346 GENETICS 



which the fertilized egg divides, then only half the body 

 will carry that mutation. If the mutation occurs in a cell at 

 a still later stage of development, only a small part of the 

 body will carry the mutated gene. Many cases of such kinds 

 have been thoroughly studied, cases in which one part of 

 the body shows the mutation, the rest not. These are com- 

 monly spoken of as somatic mutations. A somatic mutation 

 may affect but few cells. Thus, sometimes through a muta- 

 tion at a late stage of development, in the gene at locus 1.5 

 in the X-chromosome of Drosophila, a few of the elements 

 of the compound eye may become white, while the remain- 

 der of the eye is of the normal red color. ^ Such a somatic 

 mutation is not inherited by the offspring of the mutated 

 individual, since the mutation has not changed the genes of 

 the germ cells. 



The general upshot of all such studies is to show that 

 gene mutations may occur in any cell of the body, at any 

 period in life. From this it follows that there occur many 

 gene mutations 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 affects the color of the 

 eye. Eyes derived from the cell in which this mutation oc- 

 curs will be changed in color. But many of the cells of the 

 body do not produce eyes; instead 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. Doubtless 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 in- 

 dividual carrying them. 



