RULES AND RATIOS OF INHERITANCE 227 



races, and as all have the same constitution, there will be 

 little variation in size. 



Now mate together two of these individuals AaBb. These 

 will give for the F2 generation the combinations shown in 

 table 2, page 117. Of these, those that have four of the genes 

 ABAB will be largest, those with but three of them will be 

 smaller, and so on down to those having none of them — the 

 individuals with constitution abab. There will thus be a series 

 of graded sizes. To determine the relative number of indi- 

 viduals in the different grades, from smallest to largest, list 

 from table 2 (page 117) the constitutions having respectively 

 o, I, 2, 3 or 4 of the "size genes" ABAB. The result is as 

 follows : 



Number of Size Genes 01 23 4 



abab abaB abAB ABAb ABAB 

 r aBab aBAb AbAB ~ 



abAb ABaB ABaB 

 Abab ABab aBAB 



4 AbAb 

 aBaB 



Thus there are in F2 five grades as to size. The most 

 numerous individuals (6 out of 16) have the intermediate 

 size, with but two of the "size genes." There are 4 a little 

 smaller and 4 a little larger, with a single individual in each 

 of the extreme classes. The numbers of individuals in the 

 different classes are given by the binomial series resulting 

 from expanding the expression (i -j- i)*. 



Similarly, if the two original races differ in three pairs of 

 genes In different chromosomes, so that one Is AABBCC, 

 while the other is aabbcc, the Fi generation will have the 

 uniform constitution AaBbCc, and so will be Intermediate 

 between two races. The F2 generation will be found to 



