Biochemical Genetics (I) 



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effect. This, too, can be called a gene. Now 

 that we have genes which are to be defined 

 by at least two different operations, it becomes 

 inconvenient for the reader to have to decide 

 from context, each time the term gene is 

 used, whether the gene referred to is defined 

 by recombination or by phenotypic effect. 

 Accordingly, let us give these two genetic 

 units specific names, even though we have 

 no evidence so far that they differ in their 

 material basis. We can define a recon as the 

 smallest unit of the genetic material which is 

 capable of recombination, and a cistron as the 

 smallest unit of the genetic material capable 

 of producing a phenotypic effect. When the 

 less specific, nonoperational, term gene is 

 used, henceforth, it will refer to the smallest 

 unit of the genetic material as identified by 

 these, and especially other, operations. 



Note that the recon, or cistron, is not de- 

 fined in terms of properties which are beyond 

 experimental test (see p. xi). For example, 

 although the recon is part of the genetic ma- 

 terial, we do not endow it with assumed or 

 established properties of the total genetic 



material. Moreover, the recon is not the 

 smallest unit of the genetic material capable 

 of self-replication, of mutation, or of pheno- 

 typic influence. Notice also that there is no 

 restriction as to the lower limit for the size 

 of the recon or the cistron. 



The study of biochemical genetics in the 

 present Chapter (and also in Chapter 10) 

 leads us to make the simplest hypothesis, 

 namely, that a cistron produces only a single 

 primary phenotypic effect, and that all its 

 pleiotropic effects are consequent to this 

 single activity. We can call this hypothesis 

 a one cistron-one primary phenotypic effect 

 relationship. If this hypothesis can be further 

 tested and supported, we may be able to use 

 the information gained in reverse, so to speak, 

 to learn more about the size or scope of the 

 genetic material required to produce a single 

 primary effect. This would give us informa- 

 tion regarding the nature of a cistron. But 

 you should realize that the kind of informa- 

 tion we obtain about the cistron will depend 

 upon what we consider to be primary and 

 what we identify as a phenotypic effect. 



SUMMARY AND CONCLUSIONS 



While the genes detected by recombination and by phenotypic effect are related to each 

 other, their material basis need not be identical. The recon and cistron are defined, re- 

 spectively, as the smallest unit of the genetic material capable of recombination, and of 

 phenotypic effect. 



The biochemical activities necessary for the existence of protoplasm are controlled by 

 the nucleus, presumably by the cistrons it contains. These chemical reactions occur in 

 sequences that form often-branched metabolic pathways leading to the chemical, physical, 

 physiological, developmental, and morphological aspects of the phenotype. As a conse- 

 quence of this branching, most, if not all, cistrons have pleiotropic effects. 



The phenotypic differences produced by alternative cistrons can be traced by a pedigree 

 of causes back toward the cistron. Such studies demonstrate that cistrons produce their 

 effects at the metabolic level. It is reasoned that such action could involve the structural 

 components of protoplasm (organelles) or particular chemical substances, including enzymes 

 specifically, aUhough none of these effects need exclude any of the others. 



The study of inborn errors of metabolism in man demonstrates that cistrons control 

 various steps in biochemical sequences, through their influence upon enzymes. At least in 

 these particular cases, the effect on enzymes can be supposed to be the primary and only 

 consequence of the action of a segment of the genetic material. 



In view of the experimental results, a one cistron-one primary effect relationship is hy- 

 pothesized. 



