gg The Source of Variabilitij 



as thiamine, cysteine, and tryptophane. Further research demon- 

 strated the presence of many alleles, each necessary for the produc- 

 tion of a specific step in the biosynthesis of some one required 

 compound. For example, Demerec (1955) found that the action 

 of four sets of alleles in Salmonella corresponded to the four known 

 steps in the synthesis of tryptophane from anthranilic acid. 



These and other examples of biochemical genie effects have 

 given rise to the concept that genes act through controlling the 

 production of specific enzymes. Each gene is presumed to produce, 

 or cause to be produced, a specific enzyme which in turn brings 

 about a specific chemical reaction in the cell. 



MODE OF GENIC ACTION 



Cases of position effect and genes affecting single enzymatic reac- 

 tions have contributed greatly to the current view of how genes 

 act, whatever the genetic determiners are called. The following is 

 a summary of this view. The genetic determiners activate adjacent 

 substrate compounds which in turn produce enzymes catalyzing 

 still other reactions, and so on until the organism and its parts are 

 fully formed. How many such chemical steps occur between the 

 original action of the genetic material on the first substrate and 

 any one final product is not known. In some cases a few of the 

 final steps have been determined. In the bacillus Salmonella the 

 final synthesis of tryptophane is known to involve at least four 

 steps: from anthranilic acid to an unknown intermediate to indole 

 to tryptophane (Beadle, 1955). A different enzyme seems to be 

 involved at each step, and for each enzyme a different gene is 

 involved. Thus, products resulting from the action of one gene 

 appear to combine with the products of other genes to form dis- 

 tinctive substances, the whole integrated to form the mesh of 

 biological synthesis. In some cases a different character results not 

 from the action of a distinctive gene, but from interference with 

 chemical pathways due to the competition between genes for 

 identical substrates (Glass, 1957). 



It can readily be seen that character determination becomes 

 extremely complex in multicellular organisms having a well-devel- 

 oped soma. In an oak tree, the leaves, roots, and structures of the 

 flowers are removed by many physiological processes and by many 

 cell divisions from the original zygote. Yet, the genetic makeup of 

 that zygote in some fashion effected the differentiation of cells 

 and tissues through the control of chains of chemical reactions 

 leading to the completed individual. The same type of control is 



