CONTINUOUS VARIATION 



indicate the classes in which the Fg plants fell, from which the Fg's 

 came. It is clear that the F3 mean is correlated with the value of 

 the Fo individual from which it came. 



In this experiment all our predictions are verified, and we cannot 

 doubt that the polygenes controlling corolla length show segregation 

 in the way to be expected from Mendel's principles. We may observe, 

 too, that in this experiment the F^'s from reciprocal crosses are alike. 

 Male and female parents contribute equally to F^ as would be 

 expected with nuclear inheritance. 



Many experiments of this kind have been performed and all have 

 agreed, within the limitations imposed by their size and structure, 

 with the behaviour expected. 



Linkage of Polygenes 



Not a few of the experiments which show polygenic segregation 

 also reveal linkage of polygenes and major genes. The first case was 

 that of Sax (1923) in dwarf beans. He made a cross between a strain 

 with large coloured beans and another which had smaller white 

 beans. In the F., there was a clear segregation into a ratio of 3 plants 

 with coloured beans to i with white beans (Table 5). F3 families 

 were grown and by their aid the coloured plants were classified into 

 homozygotcs and heterozygotes. These appeared in the ratio of i : 2. 

 Thus seed colour was clearly under the control of a single gene 

 which has an effect large enough to be followed by the mendelian 

 method. 



TABLE 5 



AVERAGE WEIGHTS OF BEANS (in cg.) FROM COLOURED 

 AND WHITE MEMBERS OF AN F, IN PHASEOLUS 

 VULGARIS (SAX, 1923) 



Seed size, on the other hand, proved to be a continuously varying 

 character. Segregation could be detected by the greater spread of 



72 



