THE MECHANISM OF HEREDITY 71 



about 200, the grandparental sizes probably would have 

 been obtained. Furthermore, if one studies the results 

 obtained in the F 3 , F and F s generations, considering 

 only the range of their variability, it is clear they differ in 

 both type and extent of variation. 



No assumptions unproved for the inheritance of quali- 

 tative characters are necessary for thus visualizing the 

 inheritance of quantitative characters, and no facts dis- 

 covered in tracing the inheritance of other characters 

 such as those involving linkage are overlooked. But in 

 order to picture the situation easily, let us assume that 

 dominance is usually absent (often the case), that two 

 doses (i.e., the homozygous condition) of a factor have 

 twice the effect of one dose (true for all practical pur- 

 poses), that independent factors cumulative in their oper- 

 ation are allelomorphic to their absence in the hybrid 

 (linkage though it complicates matters, does not change 

 our reasoning) . 



Let us assume a case of " blended" inheritance where 

 all fluctuations due to environment are eliminated. A 

 plant 12 inches tall is supposed to be crossed with a plant 

 28 inches tall. The difference between them is 16 inches. 

 If this difference is due to one allelomorphic pair in which 

 dominance is absent, the F^ generation is all intermediate 

 about 20 inches and the F 2 generation falls into three 

 classes in which two represent the grandparental forms 

 and one represents the F l form. Twenty-five per cent, 

 are 12 inches tall, fifty per cent, are 20 inches tall and 

 twenty-five per cent, are 28 inches tall. 



But suppose this 16-inch difference between the 

 parents is represented by two allelomorphic pairs instead 

 of one. The F 1 generation is again 20 inches tall, but 



