Genie Control of Development 391 



IV. PLASTED PIGMENT 



1. Production of background efPects 



2. Inhibition 



V. LOCALIZED ACID pH 



These take care of most colors studied, and the individual known loci 

 may be assigned to one or the other chemical action. Only one 

 specific point should be mentioned: the wild-type color is the most 

 highly oxidized; mutant loci then prevent complete oxidation. Haldane 

 (1935) has drawn from such facts the conclusion which later was 

 called the one gene — one enzyme hypothesis, whereas I pointed out 

 ( 1938a ) that different rates of production of the specific enzyme might 

 be at work. 



On the basis of such a chemical situation, Scott-Moncrieff studied 

 the interaction of the different loci involved in the coloring of dahlias. 

 The loci involved are as follows: A, necessary for Hght anthocyanin 

 color produced by either cyanin or pelargonin; B, needed for heavy 

 anthocyanin; I, producing ivory flavone; and Y, producing yellow 

 flavone. A complication, which makes these experiments at the same 

 time dosage experiments, is that these dahlias are tetraploid. Thus, Y 

 and B are completely dominant in simplex condition; A is cumulative 

 from simplex to quadriplex. I is incompletely dominant; when simplex 

 it produces very little pigment; when duplex to quadriplex, the com- 

 plete amount. There is also another locus H which acts as inhibitor of 

 yellow flavone, with an additive effect from simplex to quadriplex 

 leading to cream and primrose colors. 



The interaction of these factors is, generally speaking, such that the 

 pigments suppress each other, but the flavones suppress the antho- 

 cyanin more than vice versa, the degree of suppression depending 

 upon dosages. Thus, anthocyanin intensity, controlled by A, is dimin- 

 ished in the presence of ivory flavone (I), with less pigment being 

 formed; the details depend upon dosages. Y acts strongly upon the 

 effect of I and A, suppressing them completely in some combinations; 

 Y and I similarly suppress the action of B; I changes the effect of B 

 to the type of an A effect. To this are added the dosage effects which 

 are not simple and indicate definite threshold conditions. 



The authors tried to explain the facts ( of which only a short survey 

 was given) in terms of the chemistry of pigment formation and of 

 genie action. As a balance of the different pigments is involved, it is 

 concluded that all pigments are produced from some limited common 

 source for which there is competition. The supply of this source must 



