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THE AMEBIC AN NATURALIST [Vol.LVI 



that recessive mutations— the most frequent kind— can 

 not be expressed in the individual in which tliey occur 

 except when the dominant allelomorph is simplex, while 

 such mutations may appear in a later generation of sexu- 

 ally produced progeny (East, 1917). 



Somatic Mutation of Genes 

 Several cases of vegetative variation in plants have 

 been studied with sufficient thoroughness to leave little 

 doubt that they are mutations in the strict sense, in- 

 volving the modification of particular genes. Most of 

 them are concerned with variegated color patterns of 

 flowers, leaves, or fruits, and they are more or less regu- 

 larly recurrent, a fact that makes them especially well 

 suited to quantitative studies, for it is obvious that a 

 quantitative study can be made only of variations that 

 occur with considerable frequency. For the most part 

 also these somatic mutations are dominant to the type 

 from which they spring, appearing frequently in material 

 homozygous for their recessive allelomorphs, facts that 

 exclude the possibility of their being due to any sort of 

 somatic segregation of unlike genes. Blakeslee's (1920) 

 case of a somatic variation in Portulaca is one of the few 

 examples not involving variegation. Other cases have 

 been reported by Baur (1918). 



One of the earliest cases of somatic mutation was re- 

 ported by deVries in variegated flowers of Antirrhinum. 

 Though the work was done prior to the rediscovery of 

 Mendelism and not discussed from the standpoint of re- 

 cent genetic interpretation, there is little doubt, as I have 

 noted elsewhere (Emerson, 1913), that the results can 

 best be interpreted as due to a somatic gene mutation. 



Correns's (1910) results with respect to the occurrence 

 and behavior in inheritance of green-leaved variations on 

 variegated-leaved Mirabilis and of self-colored flowers 

 on variegated flowered strains of the same species were 

 among the first to be subjected to critical genetic analysis. 

 The behavior in inheritance of green branches of varie- 

 gated Mirahilis shows this vegetative variation to be a 

 simple dominant mutation affecting ordinarily only one 

 of the duplex recessive allelomorphs. A mutated branch 



