GENE CONTROLLED PROCESSES 



187 



reaction. Each Minute has a different effect on the early part of the 

 valley along which the bristle-forming process is moving, but any of 

 these effects is sufficient to divert the course of the process out of the 

 main valley into the Minute branch valley. The course of the branch 

 valley is not absolutely fixed by the rest of the genotype, but is also 

 effected to some extent by the particular Minute used, since the final 

 effect can be more or less extreme. Schultz discovered some of the 

 modifiers which determine the course of the Minute branch valley; 

 these are factors (Delta, Jammed, etc.) which will increase or decrease 

 the effects of Minutes, and which modify all Minutes in the same way, 

 and therefore must affect the Minute valley itself and not the different 

 primary effects which originally diverted the process into that valley. 



Abnormal Abdomen in D. fujiebris,^ like the Minutes, is a character 

 brought about by several different genes, and it can also be caused by 

 chance mishaps during development. Here again the side valley is fixed 

 by the genotype, and several things may divert the stream into it. 



6. Plus- Minus Variations in Gene Effect 



The vertical dimension in our geological model represents proba- 

 bility, so that the valley bottom is really a representation of an equili- 



Fig. 88. Plus-Minus Variation in Gene Expression. — A A gene in Droso- 

 phila funebris which causes either an increase (below) or a decrease (middle) in 

 venation of the wings; the normal wing is shown above for comparison. 6 Similarly 

 aristopedia ss^ in D. melanogaster may cause the aristae to become larger and foot- 

 like (upper row) or to be reduced and even to disappear (lower row); the normal 

 arista is shown at the left of the upper row. (After Timofeeff-Ressovsky.) 



' Timofeeff-Ressovsky 1932a. 



