188 Plant-Breeding 



Parents .... Red variety X Cream variety 

 Red Sap ( D) Colorless sap (r) 



Colorless corpuscles (D) Yellow corpuscles (r) 



p ( Red sap 



\ Colorless corpuscles 



2 T 1 1 



F~ ~V~ HT ~ I 



Red sap Red sap Colorless sap Colorless sap 

 Colorless Yellow Colorless Yellow 



corpuscles corpuscles corpuscles corpuscles 



Appearance 9 red 3 red-cream 3 white 1 cream 



The ratio 9:3:4. The F* ratio, 9 : 3 : 4, is one which 

 very frequently occurs in mendelian analysis. For ex- 

 ample, as Tschermak found, when a pink-and-white 

 flowered eating pea (Pisum sativum) is crossed with a 

 white-flowered type, FI is often the original purple-flowered. 

 Then F 2 will be 



9 purple : 3 pink and white : 4 white. 



In this case the factor for purple is evidently brought in 

 by the albino. The latter contains the presence of 

 purple, which needs a factor from the other parent to 

 bring it out, and the absence of pink and white. The 

 other parent contains the presence of pink and white 

 and the absence of a factor for purple. All that is essen- 

 tial for the production of the ratio in F 2 is that F\ should 

 be heterozygous for two factors, of which one is percep- 

 tible whenever present, while the other needs the presence 

 of the first in order that its own effects may be mani- 

 fested. 



Emerson's experiments with beans. By crossing self- 

 colored varieties of beans with white varieties, Emerson 



