296 BIOCHEMICAL SYSTEMATICS 



Neurospora. In some cases mutations which appear to affect the 

 same end product in a synthesis are found to be complementary. 

 That is, when a mixture of the two mutant type nuclei is allowed 

 to become established in a single mycelium, normal growth results. 

 This result implies that the individual mutants affected a different 

 step in a series of events leading to the synthesis or utilization of a 

 substance. In non-complementary mutants normal growth is not 

 restored in the presence of mixed nuclei, and one may infer that the 

 same functional step was affected by both mutants. With more 

 refined genetic techniques it is possible to compare a series of muta- 

 tions that are non-complementary. If such non-complementary mu- 

 tants are assumed to involve, by definition, the same gene, then 

 in effect one is studying the intragenic aspects of mutation. This is not 

 the time to engage in a discourse on modern methods of genetic fine 

 structure analysis. Anyone not yet appreciative of the remarkable 

 advances in this direction will find the summary by Glass (1957) and 

 the more recent treatment by Jacob and Wolman (1961) of great 

 value. In such a vital field new discoveries are frequent, and the non- 

 specialist who can keep abreast of such discoveries must be indeed rare. 

 Two smaller volumes which summarize this aspect of modern genetics 

 in highly readable form are those by Pontecorvo (1958) and Strauss 

 (1960). 



For several reasons, microorganisms have overshadowed 

 higher plants and animals in their contributions to progress in bio- 

 chemical genetics. Notable examples are certain bacteria {E. coli) and 

 fungi (other than Neurospora, there is for example Aspergillus). 

 Populations which number in the millions, adaptability to sterile 

 culture, and short generation time combine powerful advantages. 

 There is also the additional advantage of haploidy, which guarantees 

 immediate exposure of even the recessive mutant. Since recessive 

 mutants vastly outnumber other types, this last advantage is of 

 special significance. Consider the difliculty in disclosing the true extent 

 of radiation-induced gene mutation in a population of human beings. 



With advantages such as those cited above, it is easy to 

 understand why biochemical genetics in higher organisms is lagging 

 so conspicuously. Most of the biochemical mutants detected in higher 

 plants involve secondary substances such as pigments and storage 

 products. Recently, biochemical mutants in the small crucifer genus, 

 Arabidopsis, have been studied with some success (Langridge, 1955). 

 This plant, small enough to be grown in a test tube, thus on defined 

 media, has a relatively short generation time and has now yielded 

 some biochemical mutants affecting primary metabolites. 



We may forecast the emergence of new techniques which will 



