I20 Outline of Genetics 



are self-colored. Further breeding reveals that somatic mutation 

 has occurred in the tissues concerned with the formation of these 

 grains, such that VW has become SW. The change has taken 

 place not only in the somatic tissue of the grains in question, but 

 also in the germinal tissue within the grains, and is inherited 

 accordingly. It is concluded that the recessive variegation gene 

 V has mutated to its dominant self-color allelomorph S. 



Without attempting a discussion of the breeding tests upon 

 which EiviERSON based his conclusions, it will be worth while at 

 least to mention some of the other peculiarities of this phenome- 

 non. V mutates to 5" rather frequently, but IF never mutates 

 to S. V in the heterozygous condition (VW) mutates to S five 

 times as frequently as when it is in homozygous combination (VV). 

 The mutation takes place late in ontogeny much more often than 

 in early ontogeny. As a result there are many more cases where 

 small patches of self-colored grains appear on variegated ears than 

 where large patches appear. In corn of the VV formula, only 

 one of the V genes ever mutates to 5 at a given time. Reverse 

 mutations, S changing to F, have also been noted. 



This same material provides also an example of somatic muta- 

 tion which involves the soma alone and not the germinal tissue. 

 In situations essentially similar to those described above, there 

 may appear on the variegated ears a few aberrant grains which are 

 apparently self-colored only on the crown of the seed. This 

 character has been designated as "dark-crown," and it is notable 

 that it is never inherited. Microscopic examination of the dark- 

 crown and of the fully self-colored seeds indicates that in the former 

 the epidermis alone is colored, while in the latter the epidermis 

 alone remains colorless. The conclusion seems warranted, there- 

 fore, that the two types of variation are fundamentally the same, 

 both being true gene mutations, and that the non-inheritance of 

 the dark-crown type is due to the accident that it occurs in the 

 epidermal tissue outside the germ tract. 



II. SOMATIC SEGREGATION 



It has been pointed out by several investigators that bud 

 variations appear much more frequently in plants that are hetero- 

 zygous for the genes concerned than in plants which are homozy- 



