GENETIC VARIATIONS 35 1 



revert to the original conditions; reversion occurs in a con- 

 siderable proportion of them without obvious cause. Some- 

 times the mutated genes are extremely sensitive to the pres- 

 ence of certain other genes; they revert in their presence 

 but not otherwise. Such behavior is of much interest in re- 

 lation to the question as to the essential nature of muta- 

 tions; some examples will therefore be described. 



Demerec ^° found in a species of Drosophila (D. virilis) 

 that a mutation affecting a certain gene at the locus 2.4 in 

 the X-chromosome caused the body to take on a reddish 

 tinge, in place of the normal gray. Another mutation of this 

 same gene caused the body to be yellow in color. When in 

 the descendants these two differently mutated genes were 

 brought into the same cell (where they form a gene pair, 

 one "reddish" the other "yellow"), it was found that in 

 about one-fifth of all the cases in which this was done the 

 genes that had mutated to reddish were transmuted back 

 to the original condition, so that they produced again the 

 original gray color. This reversion did not occur except in 

 the presence of the yellow gene. Thus the reddish gene is 

 sensitive to the presence of the yellow gene in such a way 

 that it reverts to normal. The reversal occurs only at the 

 time of the reduction division in producing germ cells. The 

 normal genes produced by reversal from the reddish con- 

 dition remained constantly normal, not later returning to 

 reddish. 



As breeding continued for many generations, the propor- 

 tion of reddish genes that reverted became less. At the end 

 of seven generations reversal had ceased; the reddish gene 

 had become constant. In different families there were dif- 

 ferent proportions of reversals. By selecting those in which 

 there were fewest reversals, it was possible to obtain fami- 

 lies in which the reddish genes were constant, while in other 

 families a part of them underwent reversal. 



Thus this mutation exists in two very different conditions: 



