MECHANISMS OF GENE ACTION 303 



of other modes of gene action? In Chapter 9, we have discussed the 

 hkehhood that determinants exist which influence pattern and organiza- 

 tion of such structures as chloroplasts, mitochondria, and kinetosomes, 

 and perhaps also of other structural components. Furthermore, the 

 genetic identification of nonchromosomal determinants apparently un- 

 linked with any known structures suggests the existence of virtually 

 unexplored aspects of cell heredity. 



Another line of evidence, based upon the nonrandom distribution of 

 known genes, suggests that the genetic potentialities of the chromo- 

 somes themselves have not been fully explored. The organism in which 

 the largest number of mutants has been identified is Drosophila melano- 

 gaster, in which even lethals have been mapped, and the genetic data 

 have been correlated with cytological maps of the salivary chromosomes. 

 Even in this organism, there are sizable regions, scattered throughout 

 the genome, and no different in appearance from the rest, in which no 

 mutants have been found. Sampling error cannot be invoked to explain 

 these results, in view of the large number of mutants which have been 

 obtained by all sorts of treatments. Similar evidence comes from the 

 linkage maps of maize, which also show the nonrandom distribution of 

 markers, and from Neurospora, in which no mutants have yet been found 

 on 5 of the 14 chromosome arms. 



In recent experiments with bacterial systems, a class of mutations was 

 identified which appears to control only the rate of enzyme formation, 

 not specificity. From these and related studies, a new dimension of 

 gene action, often suggested from previous work with higher plants and 

 animals, is becoming available for more critical analysis at the cellular 

 level. This new information will be discussed in the next chapter, and 

 then we shall return to a further consideration of gene action. 



BIBLIOGRAPHY 



Atwood, K. C, and F. Mukai, 1953. Indispensable gene functions in Neuro- 

 spora. Proc. Natl. Acad. Sci. Wash., 39:1027. 



Bonner, D. M., 1951. Gene-enzyme relationships in Neurospora. C. S. H. 

 Symp. Quant. Biol., 16:143-157. 



Case, M. E., and N. H. Giles, 1960. Comparative complementation and genetic 

 maps of pan-2 locus in Neurospora crassa. Proc. Natl. Acad. Sci. Wash., 

 46:659-676. 



Catcheside, D. G., 1960. Relation of genotype to enzyme content. In: Micro- 

 bial Genetics, 10th Symp. of the Soc. for General Microbiology. Cambridge, 

 The University Press, pp. 181-207. 



