RADIATION AND THE STUDY OF MUTATION IN ANIMALS 1217 



actually found. Timof(5eff-Ressovsky has studied this problem (184), 

 and came to the conclusion that he is indeed dealing with recurrences 

 of the same mutations, as far as the present technique can establish this. 

 Each member of the series presents a group of effects whose association 

 is constant. It may be noted that Friesen (44) has reached somewhat 

 different conclusions, and more recent studies of Bridges (unpublished) 

 show an unsuspected variety in the array of white allelomorphs — some 

 of which may, nevertheless, on occasion be indistinguishable each from 

 the other. Yet, as a first approximation, Timofeeff-Ressovsky's con- 

 clusion may perhaps be adequate. 



Table 4. — Mutations in Different Directions at the White Locus in Droso- 



PHILA MELANOGASTER 



Treated adult males, circa 4800 r. (Timofeeff-Ressovsky, 183) 



w; 



-bf 



,^ 



w 



w: 



w 



w 



X 



^ 



The mutation experiments are, as has been said, enormously lal)orious, 

 although the technique is simple. The rates are small, and it is difficult 

 to attach more than qualita- 

 tive value to such low per- 

 centages. Nevertheless, 

 Timofeeff-Ressovsky has 

 demonstrated (Table 4) that 

 any one member of the series 

 may on occasion change into 

 another (Fig. 3), albeit with 

 different frequencies. A 

 striking instance is afforded 

 by the differences between the frequency of mutation to different 

 allelomorphs of the two normal genes previously referred to (Table 3). 

 The extreme reverse mutation, from white to wild type, has not been 

 found; yet even here a two step reversion is possible: white to eosin, 

 eosin to wild type. This is, as it were, a limiting case; for, in general, 



Fig. .3. — (a) Mutations from the wild type, and 

 from white, to different intermediate allelomorphs: 

 (h) mutations at the white locus to and from the 

 intermediate allelomorph eosin (w*). [After 

 Timofeeff-Ressovsky (183). 1 



