258 Action of the Genetic Material 



development. Other investigators later found that many other shocks 

 like cold, mustards, X rays and other radiations, and many chemicals 

 could produce the effect. The decisive points were: (1) treatment 

 with strong agents, powerful beyond the normal range of the en- 

 vironment; (2) treatment at definite critical periods of development; 

 (3) the genotype of the material which influences the result. It may 

 be said that in Drosophila (and therefore everywhere, if adequate 

 experiments were made) every known type of mutant can thus be 

 copied as a phenocopy. The only group about which there is uncer- 

 tainty are the biochemical mutants (pigments, etc.), for which no 

 unequivocal positive result exists as yet. Some which have been 

 claimed were probably misinterpreted (e.g., straw as a chemical 

 phenocopy). But this statement applies only to Drosophila; in micro- 

 organisms, also, biochemical mutants have been phenocopied. Positive 

 results for production of phenocopies have been obtained from man 

 to bacteria; so there can be no doubt of the ubiquity of the phenome- 

 non. For Drosophila the most detailed data are found in Goldschmidt 

 (1935), Henke et al. (1941), and Ma (1943). Henke et al think 

 they can distinguish between genuine and false phenocopies, the latter 

 being those the critical period of which does not coincide with that 

 of a similar mutant studied. I do not think that this distinction carries 

 any weight. Most mutant phenotypes are known for a number of loci. 

 Innumerable mutants are being discarded all the time; others have by 

 chance not yet been observed. Thus it is impossible to say that a 

 mutant (or a few of them) checked against a phenocopy is the right 

 one. Another unchecked mutant may have the proper critical period. 

 In view of the fact that practically any kind of shock treatment 

 produces phenocopies in well-worked material, it is very difficult to 

 assume that the shock produces differentially a chemical product, 

 enzyme, substrate, active substance, identical with one produced by 

 a mutant locus. It is much more probable that the kinetics of a chain 

 of reaction, elaborating such active substances, is influenced in such a 

 way that velocities of processes, quantities of products, relative timing 

 of collaborating reactions, and concentrations amounting to threshold 

 settings will be the main factors that are being shifted in plus or minus 

 directions. If this is so, it might mean also that the mutant action 

 affects essentially such processes, though it cannot be denied that in 

 individual cases a phenocopy might take a different path from that 

 leading to the corresponding mutant phenotype. This does not mean 

 that mutant actions do not exist which directly affect the quality of 

 a substance needed for development, but it will be very difficult in 



