278 



CHAPTER 31 



FIGURE 31-5. 



Determination of precursors using mutant genes. 

 A accumulates X but makes no Y. 

 B makes no X but will make Y if X is supplied. 

 C is the normal pathway. 



mutant 1 



B 



mutant ? 



X 



added 



-^Y 



-^Y 



it has been possible to trace the pedigree of 

 causes back to a point at which only one 

 effect of the gene can be detected. This was 

 true, for example, in the case of alcaptonu- 

 ria. It is still possible (though improbable) 

 that further study of this case would reveal 

 that the gene for alcaptonuria, besides pro- 

 ducing an effect upon the particular enzyme 

 that metabohzes alcapton, has another effect 

 which has no determinable relation to the 

 enzyme effect. Such a finding would suggest 

 that this gene acts upon the phenotype in 

 more than one primary way. 



In most of the book up to this point, we 

 have employed the phenotypic effects pro- 

 duced by genetic differences to reveal the 

 nature of the genetic material. Nevertheless, 

 the properties of segments of the genetic 

 material were described not in terms of pheno- 

 typic effects, but in terms of recombination 

 and mutation. In this Chapter and certain 

 preceding ones (particularly those Chapters 

 not marked by asterisks), we were concerned 

 more with the nature and consequences of 

 the phenotypic effects produced by the genetic 



material than we were with discovering the 

 recombinational and mutational properties 

 of this material. What have we learned about 

 the way that the genetic material produces its 

 phenotypic effects? We could cite abundant 

 evidence that the genetic material does not 

 act upon the phenotype as one single indivis- 

 ible unit, but acts as though it is segmented 

 into a number of units, or genes, each of 

 which has a specific phenotypic effect. (Such 

 units or genes for phenotypic effect were re- 

 vealed because mutation made changes in 

 genes, these different genes underwent re- 

 combination, and served, in some way, to 

 produce different phenotypic effects.) 



The simplest hypothesis is that the genetic 

 unit of phenotypic effect is identical to the 

 genetic unit of recombination, for in all our 

 previous discussion the recombinational gene 

 seemed to be identical with the phenotypi- 

 cally functional gene. But, it must be em- 

 phasized, because these two views of a gene 

 are revealed through the use of different opera- 

 tional procedures, that the gene as a unit of 

 phenotypic effect, the functional gene, could 

 be larger than, identical to, or smaller than 

 the recombinational gene. (We have already 

 noted, on p. 199, the possibility that the seg- 

 ment of the genetic material defined by the 

 recombinational gene may not be identical 

 with the material affected when a single re- 

 combinational gene undergoes mutation.) 



Our study of the genetic material revealed 

 that it contains several different sized units 

 of genetic recombination, including the 

 genome, chromosome, and chromosome seg- 

 ment. The term gene came to be used to 

 refer to the smallest unit of the genetic ma- 

 terial capable of undergoing recombination. 

 The study of the genetic material also showed 

 there were different sized units of mutation 

 — the genome, chromosome, and chromo- 

 some segment. It should now be apparent 

 that we ought to embark upon a study to 

 determine the smallest unit of the genetic 

 material capable of producing a phenotypic 



