SPECIFICITY OF GENE EFFECTS 237 



Errors of Metabolism) on the inability of some humans to break down homo- 

 gentisic acid (2,5-dihydroxy])henylacetic acid), resulting in the disease known 

 as alcaptonuria. Observations reported by Gross (1914) indicated that this 

 affliction was due to the lack of a ferment (enzyme) in the serum of alcapto- 

 nurics, whereas the enzyme capable of catalyzing the breakdown of homo- 

 gentisic acid was demonstrable in the serum of normal individuals. As 

 Beadle (1945) has stated, no clearer example exists today that "a single gene 

 substitution results in the absence or inactivity of a specific enzyme and that 

 this in turn leads to the failure of a particular biochemical reaction." (The 

 writer distinctly remembers that, while he was a student in a class in physio- 

 logical chemistry, the instructor paid considerable attention to the chemical 

 explanation of alcaptonuria, but none at all to its hereditary nature.) 



Another example of gene specificity and also of gene dosage is that of yel- 

 low endosperm in corn and the content of vitamin A reported by Mangelsdorf 

 and Fraps (1931). Their study showed that the amount of vitamin A in the 

 endosperm of white corn was almost negligible, but that the presence in the 

 endosperm of one, two, or three genes for yellow pigmentation was accom- 

 panied by corresponding increases in the amounts of the vitamin. 



GENE EFFECTS IN A SERIES OF REACTIONS 



There are numerous examples which have shown that many genes con- 

 tribute to the development of a heritable character. Thus, in corn there are 

 many genes which affect the development of chlorophyll. Each recessive 

 allele, when homozygous, allows the formation of only partial pigmentation, 

 or in extreme cases no pigmentation at all, and the seedlings are albino. It is 

 generally believed that the majority, if not all, of these different genes for 

 albinism affect different steps in the process of chlorophyll development. A 

 breakdown of the process at any one of these steps results in albinism of the 

 seedling. Haldane (1942) has likened the complexity of such a synthetic proc- 

 ess to the activity of an equal number of students as there are genes, "en- 

 gaged on different stages of a complicated synthesis under the direction of a 

 professor, except that attempts to locate the professor have so far failed. Or 

 we may compare them to modern workers on a conveyor belt, rather than 

 skilled craftsmen each of whom produces a finished article." 



One of the earliest examples of the physiological bases of the specificities 

 which are the final gene products is that of the chemical analyses of genetic 

 variations in flower color. These studies were carried out in England by sev- 

 eral workers. See reviews by Beadle (1945), Beale (1941), Haldane (1942), 

 Lawrence and Price (1940) for the general results and references to specific 

 papers. 



Mention will be made here of only one of the many investigations which 

 have defined in chemical terms the hereditary differences in pigmentation. 

 Anthocyanin is one of the five types of pigments concerned in flower color, 



