INSEPARABLE EFFECTS 39 



recessive to the third. Obviously these four colours are 

 the first four groups in a set of eight, but, as their 

 effects cannot be identified, the factors which make them 

 different must be represented by " unknowns," thus : 



Grey XYZ 



Blue-grey XY z 



Black Xy Z 



Yellow xYZ 



By being recessive to colours carrying two dominants, 

 that is, to blue-grey and black, blue must carry one 

 dominant less. It may carry therefore the characters 

 Xyz, or xYz, or xyZ. But, if it carried xYz, it would 

 differ from black in three pairs, while, if it carried xyZ, 

 it would differ from blue-grey in three pairs. Conse- 

 quently, it can carry only the characters Xyz. 



Similarly, by being recessive to both black and 

 yellow, tortoise-shell must carry the characters xyZ. 



Thus the colours produced by six of the combinations 

 in which the factors X and x, Y and y, and Z and z, 

 can be combined have been identified with their factors, 

 while two still remain to be found, as follows : 



