54 



Those individuals belonging to the progeny of the above hybrid which 

 have originated as a result of a union of two black or two white gametes 

 (homozygotic individuals) can produce only black and white gametes re- 

 spectively, and their progeny in each case will be constant in succeeding 

 generations. Those individuals on the other hand, which have been pro- 

 duced by the union of unlike gametes (heterozygotic individuals) such as 

 black and white, for example, will in turn produce both black and white 

 gametes and the progeny will therefore display the same " variation " as that 

 shown in the original crossing. 



When more than one character pair is involved the result is somewhat 

 more complicated yet in full accord with the leading principle. If, for 

 example, a Bald Lax-eared wheat is crossed with a Bearded Dense eared sort 

 (dihybrid combination,) there are two character pairs to be dealt with, instead 

 of one. These may be represented as follows: 



Bald Bearded 



Lax-eared Dense-eared. 



Biffen (8) has shown that the first generation hybrids (F x ) from crosses 

 between sorts having the above characters differ according to the varieties 

 used. They are Bald or nearly so and lax-eared or strongly inclined in this 

 direction. Baldness and Laxness are here the dominant characters and hence 

 only Bald and Lax-eared forms appear in the first generation. In the second 

 generation hybrids there will be shown different combinations of the potenti- 

 alities of the parents arising through a union of egg-cells and pollen-cells. 

 Considering the two character pairs in question there may be four kinds of 

 egg-cells involved in the union, namely egg cells combining the potentialities of 

 Bald and lax-eared types, Bald and dense eared types, Bearded and dense-eared and 

 Bearded and lax-eared. The same combinations are possible in the formation 

 of the sperm or pollen cell. When these four kinds of egg-cells and four kinds 

 of pollen cells are brought together, 16 combinations are possible. These 

 mayjoe shown in the following manner allowing B to represent the Bald 

 character, b the bearded, L the lax and I the dense: (F 2 =second generation; 

 F 3 =third generation or progeny of F 2 ). 



9 d 



Description of progeny of F 2 



B L X B L = Bald and Lax forms. 



X B 1 = Bald and Lax (+ dense) 



" X b L = " " " (+ bearded) 



" X bl = " " " (+ bearded and dense) 



B 1 X B L = Bald and Lax ( + dense) 



" X B 1 = Bald and dense 



" X b L = Bald and Lax ( + dense and bearded) 



" X b 1 = Bald and dense ( + bearded) 



