250 PHYSICAL BASIS OF HEREDITY 



must have arisen at least one generation earlier, and 

 have been carried over into the two heterozygous indi- 

 viduals in question. 



It would be a point of capital importance if it could be 

 determined beyond doubt that at times recessive mutant 

 genes change back to the original (wild type) gene, or 

 even if a recessive gene could mutate to a dominant one. 

 The appearance of the wild type in a pure culture of a 

 mutant race can be accepted as good evidence of such a 

 change only when every possibility of contamination by 

 the wild type is excluded, and this is difficult to regulate. 

 In our cultures we have come across such cases, but have 

 not ventured to exploit them, since wild-type flies are 

 always present in the laboratory, and hence the discovered 

 form may have arisen through accidental contamination. 

 Thus even when a red-eyed yellow fly appeared in the 

 white-eyed yellow stock there is the barest chance that a 

 yellow red-eyed fly, or an egg of such a fly, had somehow 

 gotten into the stock. Certainty can be attained only when 

 a stock, pure for several mutant characters, reverts to the 

 normal in one of these characters, and not in the others. 

 Only one case of this kind that is above suspicion has been 

 as yet recorded. This is a mutant stock in which, as May 

 has recorded, reversion to the wild type occurs with such 

 frequency that there can be no chance of error. The stock 

 in question, bar eye, is a dominant mutant and the rever- 

 sion therefore is to the recessive wild type of eye (round 

 eye). The change back to normal is complete, since such 

 individuals give only normal offspring. When such a 

 mutant chromosome comes from the mother and goes into 

 a son he has normal (wild type) eyes ; when it comes from 

 the father, and goes to a daughter, she is heterozygous 

 for bar eye. Baur has recently recorded the appearance 

 of recessive ( ?) mutants from self -fertilized plants (snap- 

 dragon) that bred true at once. Punnett has described a 

 similar case (1919). The result can be accounted for, if a 

 mutation occurred in only a single chromosome far enough 



