Phenocopy and Norm of Reaction 259 



many cases to decide. For example, an eye-color mutant in Dro- 

 sophila which prevents the synthesis of kynurenine from tryptophane 

 may be due to the non-synthesis of an enzyme controlling the synthetic 

 step, that is, a qualitative feature. But it may also mean that the 

 enzyme or a coenzyme or a pH condition needed is not available 

 when and where needed because of pure timing changes in the set 

 of interlocked reactions which have to be in tune in order to work. A 

 quotation from Bonner (given below) shows that a prominent bio- 

 chemical geneticist has also come to the conclusion that the kinetic side 

 of genie action may be more important than the qualitative aspect em- 

 bodied in the one gene — one enzyme theory. 



To make the situation clearer, let us visualize the case of a mutant 

 and its phenocopy based upon a qualitative chemical difference. Let 

 us assume that we know chemically the substance needed to produce 

 a normal leg in Drosophila, its absence being responsible for a dachs 

 leg. This means that we take it for granted that the morphological 

 difference is based upon the presence or absence of a single gene- 

 controlled substance, enzyme or otherwise, and not upon quantitative 

 changes in the velocities of growth and differentiation (though these 

 may ultimately be caused by the presence, absence, or quantity of 

 a "growth hormone"). The same morphological effect as a phenocopy 

 could be produced by the different means mentioned. Should we 

 conclude that the substance in question is also missing in the pheno- 

 copy? 



We know now that many unspecific shocks at the proper time can 

 produce the same phenocopy, but also that one and the same shock 

 may produce different phenocopies. It is hardly conceivable how such 

 facts could be understood in terms of a substance, while they strongly 

 suggest interference with the kinetics of a system of reactions, as I 

 emphasized above. Moreover, this argumentation shows another very 

 important point. It is possible that the facts of phenocopy indicate 

 that also the mutants which are copied are based upon an interference 

 with the kinetics of determinative processes. However, this does not 

 mean that the primary causes are the same in both cases. Thus it is 

 quite conceivable that in the phenocopic experiment the shock inter- 

 feres with the velocity of some reaction by affecting its kinetics 

 directly (e.g., through the temperature increment), while in the muta- 

 tion the same effect upon the velocities is produced by a more remote 

 change (e.g., an insufficient quantity, or quality, of a growth sub- 

 stance). In this case both phenocopy and mutant are caused by the 

 same thing, a change in growth rate, but the phenocopy, by direct 



