348 



Quantitative Characters 



family would be 42 cm, which is exactly intermediate between 

 the two parents and is exactly the same as the average height 

 of the Fi plants. However, although the average height of the 

 Fi and F2 is the same, the variability would be very different in 

 the two generations. All the variation to be found in the Fi 

 would theoretically be due to the environment and is usually 



1000 r— 



Fig. 97, Frequency curve showing the distribution of the plants of the 

 F2 from a cross in which the parents differed by twelve polygenes for 

 height. 



not so great as that found in the F2. The F2 would vary from 

 the extreme of one parent to the extreme of the other, and this 

 variation would be due largely to differences in genotype. The 

 Fo generation can, perhaps, be understood better from a fre- 

 quency curve, as in Fig. 97. In all three examples discussed in 

 this chapter, the tw^o parents were the same height, but the 

 parents of each family differed from the parents of the other 

 families in the number of polymeric genes that were present. In 

 each case, however, the size and variability of the Fi plants 

 were the same. In each example, the average height of all the 

 F2 plants was the same, but Figs. 96 and 97 show the great dif- 

 ferences in frequencies of plants of different size. 



