Cases of mimic mutations and secondary mutations in the X-chromosome iJtc. 



13 



of the two mass cultures some of th(^ females and males obtained showed 

 the same modification of the forked character as that found in their 

 fathers. One of these females was tested and proved to be of normal 

 fertility in contrast to homozygous singed females which are completely 

 sterile. — That she was homozygous for the gene which was respon- 

 sible for the alteration of the forked character was clear, since all her 

 sons were forked of the modified type. 



The examination of the female offspring in the two mass cultui-es 

 mentioned revealed a relation which so far had been overlooked. In 

 one of them only ordinary forked sons were obtained, thus ])roving that 

 the mothers had not been heterozygous for the new gene. Nevertheless 

 all their daughters showed a much more extreme type of the forked 

 character than did their forked brothers. Since ordinary forked shows 

 no sex-limitation either the new mutation was allelomorphic to forked, 

 or a dominant sex-linked specific modifier of the forked character. 



Some of the original eosin vermilion modified -forked males had 

 been crossed to wild females in order to secure a stock of the mutation. 

 One of the heterozygous daughters was now outcrossed to singed males 

 from stock (Table 4). As seen from the table all the forked sons were 



Table 4. Pi, wild-type 09 X eosin vermilion forked* o'cf; 

 Fl wild-type - — - — g X singed cfcf . 



of the modified-forked type, wliich was to be expected on the allelomoipli 

 explanation view. In order to exclude the possibility that a dominant 

 modifier, very closely linked to forked, was responsible for the character 

 change described, some additional linkage tests of the same type were 

 carried out (Table 5). In all 541 forked individuals were examined and 

 they were without exception of the modified type. If we were dealing 

 with a dominant modifier w^e would expect to obtain some ordinary 

 forked individuals as a result of crossing- over between this modifier 

 and the forked gene. These tests, in connection with the fact that the 

 new gene when combined with the old forked in the female gives a 

 compound, prove conclusively that it is allelomorphic to forked. Later 



