180 



GENETICS IN RELATION TO AGRICULTURE 



factor of 28 FI plants obtained from this cross was 2.39, a value 

 which does not differ significantly from the average leaf factor of the 

 two parents, 2.49. Fig. 85 illustrates three leaves representing the three 

 types concerned in this cross. From the F\ plants of this cross 195 F 2 

 plants were grown, and their leaf factors were found to be distributed as 

 shown in Fig. 86. The striking feature of this F 2 distribution is that, 

 although practically all values for the leaf factor between the two parental 

 extremes are represented, these frequencies of values give a trimodal distri- 

 bution with modes which correspond approximately to those of the parents 

 and the F\. If these F 2 individuals be divided into three portions, as 

 shown in Table XXXII corresponding to the two parents and the FI, the 

 mean value for the upper group is 3.42 as compared with the mean value 

 of 3.47 for the corresponding parental type, the mean value of the mid 

 group is 2.59 corresponding to 2.39 for the F\, and the lower group has a 

 mean value of 1.66 as compared with 1.52 for its corresponding parental 

 group. In the upper group there were 46 plants, in the mid group 102, 

 and in the lower group 47, a satisfactory agreement with a 1:2:1 ratio. 



TABLE XXXII. RE-APPEARANCE OF PARENTAL VALUES IN THE 



(After Leake). 



OFFSPRING 



This case then gives a simple expression of the general phenomena 

 assumed to be operative in the multiple factor theory of size inheritance. 

 If it be assumed that a single factor difference be here operative, then the 

 small parent might be represented as aa and the large parent as A A. 

 Plants of the genetic constitution aa then fluctuate for their leaf factor 

 around 1.52 as a mean, whereas plants of the genetic constitution A A 

 fluctuate around 3.47. When these two races are crossed the FI is 



