384 AN INTRODUCTION TO MODERN GENETICS 



3. Mutations and X-rays^ 



The effect of X-rays is a general increase in mutation rate; there is 

 apparently no specificity in the response, and it is impossible to stimu- 

 late the production of any particular allelomorph. It is possible that 

 such a specificity may be obtainable by the use of monochromatic ultra- 

 violet radiation, since on theoretical grounds one might suppose that 

 particular genes may be sensitive to particular wave lengths, but as yet 

 the technical diflaculty of getting the radiation into the gonads without 

 injuring the more superficial tissues has prevented much work being 

 done on these Hnes.^ 



The types of mutation produced by X-rays are of the same general 

 nature as those occurring spontaneously; in fact, many, though not all. 



Fig. 154. The Proportion of Lethal and Visible Sex Linked Mutations in 



Drosophila melanogaster 



(From Timofeeff-Ressovsky.) 



the X-ray induced mutations were known previously. It is more difficult 

 to decide whether the different types of mutation occur with the same 

 relative frequencies under X-rays as they do spontaneously. At first 

 sight, one is struck by the high percentage of lethals in X-rayed flies 

 (over 90 per cent of all mutations produced) but when the spontaneous 

 mutation is carefully investigated by the same methods, it is found that 

 there too lethals make up as high, or nearly as high, a proportion of the 

 total. Timofeeff"-Ressovsky^ has recently shown that there is an even 

 more frequent class of sublethals whose only effect is a slight lowering 

 of viability. These are normally missed by ordinary m.ethods of investi- 

 gation (they are only detectable by their effect on the sex ratio in F2's 

 from CIB cultures), but there is no reason to doubt that they occur in 

 an equally large proportion in untreated flies. They have only recently 

 been detected, and are not included in the figures for mutation rates 

 given in other parts of this section. Chromosome aberrations are also 

 plentifully produced by X-rays. 



^ General references: Muller 1934, Timofeeff-Ressovsky 1934^, 1937. 

 ^ Altenburg 1934, Reuss 1935. Extensive work on maize by Stadler is not 

 yet published. Cf. Stadler and Sprague 1936. ^ Timofeeff-Ressovsky 1935. 



