SPECIFICITY OF GENE EFFECTS 239 



the series of steps would have proceeded only to that j)oint. Following the 

 initial work in Neurospora by Beadle and Tatum (1941) on mutants which 

 blocked certain metabolic processes, this type of approach has expanded 

 enormously and profitably. Attention can be called here to but one very sig- 

 nificant example of this kind of experimental study in microorganisms. A 

 report by Srb and Horowitz (1944) shows clearly how many genes act in the 

 synthesis of arginine. Of fifteen mutant strains studied, there were seven dif- 

 ferent steps represented in the synthesis of arginine. One of the forms grew 

 only if arginine was supplied. Two others required either arginine or citrul- 

 line, and these two strains were genetically different. Four other strains, 

 genetically different from the first three strains and from each other, would 

 grow if arginine, citrulline, or ornithine were provided. For a diagrammatic 

 representation of these steps, see Beadle (1945). 



DIRECT EFFECTS OF GENES 



The preceding examples are but a few^ of the many which could be cited to 

 illustrate the gene specificities in the development of a genetic character 

 which involves the successive activities of many genes. Are there any genetic 

 characters which may be the immediate products of the causative genes? An 

 example almost unique in higher plants is that of the waxy gene in corn 

 (Collins, 19C9) in its effects upon the starch of the pollen grain and the endo- 

 sperm reserves. As is well known, the starch granules in the pollen grains 

 bearing the waxy gene are stained reddish-brown with iodine, as are the 

 endosperm reserves of waxy seeds, in contrast to the typical blue reaction of 

 the starch granules of non-waxy pollen and of the endosperm reserves of 

 non-waxy seeds. Following studies of the physiological effects of the waxy 

 gene, Brink (1929) proposed that this gene has its effect on the enzyme amy- 

 lase which functions directly in the synthesis of starch. 



Another class of hereditary characters which in some respects appears to 

 satisfy some of the criteria for a direct effect of the causative genes is that of 

 the antigenic characters of the red blood cells of animals. With only rare 

 exceptions, to be considered later in more detail, each of the known antigenic 

 substances has appeared in the cells of an individual only if one or both 

 parents also possessed it. If there is but a single pair of contrasting charac- 

 ters, each is expressed in the heterozygote. Further, the cells which give rise 

 to the hematopoietic tissue from which the red blood corpuscles are derived 

 are laid down shortly after the first division of the fertilized egg. The possibil- 

 ity cannot be excluded, of course, that there is a chain of reactions within 

 each cell leading to the formation of the antigen, but no block in such a chain 

 of reactions has yet been observed. There are two statements concerning the 

 cellular antigens which are of interest: (1) the antigenic substance must be 

 located at or near the surface of the cell in order to be detectable, and (2) 

 there is no known effect of the environment upon them. 



