NATURE OF THE GENETIC EFFECTS 443 



Thus the decision as to whether the radiation technique will be helpful 

 must depend upon a number of factors which differ according to the 

 species and variety in question. In general, the larger, the slower grow- 

 ing, and the more expensive to raise a type of organism is, and the smaller 

 the potential number of offspring per individual, the less suited it is to 

 the use of the radiation technique for its improvement, since the indi- 

 viduals are less expendable. For the present this rules out mammals in 

 most but not all cases. Furthermore, the more readily available large 

 populations of the organism are, for a search for spontaneous mutations, 

 the less advantage, other things being ecjual, is afforded by the applica- 

 tion of radiation for mutagenesis. Where definite recessive mutations 

 are wanted, in organisms which are mainly crossbred but can be readily 

 inbred, these mutations, of spontaneous origin, can often be found with 

 as high a frequency merely by inbreeding (and especially by selling) as 

 that with which eciuivalent mutations produced by radiation can be 

 found. To match the fact that one irradiation may cause, say, 20 times 

 as many mutations as arise spontaneously in one generation, the spon- 

 taneously arisen mutant genes have usually accumulated for a good many 

 more than 20 generations, on the average, so that although they are sel- 

 dom seen without inbreeding, they have nevertheless attained a corre- 

 spondingly high frequency in the population. Inbreeding will then bring 

 them to light. Moreover, it is sometimes true that the very mutations 

 which are more desirable are more likely, through natural selection, to 

 have attained a higher natural frequency. 



Radiation is accordingly most likely to be found advantageous for 

 small, rapidly multiplying organisms, including especially those which 

 are habitually selling (like many higher plants), or which have major 

 haploid phases (as in Hy menoptera, Rotif era, and many microorganisms) , 

 and which have therefore been weeded comparatively free of spontaneous 

 mutants. Where it is much easier for the breeder to obtain such organ- 

 isms by cultivating them himself than by searching extensively for them 

 abroad, the advantage of using radiation on them becomes emphasized. 

 Where mutations are desired within restricted varieties that it is either 

 impossible (e.g., because of their asexual nature) or impracticable (e.g., 

 because of their complex of desired characteristics) to outcross, this cir- 

 cumstance constitutes another feature which favors the use of the irradi- 

 ation technique. 



Where the circumstances for irradiation are favorable, one is not 

 necessarily confined to hunting, among the descendants of the treated 

 organisms, for mutations which individually produce large, definitely 

 recognizable effects, like most of the mutations dealt with in conven- 

 tional genetic studies. Neither need the mutations be recessives. Thus, 

 as the already cited work of Buzatti-Traverso on Drosophila has shown, 

 the practice of ordinary selection, even without inbreeding, can, if inten- 



