Primary Actions 279 



enters the picture. However, as it deals primarily with difiFerentiation, 

 it will be discussed in a later chapter. 



C. SPECIAL THEORIES OF GENIC ACTION 



In an introductory discussion to the chapter on genie action I 

 presented a very generalized concept of genie action, which probably 

 fits all sides of the problem, including all the more special theories. 

 We shall now discuss such special theories, which attribute specific 

 effects to the genie material. In these theories it is of no great im- 

 portance whether the primary gene products act at the chromosomal 

 site or within the nucleus or in the cytoplasm as long as their effect 

 can be described in biochemical terms characterizing one decisive 

 result of genie action. The most elaborate theory of this type is 

 Beadle's original one gene — one enzyme theory and its subsequent 

 formulation as one gene — one function. The general theory was first 

 conceived (but overlooked by all of us) by Garrod (1909) for the 

 genetics of human metabolic disturbances, and was also briefly stated 

 by Haldane (see 1954), but was independently derived later as a 

 result of different attacks upon problems of biochemical genetics in 

 more or less concrete form by Kiihn and Caspari and by Beadle and 

 Ephrussi for the determination of eye pigments, by Lawrence and 

 Scott-Moncrieff for plant pigments, and by Moewus for sex-determin- 

 ing stuffs in Chlamydomonas. However, its strict formulation and 

 experimental underpinning on a large scale are due to Beadle and 

 Tatum (1941a) and their students and followers in their famous work 

 on nutritional mutants in Neurospora and bacteria. 



a. The one gene — one action theory 



The general trend of the facts and conclusions is well known 

 and has been reviewed many times, especially by Beadle (1945, 1949), 

 Horowitz (1950), and Catcheside (1951). It will suffice here to 

 mention the general setting for facts and conclusions. If it is known 

 that a certain material, vitamin, or amino acid is needed for normal 

 growth, a deficiency of the stuff or its absence prevents growth. 

 Mutants are found which have such specific effects; for example, 

 nicotinic acid cannot be synthetized, and growth occurs only where 

 this is added to the medium. Of course, such additions provide the 

 method of finding the specific deficiency. If it is known that nicotinic 

 acid is normally synthetized in a series of consecutive steps — trypto- 

 phane, kynurenine, 3 hydroxykynurenine, 3 hydroxy anthralinic acid, 

 nicotinic acid, or, in short, A-B-C-D-E — it can be tested by the afore- 



