DI-HYBRID CROSSES 177 



Let us consider the external appearance of these 

 various types in the particular example before us. 



Nine of the above sixteen terms include A and B, 

 and are therefore smooth yellow in appearance. (We 

 need not stop to consider whether a or b or both are 

 present in addition, since these are recessive.) 



Three terms include A and b, B being absent. These, 

 therefore, appear wrinkled yellow. 



Three include a and B, A being absent. These, 

 therefore, appear smooth white. 



One contains a and b only, and is, therefore, wrinkled 

 white. 



With regard to internal constitution : 

 The nine individuals of appearance AB include the 

 following types : 



One pure, ABAB, breeding true to the smooth 

 yellow type on self-fertilization. 



Two ABAb, heterozygous in respect of the 

 pair B-b, but pure yellow. 



Two ABaB, heterozygous in respect of A-a, 

 but pure smooth. 



Four A Bab, heterozygous in respect of both 

 pairs of characters. 



The three individuals of appearance Ab include the 

 following types : 



One pure, AbAb, breeding true to the (new) 

 wrinkled yellow type. 



Two Abab, giving both wrinkled yellow and 

 wrinkled white. 



The three individuals of appearance aB include the 

 following types : 



12 



