130 GENETICS IN RELATION TO AGRICULTURE 



developing is white as respects aleurone color. Here apparently only 

 aleurone color is concerned. Similarly in other cases much more insig- 

 nificant changes may be connected with definite factor differences. Thus 

 a forked condition of the spines in Drosophila is dependent upon a 

 definite factor difference, a recessive factor in this case. One could 

 go on and recount indefinitely factors which cause only very slight 

 character changes. Any character change, therefore, however slight, 

 may be based on genetic factor differences. The only valid genetic 

 test is the pedigree breeding method, at the same time giving due con- 

 sideration to environmental influences which may obscure or temporarily 

 cover entirely the underlying genetic differences. 



Very great somatic differences may also be dependent upon differences 

 in single factors in individuals. Perhaps the most striking of these are 

 large size differences such as are found in beans, peas, and even in animals 

 at times. Thus in beans the main difference between pole and bush 

 beans is dependent upon a single factor difference. The difference 

 between tall and dwarf varieties of peas is of a similar nature and has 

 been fully discussed above. Certain types of dwarfing in man appear 

 to depend upon single factor differences and in Drosophila there are 

 factors which determine the production of giant races and others of 

 dwarf races. Moreover, factor differences show striking relations to 

 one another. Thus in Drosophila there are factors for eye color which 

 change the shade of red in the eye, some resulting in a darker and many 

 in lighter shades, but there is also a single factor difference which results 

 in white eyes or in other words in the entire loss of color in the eyes, 

 and even further there is a factor for an eyeless condition, which when a 

 part of the genetic constitution of a fly results in the production of mere 

 rudiments of eyes or even none at all. 



Very frequently single factors may cause such profound changes as 

 to alter the entire appearance of the individual and interfere more or 

 less with all its functions. Such, for example, is the case with fasciated 

 forms in plants, some of which at least are dependent upon simple factor 

 differences. A striking case of this type has been reported by O. E. White 

 in tobacco. In this fasciated variety the number of leaves is greatly 

 increased, from 24 to as high as 80, the stem is flattened and exhibits 

 a characteristic fasciated condition, and the flowers are very abnormal. 

 The abnormality of the flowers extends to every part, the numbers of 

 sepals, petals, stamens, and ovary locules are increased, and striking 

 deformities of these parts give evidence of the disturbing effect of the 

 factor. The abnormal effects of the factor are not confined to external 

 characters, but cytological studies show that the division figures, par- 

 ticularly in reduction, show marked irregularities which may be expressed 

 in an increase in the number of chromosomes, or in a breaking down of 



