332 Interaction of Genes 



crossed with a white, riVi r2r2, the Fi is red but is of a Ughter 

 shade than its red parent. The F2 plants are: 



1 RiRi R2R2 — very deep red — 4 genes for red 



2 RiRi i?2^2 — deep red — 3 genes for red 

 2 Riri R2R2 — deep red — 3 genes for red 

 4 Riri R2r2 — intermediate red — 2 genes for red 



1 RiRi r2r2 — intermediate red — 2 genes for red 



2 Rivi r2r2 — pale red — 1 gene for red 



1 rir\ R2R2 — intermediate red — 2 genes for red 



2 riri i^2^2 — pale red — 1 gene for red 

 1 riVi r2/'2 — white — genes for red 



If like phenotypes are classed together, the F2 would segre- 

 gate into YiQ very deep red : ^4 deep red : %q intermediate 

 red : % pale red : %6 white. When there are more than two 

 duplicate genes with a cumulative action, and dominance is in- 

 complete, the F2 ratio of phenotypes becomes even more complex. 



Interaction in Polyhybrids 



The simplest examples of gene interaction involve two pairs 

 of genes, but in many instances three or more pairs interact in 

 various ways. 



Gene Interaction in Primula. An excellent example of inter- 

 action in polyhybrids is found in the Chinese primrose, Primula 

 sinensis. The form of the leaf is due to the interaction of at 

 least seven pairs of genes, and the normal type is produced by 

 all the dominants together. The normal leaf form is called 

 "palm" (Fig. 94). In this type the clefts or incisions are sharp 

 and are arranged so that the leaf is reminiscent of the fan- 

 shaped type of palm leaves. Other types are the "tongue" and 

 "oak." In the "tongue" the leaf is elongated and the clefts are 

 slight. In the "oak" the lobes are fewer than in the "palm" 

 and the clefts between the lobes are so deep as to extend almost 

 to the midrib. If an "oak" is crossed with a "tongue," the Fi 

 is "palm" and the F2 consists of 9 "palm" : 3 "tongue" : 3 

 "oak" : 1 "tongue-oak." The last type is easily distinguishable 

 from the others and resembles a combination of a "tongue" and 

 an "oak" type. The "tongue" plants are homozygous for the 

 recessive gene t and also have the gene in homozygous or 

 heterozygous condition. The "oak" plants are 00 and have one 



