THE NATURE OF THE GENE 289 



A continuous series of quantitative variations ranging from apparently- 

 deep self colors with only occasional color changes that are apparent 

 through a series of dilute self colors with all gradations of color from 

 deep red to whitish and with increasing numbers of dots, splashes, lines, 

 bands and larger segments of darker and lighter colors; and through a 

 series of variegations varying in pattern from very heavy to extremely 

 light. 



It is obvious that a hereditary return to Wild type is effected 

 when an area with the return mutation embraces the germ cells. 

 Only rarely reversible mutations of these unstable genes occur, 

 but changes to different alleles are found. Many factors seem to 

 influence rate and type of somatic mutation in these unstable 

 genes. 



Correns (1919) described this instability as a disease of the 

 gene and makes the following assumptions: The gene consists of a 

 large molecule to which the same side chain of atoms (same 

 residue) is attached, say, ten times. This number might undergo 

 changes in the plus or minus direction under unknown condi- 

 tions, external to the gene. In the case investigated by him, a 

 definite ratio of white and green mosaic spots in the plant would 

 be correlated with the different number of side chains in the 

 molecule. Though Correns did not develop from this a general 

 theory of gene mutation, and though he did not actually consider 

 these unstable genes identical with normally mutating genes (he 

 actually was opposed to this idea; unstable genes were sick genes 

 to him), it is obvious that one might conceive of mutation also 

 as of a quantitative change in the side chains of a gene molecule. 

 (The facts could of course be as well described in terms of insta- 

 bility of the number of whole gene molecules (see Goldschmidt, 

 19286). From the same body of facts Demerec (19316) derived a 

 related theory of gene mutation, probably also held by many 

 other geneticists who have not enlarged upon it. 



Demerec starts from the rate of mutations. This is normally 

 rather low, but all transitions lead from rather stable genes to 

 very unstable ones. In the latter, the rate of change is suffi- 

 ciently high to be measured, and the effects may be easily 

 observed in regard to time of occurrence (the coarser or finer 

 type of variegation), factors affecting the rate, and the end 

 products. According to Demerec, the end product of the change 

 is always the same, viz., change from mutant into Wild-type 



