268 CALVIN B. BRIDGES 



THE DIFFERENTIATION OF PURPLE BY VERMILION— DISPRO- 

 PORTIONATE MODIFICATION 



While the difference between the color produced by the purple 

 gene and the color produced by its wild-type allelomorph (red) 

 is distinct, it is neither great nor striking, since in tone purple is 

 first slightly darker and later somewhat lighter than red. How- 

 ever, in classifying the eye colors in F2 from the cross of ver- 

 milion by wild, it was observed that the difference between 

 vermilion purple and vermilion not-purple was not only con- 

 stant in direction, but also conspicuous in extent. The sepa- 

 rability of purple versus not-purple is favored by the presence of 

 vermilion, which may therefore be called a 'differentiator' of 

 purple. Regarded in the converse relation, namely, the effect 

 of purple on vermilion rather than the effect of vermilion on 

 purple, purple is a much stronger modifier of vermilion than of 

 not-vermilion. Purple may be described as a 'disproportionate 

 modifier' of vermilion, since from the small amount of its effect 

 on eye color when acting alone one would not have expected the 

 great effect it produces when acting in the presence of vermilion. 



This type of intensification — disproportionate modifier and, 

 conversely, differentiator — stands midway between the normal 

 relations where combination effects are roughly proportional to 

 the separate effects so that both genes may be called 'general 

 modifiers,' and the special relation where a given gene, 'specific 

 modifier,' produces by itself no visible effect whatever, but 

 which gives a more or less marked effect when acting in con- 

 junction with some other gene, its specific base, sensitizer, or 

 differentiator. 



In order to make full use of this differentiation of purple 

 versus not-purple by vermilion, it is necessary that all flies used 

 in the experiment should be made homozygous for vermilion. 

 This is often inconvenient, and accordingly only in the early 

 and comparatively simple experiments was this method em- 

 ployed. It was soon found also that the separation of purple 

 from red was not causing any trouble, so that the differentiation 

 in this case has little net advantage, though it is still of interest 

 as being the first example in Drosophila in which intensification 

 was recognized and deliberately made use of. 



