3o6 PLANT HYBRIDIZATION BEFORE MENDEL 



CLASS III 



The character enclosed in parentheses, according to Mendel's original 

 conception, is the latent one in the hybrid. 



Mendel then observed that the whole of these different groups 

 of plants could be arranged into three different classes, as follows : 



1. The first class included only groups 1, 2, 3, and 4, with the 

 signs AB, Ab, aB, and ab. It is evident that round yellow seeds 

 (AB) will come true, as both the characters are dominants of 

 different character-pairs; likewise with wrinkled green seeds (ab), 

 since both of these characters are recessives of different character- 

 pairs. So also round green seeds (AAbb) and wrinkled yellow 

 (aaBB) will also come true and be constant, since they combine 

 the dominant of one character-pair with the recessive of another. 



2. The second class of plants includes groups Nos. 5, 6, 7, and 

 8, AABb, aaBb, AaBB, and Aabb. 



The above groups, as Mendel puts it, "are constant in one char- 

 acter and hybrid in another, and vary in the next generation only 

 in the hybrid character." (p. 352.) 



This means, for example, that the plants in group No. 5, which 

 bears the sign AABb, are round (A) and yellow (B) in appear- 

 ance, but since they bear also the hidden recessive character (b), 

 they are hybrid with respect to color. 



Likewise with the plants in group 6, which bear seeds that all 

 appear wrinkled 3^ellow, but which are hybrid (Bb) as to color, 

 since the B and the b occur together. 



3. Finally, the third class includes only group 9, in which 138 

 plants bear round yellow seeds so far as appearances go, but, 

 since Aa and Bb are confined together, it is apparent that these 

 seeds are not pure round, but hybrid round, and that the yellows 

 are not pure yellow but hybrid yellow. 



Now, by comparing the average number of plants of the groups 

 in the three classes, we get a very close ratio of 1 :2:4, since the 



