380 PRINCIPLES OF EMBRYOLOGY 



into the so-called 'one-gene-one-enzyme' theory. This supposes that all 

 genes operate through the medium of enzymes, and that each gene is con- 

 nected in the first place with one and only one enzyme, hi one form of the 

 theory the gene is supposed actually to produce the whole enzyme; in 

 another the suggestion is only that the gene determines the specificity of the 

 enzyme, and thus its abihty to react, and its efficiency in reacting, with the 

 appropriate substrates. 



This hypothesis has been widely beheved in recent years, and even when 

 some doubts are expressed as to its adequacy, it has usually been given 

 credit for having stimulated a great deal of valuable work. Actually, 

 however, it is probable that the stimulus to the work came rather from the 

 experimental technique of identifying mutations which lead to the block- 

 age of a series of synthetic reactions rather than from the one-gene-one- 

 enzyme hypothesis itself. In fact, almost the first problem which the 

 hypothesis would raise is whether a given gene necessarily controls the 

 formation of the same enzyme in different tissues, in which it is reacting 

 with different cytoplasms. This question, however, still remains quite 

 unanswered and hardly tackled, not, surely, because it is far removed 

 from theory but because it demands a different experimental approach. 



The gravest criticism of the one-gene-one-enzyme theory is that it 

 draws its support almost entirely from studies of unicellular or very 

 primitive organisms and thus leaves out of account of the whole range 

 of phenomena involved in regionahsation, which may or may not fall 

 into hne with it. Even within the realm of the micro-organisms, it seems 

 that at the present time the theory is beginning to appear as an over- 

 simplification (cf Haldane 1954). However, work of the kind associated 

 with it has led to the production of numerous genes which have thrown 

 light on a wide variety of problems. For instance, it has been shown that 

 a gene which has mutated to an apparently inactive form may many 

 generations later mutate back again, and the corresponding activity 

 reappears in the cells. This shows that a gene may continue to multiply 

 in a sequence of dividing cells even though it shows no signs of activity; 

 though of course it remains doubtful whether the gene is truly inactive or 

 is continuing all the time to produce an enzyme of altered specificity. 



Again, this material provides some relatively clear-cut examples of 

 secondary interactions between gene-controlled processes of the kind 

 which seem necessary as a basis for a general theory of development (cf. 

 Chapter XIX). Thus in Neurospora a certain gene blocks the formation 

 of isoleucine and this leads to the accmnulation of its precursor, which 

 cannot be utiUsed but increases in concentration until it inhibits a different 

 but connected reaction by which a-ketoisovaleric acid is changed into 



