50 GENETIC VARIATIONS 



without the action of radiation. A considerable number of 

 mutations that behave in this way have been discovered. This 

 is a matter of much interest in relation to the nature of muta- 

 tions; some examples will therefore be mentioned. 



Demerec* found in a species of Drosophila that a mutation 

 affecting a certain gene located at a particular point in the 

 X-chromosome caused the body to take on a reddish tinge, 

 instead of the normal gray. Another mutation of this same 

 gene caused the body to become yellowish in color. When 

 these two differently mutated genes were brought together in 

 the same cells, it was found that in about one-fifth of all the 

 cases in which this was done, the genes that had mutated to 

 reddish were transmuted back to the original condition, so 

 that they produced again the original gray color. Thus the 

 mutation that produced the reddish color was not a perma- 

 nent change. In some way the presence with it, in the same 

 cell, of the gene that had mutated to yellow caused in some 

 cases the reversal of the change to reddish. 



The proportion of the cases in which the reddish gene 

 changes back to normal was found to differ in different fam- 

 ilies. By selection from the different families and further 

 breeding, Demerec obtained families in which the reddish 

 mutation never returns to normal ; others in which the rever- 

 sion occurs very frequently. 



Other reversible mutations were found in the same animal. 

 A mutation of a certain gene ("miniature") in the X-chromo- 

 some caused the wings to be small.^ In later generations this 

 mutated gene frequently reverts to normal. By selection from 

 different families it was possible to obtain some stocks in 

 which the return of the mutated gene to normal takes place 

 very rarely, the mutation remaining permanent in most cases. 

 In such a group there were but five reversions in 11,609 cases. 

 In other stocks the reversion is very frequent, occurring in up 



