No. 588] 



F, BLEND AND GENIC PURITY 



743 



traits lying in that particular chromosome, which each J? t 

 individual as a parent is capable of passing on. There- 

 fore, by turning the spools so that all possible combina- 

 tions are made, one can read off directly all of the dif- 

 ferent hereditary potentialities to be had by inbreeding 

 the F x generation. Consequently the F 2 line (which is 

 charted on a flat surface) is simply a record of such 

 combinations. 



For the purpose of this study a case of blended inher- 

 itance is one in which the development in F x of a given so- 

 matic trait — regardless of whether it develops from one 

 or more genes — is about midway between its development 

 in the two parents, each of which is of pure stock in refer- 

 ence to the trait concerned. Until about the year 1910 

 students of heredity were unable to coordinate the general 

 rule of dominance and segregation on one hand, with the 

 frequent exception of blending and segregation on the 

 other. Now the existence of at least three different routes 

 by each of which nature arrives at the somatic blend in F x 

 are recognized, and each finds ready interpretation in con- 

 sonance with the theory of the pure gene. The first of 

 these is the dilution or true blend route, by which nature 

 appears to travel in the classical cases of the Blue Anda- 

 lusian 1 fowl resulting from the crossing of splashed- 

 white and black parents, and of the pink four o'clock 

 (Mirabilis jalapa) resulting from the crossing of red and 

 white parents. 



The ordinary mode of inheritance is strongly duplex — 

 that is, the zygote normally possesses two genes for each 

 trait, either one of which genes is usually sufficient — with 

 possibly a liberal surplus of valence — to give full somatic 

 expression to its correlated trait. In such cases complete 

 dominance in F x and clear-cut segregation in F 2 are the 

 rule. Occasionally, however, in cases wherein a duplex 

 parent possesses a strong somatic development of a trait, 



i "Mendel's Principles of Heredity" (3d Impression, 1912), p. 51, by 



