THE MUTATED GENE 93 



of the oxidase; bases, except in high concentration, increase the 

 rapidity of melanin formation. There is no influence of light, 

 and a considerable one of temperature. This seems dependent 

 mostly upon the presence of a second catalyzer which is heatproof. 

 Systematically all types of substances were studied in regard to 

 their action upon melanin formation; some of them enhanced it, 

 while others interfered in different quantitative degrees (grades 

 from light grey through brown to black), which were measured. 

 Certain chemical rules were found, but these do not mean much 

 for the genetical analysis. The only conclusion that is warranted 

 is that, given chromogen and ferment, the amount and color of 

 pigment may depend upon a number of chemical conditions 

 which might be produced by other genes controlling them. 



More suitable material is found in plants, because here the 

 basic chemistry of the pigments is thoroughly known after the 

 work of Wheldale, Willstaetter, Robinson, Karrer, and others. 

 Most of the work relating mutant genes to actual chemical 

 products has been done by Euler and his collaborators on the one 

 hand and the collaborators of J. B. S. Haldane (see 1935) on 

 the other, after the pioneer work of Miss Wheldale (later 

 M. W. Onslow) in early Mendelian days (see Onslow- Wheldale, 

 2d ed., 1925). 



Euler and his collaborators studied in a long series of papers 

 (Euler et al., 1929-1935) the chemistry of chlorophyll-defective 

 mutants of barley, which had been genetically analyzed by 

 Nilson-Ehle and Hallquist. The results were checked with 

 similar mutations in other plants. There are the completely 

 white albino types of which seven genetically different ones were 

 tested; also the yellowish xantha and the pale alboxantha types. 

 The chemical differences between these types were analyzed. The 

 main results thus far are: 



1. No chlorophyll whatsoever is present in the albino forms. 

 The heterozygotes, however, have normal chlorophyll content 

 and complete dominance. 



2. The base of chlorophyll is the porphyrin-nucleus which 

 contains about 70 per cent of the mass of the molecule. The 

 white mutants produce only a small amount of porphyrin, 

 indicating that the disturbance occurs earlier than the porphyrin 

 synthesis. Carotin synthesis, which is chemically related, suffers 

 in the same way. 



