396 RADIATION BIOLOGY 



mentioned in Sect. 1) that in a living thing, as in any complicated organ- 

 ization the parts of which have been selected so as to interact nicely for 

 the accomplishment of a given difficult end result (this result in the case 

 of all living things being survival and reproduction), any change of a 

 part, initiated in a random way (i.e., without foresight or selection for 

 that end), brings about in the vast majority of cases a less efficient func- 

 tioning of the system (Muller, 1918, 1923). 



We have already seen (p. 395) that the lethals greatly outnumber the 

 visibles. It had long been suspected, however, that there are many more 

 detrimentals without visible effects than lethals, in line with the idea 

 that for viability, as for other characters, "small" mutations (i.e., those 

 with small effects) are more numerous than "large" ones. Because of 

 the difficulty of detecting such invisible detrimentals, their frequency 

 among spontaneous mutations has not yet been investigated. In the 

 case of radiation mutations, however, the matter has been studied. The 

 results obtained in two independent, simultaneous series of investiga- 

 tions, one by Timofeef-Ressovsky (1935a) and the other by Kerkis work- 

 ing under the direction of Muller (Muller, 1934; Kerkis, 1935, 1938), agree 

 in showing that in the X chromosome of Drosophila invisible detri- 

 mentals are induced with some 3 to 5 times the frequency of the com- 

 bined class of lethals and sublethals. The ratio may indeed be' consider- 

 ably higher than this, since the technique was hardly refined enough for 

 the detection of detiimentals with a viability greater than some 85 per 

 cent of the normal. Other studies have shown that "invisible" mutants 

 causing sterility or lowered fertility of some degree also form a very large 

 group. This group, however, overlaps, to an extent not yet well investi- 

 gated, that of the detrimental mutations. 



It is evident from this discussion that if mutations could be arranged in 

 order of the conspicuousness of their effects on the organism, so as to form 

 a kind of spectrum, there would be a comparatively narrow "visible" part 

 of this spectrum, with a much larger region to "the right" of it, compris- 

 ing the lethals, with effects so drastic as to remove the individuals from 

 our view before maturity, and a still larger region to "the left," compris- 

 ing the genes with invisible effects. This principle, first encountered in 

 the case of spontaneous gene mutations (Muller, 1923), has since then 

 been shown, through the much more detailed studies cited, to hold also 

 for those induced by ionizing radiation. 



13. EFFECTS OF CHANGING THE RELATIVE QUANTITIES (DOSAGE) 



OF GENES 



All the classes of mutations mentioned in the preceding section, dis- 

 tinguished according to their kind and degree of expression, i.e., according 

 to the set of observable characteristics or phenotype of the individuals 



