PERCENTAGE 



OF PLANTS 



-50 



-25 



-M) 



1 



40 



55 



70 



85 



100 



COROLLA LENGTH IN MMS. 



Fig. 17. — Frequency distribution of corolla length in families of Nicotiaim longijlora, 

 showing segregation for the polygenes controlling the differences between the twu 

 parent strains (P) in this character. Variation in P and F^ is wholly non-heritable. 

 To this is added in F, the heritable component depending on segregation. The spread 

 of the frequency distribution is thus higher in Fo than in F^ and P. In F3 the average 

 heritable component of variation is half that in Fj, but it varies from family to family 

 according to the number of genes for which the Fg parent was heterozygous. The 

 spread of the frequency distribution is thus variable in F^ though always lying between 

 that of F2 on the one hand, and of F^ and P on the other. The mean corolla length 

 of each F3 is related to the corolla length of its F2 parent, indicated by the source 

 of the relevant arrow (based on East, 191 5). 



71 



