GENE MUTATIONS 49 



From this it follows that many gene mutations occur that 

 have no manifested effect whatever; they do not change the 

 characteristics of the individual in which they occur, nor those 

 of his descendants. For example, consider a mutation in a 

 gene that aflfects the color of the eye. Eyes derived from the 

 cell in which this mutation occurs will be changed in color. 

 But many of the cells of the body do not produce eyes; in- 

 stead they produce legs, wings, parts of the body, or the like. 

 Mutation of an eye-color gene in such cells will thus have no 

 effect on eye color, no visible effect on any characteristic. It 

 can probably be said with truth that most eye-color gene 

 mutations have no effect on eye color, for most of them 

 occur in cells that do not produce eyes. Doubdess there occur 

 great numbers of such mutations that yield no visible effect; 

 any individual may carry many such mutated genes. All such 

 mutations, present in body cells only, are totally lost at the 

 death of the individual carrying them. 



When a mutation has occurred, is the change a permanent 

 one? Or may the genetic system later return to its original 

 condition ? 



The great majority of mutations appear to be permanent 

 changes. They are inherited by the progeny of the mutated 

 individuals, and such inheritance continues for an indefinite 

 number of generations. 



But in recent years a considerable number of cases have 

 been discovered in which the change is not permanent. Hav- 

 ing mutated in a certain way, producing its usual effects on 

 the characteristics, the genetic system later changes back to 

 normal, so that the original characteristics are restored. Some- 

 times such a reversion to normal is produced by subjection of 

 the mutated genes to radiation. In other cases the mutated 

 genes show a marked tendency to revert to the original condi- 

 tion; reversion occurs in a considerable proportion of them, 



