352 GENETICS 



in one it reverts frequently to normal; in the other it is con- 

 stant. The unstable condition passes into the constant con- 

 dition. 



In the same species of Drosophila occurs another incon- 

 stant mutation that is of much interest. A certain gene in the 

 X-chromosome was changed by a mutation in such a way 

 as to cause the wings to be small; it is known as miniature. 

 This mutation is in most individuals constant. But in a cer- 

 tain case it was found that this mutation (or another with 

 the same effect) frequently reverts to normal, yielding again 

 large wings. After it has reverted to normal it remains con- 

 stantly in the normal condition. By selection from different 

 families it was possible to obtain some families in which the 

 return of the mutated gene to normal takes place very in- 

 frequently. In a group of such families there were but five 

 reversions in 11,609 cases. In other stocks the reversion is 

 frequent, occurring in up to 50 per cent of the cases. The re- 

 version to normal may occur either at the time of the forma- 

 tion of germ cells, or in certain of the body cells. In certain 

 families however the reversions occur only in body cells, not 

 in germ cells. In the body cells of males these reversions oc- 

 cur about twice as frequently as in those of females. The 

 frequency of reversal to normal is increased by the presence 

 of certain other mutated genes in the same individuals. It is 

 likewise increased by high temperatures. 



Thus again we have a mutation that is constant in some 

 stocks, inconstant and reversible in others. And in some 

 stocks the reversal occurs only in body cells, while in others 

 it occurs in both body cells and germ cells. This mutation is 

 sensitive to the presence of other genes, and to the differ- 

 ence between males and females. Other cases of unstable 

 mutations are known in Drosophila. 



Extreme cases of unstable mutations, of a different type, 

 are found in certain variegated plants. Here different parts 

 of the plant show different colors of leaves, flowers, fruit 



