GENETIC VARIATIONS 355 



the normal wild-type condition, though they do it rarely. 

 The mutational change is not final nor irreversible. 



What is the nature of the change in the gene when muta- 

 tion occurs? 



One line of evidence on this question is derived from the 

 relation of the mutated genes to characteristics. On this 

 there are several points that appear of significance: 



Injurious Effects of Mutations: First there is the fact that 

 when carefully examined most mutations are discovered to 

 be defects. The organism is the worse for their occurrence. 

 These relations are shown in two ways. 



On the one hand, the specific effect of the mutations on 

 some particular part or function of the organisms is, in most 

 cases, to bring about a loss or abnormality. Certain muta- 

 tions cause the body to be distorted, or imperfect in parts, or 

 to lose certain typical structures. Others make the limbs ab- 

 normal or small, or result in their total loss. By other muta- 

 tions the eyes are made imperfect, or very small, or are 

 totally lost. In general it is true that a part that has been 

 altered by a mutation is not so well adjusted to the rest of 

 the organism or to the surrounding conditions, as it was be- 

 fore the mutation occurred. The general impression from a 

 group of mutated organisms is one of defectiveness and 

 abnormality. 



Further, in addition to their specific effects on particular 

 parts of the body, mutations usually have an injurious effect 

 on the organism as a whole. They weaken the constitution 

 of the individuals in which the mutated genes exist. The re- 

 sult is that the mutated individuals lack resistance to severe 

 conditions, and have a shorter life and a higher mortality 

 rate than those not mutated. Individuals showing the mu- 

 tated characteristics are difficult to keep ahve; specially fa- 

 vorable conditions must be supplied or they perish. Under 



