324 



Interaction of Genes 



substance to a purple one. Thus PB plants are purple. Since 

 Pb plants have no color-changing enzyme, they are yellow. 

 Plants with the genes pB and pb have no colored substance to 

 begin with and are white. They are alike because B, the enzyme, 

 has no colored substance with which to react and does not affect 

 the gene product of p any more than b. 



Pi 



X 



PPBB 



ppbb 



F, 



PpBb 



Pb 



pB 



pb 



Pb 



PPBB 



PPBb 



PpBB 



PpBb 



PPBb 



PPbb 



PpBb 



Ppbb 



pB 



pb 



PpBB 



PpBb 



a:) 



ppBB 



CD 



ppBb 



PpBb 



Ppbb 



CD 



ppBb 



ppbb 



Total: 9PB = purple 

 3Pb = yellow 



3Pf 1= white 



Fig. 90. Checkerboard showing the 9:3:4 ratio in beans. A cross of 

 a homozygous purple, PP BE, and a white, pp bb, produces a purple Fi 

 (PpBb). The F2 segregates into 9 purple {P B) : 3 yellow (Pb) : 4 white 

 (SpB and Ipb). This can possibly be explained by assuming that P 

 produces a colored and p a colorless chemical compound and that the 

 colored compound is j^ellow in the presence of gene b but can be changed 

 to purple by the enzyme B. (Based on the work of Dr. G. H. Shull.) 



