354 GENETICS 



in different parts of the body, such as occur in maize. But 

 in animals there is no way of determining whether this is 

 true, because the different parts of the body do not produce 

 germ cells; the germ cells all come from the germ gland in 

 the interior of the body. This therefore remains an unset- 

 tled question. 



Are the more common mutations, that appear quite sta- 

 ble, phenomena of a different type from the unstable muta- 

 tions above described? There are a number of considera- 

 tions that bear on this question. First, the typical unstable 

 mutations vary in the degree of instability. Second, the same 

 mutation may exist as a stable condition, and in other cases 

 as an unstable one. Third, the unstable mutations may in the 

 course of time pass into the stable condition. Examples of 

 these three types of phenomena we have already seen. 



Furthermore, it has been found of late that some of the 

 typical stable mutations may rarely and under special con- 

 ditions revert to the original normal or wild-type condition. 

 Under the action of X-rays, Patterson and MuUer observed 

 in Drosophila two reversals of the scute mutation, and seven 

 of the forked mutation. TImofeef-Ressovsky made exten- 

 sive investigations on a number of mutated genes In chromo- 

 somes X and III of Drosophila. ^^ Under radiation he ob- 

 served 1 8 reversals to normal out of 289,000 genes radiated. 

 In certain other cases a mutation was transmuted to another 

 condition; thus white eye was transmuted in different cases 

 to eosin, blood, and buff. Johnston and Winchester ^^ radi- 

 ated 713,000 mutated genes in the X-chromosomes of Dro- 

 sophila; they observed 24 reversions to normal. The par- 

 ticular mutated genes that reverted were: yellow, scute, cut, 

 vermilion, miniature, garnet, forked, and carnation. Re- 

 version of the gene forked occurred 1 1 times. 



Thus It appears that many of the typical constant muta- 

 tions have not completely lost the capability of returning to 



