1 86 GENETICS 



in the plants having the genes aabb, while all the other 

 combinations of table 2 on page 117 give the triangular 

 pod.^ Similar cases of duplicate genes are known in a number 

 of organisms. 



10. "Modifying Factors'' : In some cases the effect of 

 some of the genes is merely to modify in detail a character- 

 istic that is due primarily to another gene. Such are known 

 as modifying genes or factors. Thus, in Drosophila, a cer- 

 tain modified gene at I, 1.5 causes the eye to assume the 

 pinkish color known as eosin, but the depth of this eosin 

 color depends upon certain other genes present in other 

 parts of the genetic system. One of these, known as cream 

 b, is located at II, 22.5; another, cream III, at III, 36.5; 

 and several other such genes modifying the eosin color are 

 known. If these genes are present in the normal or unmod- 

 ified condition they do not affect the eosin color; when mod- 

 ified in certain ways they do. Again, in the rat, the presence 

 of a certain gene causes the fore part of the body to be 

 colored brown, forming what is called a hood, the rest of 

 the body being white. The extent of this hood depends on 

 the presence in the individual of a number of other genes; 

 some of them increase its extent, others decrease it.^ Such 

 modifying genes or "modifying factors" play a role in many 

 characteristics. They may produce no visible effect except in 

 the presence of a particular character that they modify. 

 Thus the modifiers of eosin eye color produce no visible 

 effect in an individual with red eyes. 



11. The same phaenotypic character — that is, the same 

 characteristic in outward appearance — may be due in dif- 

 ferent cases to the alteration of diverse genes, in different 

 parts of the genetic system. This is illustrated in numerous 

 cases in Drosophila. 



Thus, modification of a certain gene at II, 54.5, causes the 

 color of the eye to change from the usual red to a darkish 



