.132 THE FIRST PRINCIPLES OF HEREDITY 



50 per cent. " wasters." Thus was explained the puzzHng 

 phenomenon, which could not be solved for a long time, 

 that, while the crossing of two blue birds only gives 50 per 

 cent, of blues, the crossing of a black with a white bird 

 produces a progeny all of which are blue. The explanation 

 is that the black and white are the pure breeds, while the 

 blue is a Mendelian hybrid. The following diagram wiU 

 make this clear : 



Black— D White— R P 



Blue— D(R) Fi 



I DD 2 D(R) I RR F2 



I Black 2 Blue i White 



Fig. 54. — Andalusian Fowls. (After Bateson.) 



{a) Compound Characters. 



So far, when we had to deal with two pairs of allelo- 

 morphs, the units of each allelomorphic pair expressed a 

 character of the organism distinct from the character 

 expressed by the second allelomorphic pair. We had, e.g., 

 the allelomorphs yellow-green, as affecting the colour of a 

 part, distinct from the allelomorphs round- angular, as 

 affecting the form of that part, the one effect being nowise 

 dependent on the other. We now come to cases where 

 the units belonging to different allelomorphic pairs do react 

 on each other, thereby producing new forms. Such 

 allelomorphs have been called " compound allelomorphs." 

 The character resulting from them are compound characters 

 — i.e., they are single characters produced by the con- 

 currence of more than one allelomorphic factor. 



A case in point is that of crosses between red-flowered 

 and cream-flowered varieties of sweet-peas or stocks. 

 By crossing the red variety with a cream-coloured variety 

 we get a hybrid which, being a dihybrid, produces, on 



