GENETICS IN RELATION TO AGRICULTURE 

 The males, however, produce the following series of sperms: 



v(WX) VY vY. 



90 



Mating Fi flies of this cross, therefore, results in the F 2 population shown 

 in the checkerboard in Fig. 44. When these are collected into like pheno- 

 types the ratio obtained is 9 long red: 3 long white: 3 vestigial red: 



1 vestigial white, but this agreement with the typical dihybrid ratio is only 

 apparent. When the females alone are considered the ratio is 6 long red : 



2 vestigial red, and the males are in the ratio 3 long red: 3 long white: 

 1 vestigial red:l vestigial white. The disturbance in the ratio is 

 occasioned by the unique behavior of the white-eye character which 

 behaves exactly as it did in the simple case which was analyzed previously. 

 The reciprocal ratio, therefore, is additional evidence as to the adequacy 

 of the chromosome theory. 



The Trihybrid. The same line of reasoning of course applies to 

 cases in which three pairs of factors are involved. Such for example is a 

 case which Baur has described in the common garden snapdragon, 

 Antirrhinum majus. In this particular case the factors involved have 

 the following relations. 



Z a factor which conditions the development of the zygomorphic 

 type of blossom which is characteristic of the species. The factor z, its 

 allelomorph, conditions the production of peloric blossoms, i.e., blossoms 

 which display radial symmetry. The normal form is nearly completely 

 dominant. 



R a factor for red color of the blossoms. The allelomorph r gives 

 flowers which are flesh colored. 



/ a factor for ivory coloration of the blossoms. The allelomorph 

 i in this case conditions the production of yellow flowers. R with / gives 

 flowers red on an ivory background, a magenta type of coloration, 

 whereas R with i gives flowers which are red on a yellowish background. 

 It is possible to distinguish these two classes in a mixed population. 



TABLE XIII. DISTRIBUTION OF CLASSES AMONG THE PROGENY OF AN Fi HYBRID 

 SNAPDRAGON OF THE COMPOSITION ZzRrli 



