Genie Control of Development 393 



the straightforward quaHtative biochemical approach does not lead 

 very far. 



Other examples of a comparable type could be given. One of the 

 best elaborated is Wright's analysis of coat colors in guinea pigs, 

 carried out from 1916 to 1949, involving the interplay of at least seven 

 mutant loci. Without going into the details (which are given in all 

 reviews on biochemical genetics), we may say that again different 

 t\'pes of genie actions are involved which must be properly attuned 

 to give the final results. There is present a genie control by a rate 

 process of the quantity of pigment, interfering with the quantity of 

 two different pigments; another control determines which of the tsvo 

 pigments is formed, probably from the same precursors, with compe- 

 tition for a common substrate. This control is assumed to work by con- 

 ditioning the hair follicles in three different, genically controlled ways; 

 two other processes control the distribution of pigment over the skin 

 (spotting) and within a hair (agouti); this means interaction of the 

 pigment-forming processes with an independent patterning process. 

 This is another system in which an exact interplay of primary syn- 

 thesis of biochemical precursors, different enzyme systems for further 

 synthesis, rate processes controlling quantities, processes involving 

 competition for substrates, and processes of cellular diversification 

 according to a pattern are found and can be visualized in the way we 

 have frequently discussed before. I do not think that more examples 

 are needed. 



b. Pleiotropy 



Pleiotropy, the production of manifold effects by one mutant 

 locus, has been mentioned earlier. Strictly speaking, there are prob- 

 ably no mutants in existence which do not affect more than one 

 structure. A close inspection usually re\'eals such effects, either mor- 

 phological or physiological (e.g., viabilit)'). But if we speak of pleio- 

 tropy, we usually mean cases in which manifold effects are rather 

 obvious. We have only to go over Bridges and Brehme's (1944) 

 catalogue of Drosophila mutants to find numerous examples of all 

 grades. The same is true for maize mutants. For our present topic, the 

 different examples of pleiotropy are not of equal value; some give us 

 more, some less, or no information on the interplay of genie action. 



There is one group which actually should not be called pleiotropy 

 without qualification. Griineberg (1938) called it "spurious pleio- 

 tropy"; I prefer to call it "syndromic pleiotiopy," because the effect 



