Gene Action and Polypeptides 



407 



FIGURE 32-2. 



Formula for 

 phenylpyruvic acid. 



H 



I 



H— C C— H 



H— C. JZ— H 

 C 



H— C— H 



c=o 



I 

 COOH 



ated or circumvented if phenylalanine — 

 which is essential to proteins — is reduced in 

 the diet to an amount sufficient for protein 

 synthesis but insufficient for any appreciable 

 quantity to be converted into phenylpyruvic 

 acid. Since tyrosine is also needed for hu- 

 man protein, it must be present in sufficient 

 quantity in the diet of phenylketonurics. 

 Finally, it should be noted that a parahy- 

 droxylase, which converts phenylalanine to 

 tyrosine and is normally present in the liver 

 (where most of the phenylalanine is nor- 

 mally metabolized and oxidized), has not 

 been found in phenylketonurics. 



Inborn metabolic defects are of great help 

 in identifying the places where genes direct 

 metabolic processes. They also permit the 

 determination of precursors of a genetically- 

 defective step and aid in the study of chains 

 of biochemical reactions and metabolic path- 

 ways. For example, if mutant 1 cannot form 

 substance Y but accumulates substance X, 

 and if mutant 2 can only form Y when X 

 is supplied, then X must be a precursor of 

 Y (Figure 32-3). 



Biochemical genetics is of special inter- 

 est in another respect. In the cases most 

 thoroughly investigated, one can trace the 

 pedigree of causes back to a point where only 

 one effect of the gene is detected, for exam- 

 ple, as in alcaptonuria. It is quite improb- 

 able that further study of the gene for al- 



captonuria will reveal another phenotypic 

 effect which, when tested adequately, will 

 prove to be produced independent of the 

 effect upon homogentisic oxidase. Present 

 findings suggest, therefore, that this gene 

 acts upon the phenotype only in one, pri- 

 mary way. 



One Gene-One Primary Effect Hypothesis 



The study of biochemical genetics in this 

 chapter (and also in Chapter 6) leads us to 

 hypothesize that each gene has only a single, 

 primary, phenotypic effect and that all the 

 pleiotropic effects of a gene stem from this 

 single activity. If the hypothesis of one 

 gene-one primary phenotypic effect is sub- 

 stantiated, we may be able to determine the 

 size or scope of the genetic material whose 

 action produces a single, primary effect. Such 

 information would reveal the nature of the 

 functional genetic unit, but it should be real- 

 ized that this kind of information will de- 



figure 32-3. 



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 2 



I added 



— Y 



