52 SUMMARY OF CURRENT RESEARCHES RELATING TO 



dominants ; secondly, hybrids ; i.e. into groups A and A (B) in ratio 

 of 1 : 2. 



Mendel found bis results to come very close to tbe theoretical pre- 

 supposition, when he crossed yellow with green peas. He found the 

 numbers to be 3:1, and the recessive or green seed produced only 

 green seed ; while of the yellow or dominant, one in three produced 

 only dominants, but two out of three produced hybrids, and this gave 

 him A, 2 A (B), B, the theoretical proportions. Mr. G. M. Allen has 

 found the same proportions to be true in the case of crossing white 

 mice with grey mice. 



The correctness of Mendel's hypothesis of the purity of the germ- 

 cells and of their production in equal numbers, is shown by back- 

 crossing of a hybrid with one of the parental forms. For example, in 

 a case of simple dominance the first generation will all be D (R). Any 

 one of them back-crossed with the recessive parent will produce 50 p.c. 

 pure recessives and 50 p.c. hybrids. 



For hybrid produces germ-cells . . D + R 



For recessive parent produces germ-cells R + R 



The possible combinations are 2 D (R) + 2 R 



And this has been proved for peas and for mice. 



In dealing with cross-breeding between parents differing in more 

 than one character, we find in the second and later hybrid generations, 

 individuals possessing new combinations of the characters found in the 

 parents, indeed, all possible combinations of these characters will be 

 found, and in the proportion demanded by chance. 



Take parents differing in two characters A, B (the recessive phases 

 a, b). The immediate offspring resulting from the cross will be all 

 alike, A B (a b). The second and later generations of hybrids will 

 contain the stable classes A B, A b, a B, a b, also the unstable forms 

 A B (a b), A (a) b, and a B (b). One, therefore, of each of the stable 

 combinations occurs in every sixteen " second-generation " offspring ; 

 and only the individual which possesses both recessive characters can at 

 once be set aside as pure. Moreover, nine out of every sixteen 

 " second generation " hybrids will possess the two dominant characters, 

 but only one will be pure with, regard to those characters, for four will 

 be hybrid in one character, and four will be hybrid in both characters. 



Mendel generalised these statements as follows : In cases of com- 

 plete dominance (parents differing in n ways), the number of different 

 classes in the second generation will be 3", of which 2" will be stable ; 

 the remainder will be hybrid, though indistinguishable from pure in- 

 dividuals, and the smallest number of individuals which, in the second 

 hybrid generation, Avill allow of one pure individual to each visibly 

 different class will be 4". This gave rise to a new conception of 

 " purity " : An animal or plant is pure if it produces gametes of only 

 one sort, even though its grandparents may among themselves have 

 possessed opposite characters. 



Several exceptions to Mendel's law have been observed, for example : 



(1) Mosaic inheritance, in which a pair of characters usually related 

 as dominant and recessive occur in a balanced relationship side by side 



