that of the control. The use of wave length 297 m^ 



