CONTINUOUS VARIATION 



Four important conclusions follow from this experiment: 



In the first place a character can (and in fact nearly always does) 

 show heritable and non-heritable variation simultaneously. 



Secondly, the two kinds of variation are indistinguishable by 

 anything but a breeding test. Only experiment can tell us 

 whether the bean is large because of its genotype or in spite of its 

 genotype. 



Thirdly, the effects of the environment can either reinforce or 

 counter-balance genetical differences. Thus 70-cg. mother beans from 

 line II are larger than 20-cg. mother beans from line XVIII, partly 

 because of the genetical difference, which their offspring reveal as 

 averaging 15-cg., and partly because of non-heritable or external 

 effects. At the same time 6 of the 9 lines of Table 4 B included mother 

 beans in the 60-cg. class, even though the genetical values of these 

 beans must have differed by nearly 20 cgs. in the extreme case. 

 These genetical differences were counter-balanced by non-heritable 

 effects. 



Finally, the genetical values of the lines show a graded series from 

 35 -I to 64*2 cgs., with no single great discontinuity. Indeed, the 

 genetical differences that are here proved to exist between these 

 lines are much smaller than the effects that environmental agencies 

 are producing within each line. The genes that are acting must be 

 genes of relatively slight effect. 



Cumulative Effects of Genes 



Johannsen's experiments with true-breeding lines enabled him 

 to demonstrate all these effects and properties of non-heritable 

 variability. They also, however, tell us that there must be many 

 genotypes having different effects on the character; for, even when 

 separated from the non-heritable, the genetical differences are not 

 simply due to a single gene. 



Nilsson-Ehle showed by hybridization experiments how this 

 could come about. He crossed varieties of wheat and of oats differing 

 in various characters, which segregated clearly in Fg and later 

 generations; but not always into the familiar mendelian ratios. For 

 example, in wheat, an F2 from crossing red- and white-grained 

 varieties might segregate in a ratio of 63 plants with red grain to i 



64 



