46 GENETIC VARIATIONS 



fest effect on the individual; the latter remains quite un- 

 changed. Thus one or many gene mutations may have 

 occurred in the chain of genes of a given individual, but so 

 long as only one gene of any pair is affected, there may 

 be no manifest effect. Only vv^hen both members of a pair of 

 ,genes are mutated does the recessive mutation produce its 

 bodily effect. A small proportion of the mutations that occur 

 are dominant; in this case even when but one member of a 

 pair of genes has mutated, the individual is changed by the 

 mutation. 



As to their absolute frequency, in ordinary observation the 

 changes knov^^n as gene mutations seem to occur very rarely. 

 Yet when large numbers of individuals are examined for 

 long periods, and the eye is carefully trained for the detection 

 of mutations, the number occurring in even a short period of 

 years is very considerable. In the fruit fly, during twenty-five 

 years of observation, such changes have been seen to occur in 

 several hundred different genes. In relation to the course of 

 evolution, twenty-five years is an infinitesimally small period, 

 so that relative to the course of evolution, gene mutations are 

 numerous and of frequent occurrence. If they fulfill other 

 requirements, their numbers are sufficient to account for 

 evolution. 



As to the occurrence of gene mutations, a mutation occur- 

 ring at a given time appears to affect only a single one of the 

 genes in the chain of genes. Further, a mutation occurs in 

 only one of the two like genes that are present in any cell. 

 The genes are in pairs, the two members of a pair having 

 similar or identical constitutions and effects. But the occur- 

 rence of a mutation affects but one of these, leaving the other 

 unchanged. It can even be shown that when a single gene has 

 split in two, in preparation for cell division, one of the two 

 halves may become mutated while the other does not. All 



