CHAPTER VII 

 MUTATION 



When De Vries ''discovered" the phenomenon of 

 mutation in Oenothera Lamarckiana, he stated that 

 mutations were quahtative, discontinuous, constant 

 changes in the germ plasm (see p. 6). These three 

 fundamental characteristics still hold true, but some of 

 De Vries' other ideas have been considerably qualilied 

 by later work. The critical analysis of the germ plasm 

 that has been effected during the last decade has made 

 it possible to describe mutation in a much more exact 

 way than before, and to describe it in terms of the 

 Mendelian mechanism. 



For convenience our discussion of this general subject 

 will be put into the form of a classification. (The 

 sequence followed in this classihcation is that of the 

 increasing magnitude of the "area" of the germ plasm 

 affected by the change.) 



I. gene changes 



I. Locus changes. — These are changes restricted 

 to a single locus of one of the chromosomes, so that they 

 involve only one gene, without affecting even its nearby 

 neighbors. Usually they are effective on only one chro- 

 mosome of a pair, without affecting the corresi)onding 

 locus of its allelomorpliic mate. Consequently, the 

 change first appears in the heterozygous condition. 

 (Baur estimates that such changes originate in the 

 heterozygous condition four hundred times as frecjuently 



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