10 



Some Mechanisms of Gene Action 



How are the hereditary potentiaUties of a cell expressed? What are 

 the chemical pathways leading from genotype to phenotype? In the pre- 

 ceding chapters, emphasis has been placed primarily upon analysis of the 

 genetic material itself. Nonetheless, phenotypes have been intimately 

 involved in all experiments, since the presence, absence, or mutated 

 state of the genes could only be assessed in terms of their expression. 

 Even experiments in which nucleic acids were treated in vitro, required 

 a resulting phenotypic change to permit a genetic interpretation. 



Thus, in fact, all genetic investigations have involved considerations 

 of gene action, but investigators of transmission genetics have usually 

 preferred to regard phenotypes merely as formal "markers" by means of 

 which to follow the genes. This approach resulted in part from the dis- 

 maying complexity of the problem, for most mutations studied in higher 

 organisms have involved intricate morphological changes such as altered 

 bristle number or wing shape in Drosophila, or taillessness in mice. 



THE SEARCH FOR ANALYZABLE TRAITS 



The first problem in the analysis of gene action has been the choice of 

 suitable phenotypic traits, amenable to study. With higher organisms, 

 the most extensive studies have dealt with pigment changes, for example, 

 alterations in the content of anthocyanins and anthoxanthins in plants 

 following mutation at individual loci. It was shown in a series of re- 

 searches, which began as early as 1913, that a single gene mutation could 

 block the formation of a single pigment and, in some instances, of more 

 than one pigment. In the primrose, for example, a magenta-colored 



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