GENE MUTATIONS 47 



these relations show that whatever is causing the mutation is 

 precisely localized and extremely restricted in its action. 



Since most mutations are recessive in their effects, and since 

 only one gene of the pair of genes present is mutated, as a 

 rule the mutation produces no manifest effect on the indi- 

 vidual in which it occurs. The fact of its occurrence cannot be 

 detected in that individual. It is only when by the processes 

 of mating in ordinary reproduction, two of the recessive 

 mutations get together in the same pair, in some of the de- 

 scendants of this individual, that the mutation appears to view. 



Thus, when the recessive mutation becomes manifest, it 

 has actually occurred, as a rule, at least two generations earlier, 

 in one of the ancestors of this individual. This relation intro- 

 duces much difficulty into the study of the time and place at 

 which mutations occur. Many mutations do not come to light 

 for several or many generations; indeed there must exist in 

 any organism many mutations that have never come to light. 

 In the course of generations a large number of recessive muta- 

 tions may have collected in the chromosomes, none of them 

 producing any efTect, because each is accompanied by a 

 normal or unmutated gene of the same pair. If now inbreed- 

 ing — the mating of close relatives — occurs, a number of these 

 mutated genes may be brought together into the same pairs, 

 in one individual ; thereupon the effect of all these mutations 

 suddenly become manifested in that individual. Since most 

 mutations are harmful in their effects, this individual will be 

 weak and defective, perhaps in several different ways. This 

 is the reason why inbreeding is harmful; if there were no 

 gene mutations, apparently there would be no disadvantage 

 in the mating of close relatives. One of the chief results of 

 biparental reproduction — what might be called one of its 

 functions — is that it largely nullifies the effect of harmful 

 mutations. 



