NATURE OF THE GENETIC EFFECTS 441 



and/or activity. It is probable that a similar genetic mechanism is opera- 

 tive — this time on the foreign cells present — when radiation is used 

 against certain parasitic invaders, notably fungi, when they are so super- 

 ficially located as to be reached with a high enough dose without damaging 

 too much human tissue. Sterilization by irradiation — another example 

 of the same kind of effect — is, as earlier pointed out, inadvisable when it 

 is intended to be temporary, and it is usually more surely and safely 

 accomplished by other means when it is intended to be permanent. 



There is, however, an interesting possible use to which sterilization by 

 irradiation might be put, provided preliminary work along the given lines 

 on the screw-worm fly Callitroga americana by Bushland and Hopkins 

 (1951) is borne out in extensive field tests. The objective in such cases is 

 the checking, and perhaps in given areas even the extermination, of cer- 

 tain noxious species (in the given case a parasitic species) which are sub- 

 ject to great seasonal variation in numbers. The principle of the method, 

 which was suggested by E. F. Knipling, is to irradiate very heavily great 

 masses of individuals, artificially bred on a vast scale for that purpose, 

 and then to sow them widely throughout the breeding ground of the 

 species during periods when the natural population, at its lowest ebb, 

 is just about to multiply again. The irradiated individuals, being more 

 numerous than the wild ones and mating with them, should tend to 

 swamp out the latter 's multiplication. A factor contributing to this 

 result is that the spermatozoa of the recently irradiated males are still 

 functional, so as to compete with normal sperm, but carry structurally 

 changed chromosomes which cause the death of the great majority of 

 zygotes resulting from the cross with untreated females, while the few 

 offspring which do survive, although phenotypically normal, are never- 

 theless laden with structurally changed chromosomes which in turn kill 

 off a large proportion of the zygotes in the next generation. By repeated 

 large-scale application of this method to a population already depleted 

 by it, it might be possible progressively to reduce its numbers, provided 

 the foci of natural breeding are sufficiently accessible to be adequately 

 reached in the process of distributing the irradiated individuals. 



Another method of utilizing individuals which have suffered genetic 

 damage from irradiation — this time probably in the form of gene muta- 

 tion, however — for checking the ravages of those of the original type has 

 often been proposed in the case of parasitic microorganisms. This 

 involves the production of strains whose virulence has been so decreased 

 that they may be used as live vaccines, in the manner of cowpox or BCG. 

 The chief difficulty lies in obtaining varieties which can be relied upon 

 never, despite their vast multiplication, to change back again via reverse 

 or "suppressor" mutations to a virulent form. Doubtless a whole com- 

 bination of mutations would be required for this. Tests for the presence 

 of these, after the first mutation had already made the organisms non- 



