GENETIC VARIATIONS 359 



other gene of the pair being completely absent). If two of 

 the mutated genes are present, the forking is still perceptible, 

 but is not quite so marked. But if three mutated genes are 

 present, the bristles are much less distinctly forked; they are 

 more like normal simple ones. It appears that the forking 

 is due to the fact that certain normal developmental proc- 

 esses are in the mutated individuals not fully carried out; 

 the effect of the mutation is to reduce the normal action. 

 The greater the number of mutated genes present, the more 

 nearly the normal processes are to completion. 



Tests of this kind, so far as carried out, indicate that most 

 mutant genes are of this type. The mutational change con- 

 sists in a reduction of the activity of the normal gene. 



In the reversal of mutations, described earlier, in which 

 there is a restoration to the normal condition of the gene, 

 the change is obviously not an inactivation; on the con- 

 trary the inactivated gene recovers its activity. Consider- 

 ing such a reactivation as itself a mutation, it is sometimes 

 held that this demonstrates that progressive mutations oc- 

 cur, as well as those that are the reverse of progressive. Re- 

 covery of an inactivated gene however is obviously a dif- 

 ferent matter from a progressive change by which a gene 

 would pass into a more active condition that had not before 

 existed. 



With the usual nature of gene mutations as inactivations 

 agrees the fact that most mutations are recessive, as com- 

 pared with the effect of the unmutated gene, so that when 

 the mutated gene and the normal gene are present together, 

 the effect on the characteristics is that of the normal gene 

 alone. The injurious effect of most mutations is likewise in 

 agreement with this. The mutation is a reduction in the ac- 

 tivity of the gene, leaving certain developmental processes 

 imperfectly performed. 



But in addition to the great majority of gene mutations 

 that are recessive, and result from partial inactivation of the 



