THE MIT AT ED GENE 169 



the Bar reaction will influence the effect of the Bar reaction by 

 + changing its time of onset, its speed, its end or threshold value. 

 In short, these facts again fit best into a general picture of the 

 type as expressed in the theory of reaction velocities in tune. 



Interaction of genes, in the cases reported, was analyzed by 

 a comparison of the phenotypes, though the reactions leading to 

 these phenotypes were more or less inaccessible. In plants, an 

 analysis is available which is based upon the study of the chemism 

 of the process, viz., the work of Lawrence and Scott- Moncrieff 

 (1935) on flower color in Dahlia. We have reported (see page 

 95) the chemical side of this work. A special study was made 

 of the interaction of the different genes controlling the chemism of 

 flower color. (This is the work alluded to in connection with 

 dosage differences in polyploids, page 150.) The genes in 

 question are as follows : A is necessary for light anthocyanin color 

 produced by either cyanin or pelargonin. B is needed for heavy 

 anthocyanin pigmentation. I produces ivory fiavone and Y 

 produces yellow fiavone. As these Dahlias are tetraploid, each 

 factor is quadriplex. 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 complete amount of 

 pigment. A fifth gene H acts as an inhibitor of yellow fiavone, 

 with a cumulative effect in the four quantities 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 anthocyanin more than vice versa. The degree of suppres- 

 sion depends upon the total number and the proportion of the 

 flower factors present. We may choose a few examples from the 

 large array of facts. Anthocyanin intensity (by A) is diminished, 

 when ivory fiavone (by /) is present, less pigment being actually 

 present. If the number of these genes is changed (1 to 4), the 

 effect is proportional; e.g., more /-genes increase the paleness 

 of the color by proportionally reducing the amount of antho- 

 cyanin formed. The relative effect of / is higher than that of 

 A, as the combinations of different quantities of both show. The 

 following grouping from low to high anthocyanin color (left to 

 right) in different combinations of A and / in 1 to 4 quantities 

 illustrates the point: 



