60 Compound Characters [CH. 



So in general 



4" =3" 



+ 3*~ l +3*~ 1 + ...... n times 



3 + ...... i (-i) (-2) times 



&c. 



COMPOUND CHARACTERS AND NOVEL TYPES PRODUCED BY 

 RE-COMBINATIONS. 



Thus far we have dealt only with cases in which the 

 characters of each allelomorphic pair have independent 

 effects on the visible appearance of the zygotes. In peas, 

 for instance, we have seen a pair of characters, tallness and 

 dwarfness, producing their effects quite independently of 

 other pairs of characters, such as those which determine 

 the flower colour or the seed-shapes. Each can be 

 separately perceived by its effects, and the presence of the 

 one in no way influences the development of the other. 

 Such are the imaginary characters A, a, and B, b, whose 

 distribution is represented above, and the four types pro- 

 duced by their several combinations are each distinguished 

 without difficulty, as AB, aB, Ab, ab. 



We now pass to a class of cases manifesting greater 

 complexity. The essential phenomenon in these cases is 

 that definite characters are produced by the mutual inter- 

 action of factors belonging to distinct allelomorphic systems. 

 Such interactions, as we now know, are of the greatest 

 importance in heredity, and the progress of genetics will 

 consist largely in disentangling the elements to which these 

 combination-effects are due. 



We may speak of characters thus produced as compound 

 characters. The nature of such compound characters is 

 well exemplified by phenomena which have been observed 

 respecting the inheritance of several types of combs seen 

 in various breeds of poultry, and as these cases are illustra- 

 tive of many others I propose to consider them in some 

 detail. 



