90 PHYSIOLOGICAL GENETICS 



again, others may defy a simple explanation. This, however, 

 would not change the fact that most multiple-allelomorphic 

 scries give definite information regarding the action of the 

 respective genes upon development. More facts will be pre- 

 sented in other chapters. 



C. Genes Controllincx Known Chemical Processes 



The effect of a genie action may be a morphogenetic process of 

 any kind; it may also be the production of a physiological condi- 

 tion; and it may be the final production of a definite chemical 

 substance deposited somewhere in the body. In general terms, 

 we might assume that all genes act primarily in the same way. 

 The different types of end effects will therefore probably furnish 

 no different clues in regard to the primary reactions set in motion 

 by the genes. But there is a long chain of processes leading from 

 the gene to the final phenotypic expression of its activity. When 

 morphogenetic processes are involved, usually the members of the 

 chain that may be unraveled (see later chapter) cannot be defined 

 chemically but only in such general terms as growth substances, 

 evocators, hormones of differentiation. If, how r ever, the end 

 of the chain is a definite chemical situation, there might be the 

 possibility of finding some members of the chain and of defining 

 actions of mutant genes in terms of definite chemical processes. 



Strictly speaking, all the processes of deposition of pigments in 

 eyes, hairs, feathers, skin, many of which have already been 

 mentioned, belong here. But in most cases, these processes 

 have been studied only in a rather general way from the stand- 

 point of chemistry. In certain cases, it is known whether 

 melanin pigments are involved or pigments of the carotinoid type, 

 whether different pigments may be different grades of oxidation 

 or reduction of one substance, and similar rather general facts. 

 Such cases have been mentioned, and the fact was noted that the 

 phenotypic result might be dependent upon time or quantity of 

 formation of the chromogen (tyrosin, etc.) or similar quantitative 

 variants of the action of the catalyst (dopa, oxidase) or both. 

 More examples will be discussed later on. But as a rule the 

 chemical elements of these processes could not be clearly defined 

 (see page 91). 



There are, however, a few actual chemical attacks on these 

 problems in animal genetics. 



