250 UADIATIO.N lilol.oCY 



sufficient to advance malctinllx' tlic uiKicrstandiiiji; of I lie nature of 

 induced hereditary chanjies. 



'I'li<> genetic action ot ult ia\iolel radiation hears on threi- major proh- 

 li-nis. riie lirst ot" these is the analysis of the coniph'x of }:;<*"<'tie varia- 

 tions induced l)y the ioni/ins radiations. It is invariably found, where 

 adeciuate cytogenetic tests can he made, that hoth mutations and chro- 

 mosomal aberrations are induced by X radiation. A clear-cut separation 

 of the two phenomena has not yet been accomplished, and the question of 

 their similar or dissimilar nature and origin remains unanswered. If the 

 induction of mutations and chiomosomal rearrangements by the ionizing 

 radiations results from some common, fundamental effect, the two types 

 of genetic alteration should have a common spectral limit. The analysis 

 of induced hereditary variations would then involve determining the 

 alternate pathways of reaction which culminate in a variety of genetical 

 and cytological expressions. If, on the contrary, the diverse effects of X 

 radiation are of independent origin, it is possible that their spectral rela- 

 tions may be sufficiently different to permit the separation and perhaps 

 the selective induction of one type of variation to the exclusion of others. 

 The individual phenomena should then pro\e to lie more amenable to 

 analysis. 



The second problem is concerned with the determination of genetic 

 effectiveness of specific wave lengths of the ultraviolet spectrum, as a 

 ckie to the chemical nature of the substance within which the energy 

 absorption leading to genetic change takes place. With ionizing radia- 

 tions such a study is not possible since their absorption is independent 

 of molecular organization. 



The third problem is concerned with the nature of mutations in general, 

 whether spontaneous or induced. Mutations as experimentally identified 

 are a residual class, identified by negative criteria. The analysis of spe- 

 cific X-ray-induced mutations has shown that character changes inherited 

 as if due to gene mutations may be in some instances the result of chromo- 

 somal rather than genie alteration. The category "mutations," as 

 experimentally defined, must therefore be a complex one, including vari- 

 ous extragenic as well as intragenic alterations. Since the mode of action 

 of ultraviolet radiations is so different from that of ionizing radiations, the 

 comparativ^e study of mutations induced by these agents is promising. 

 The relative infrequency of chromosomal derangements induced by 

 ultraviolet suggests that certain extragenic alterations simulating gene 

 mutation may be infre(|uent or absent among the mutations from ultra- 

 violet treatment. The possibility of qualitative differences in the chro- 

 mosomal alterations must also be considered. 



The spectral analysis of the complex of genetic effects induced by X 

 rays, as outlined above, implies the as.sumption that the mutations and 

 chromosomal alterations induced by X rays are qualitatively analogous 



