V INTERACTION OF FACTORS 47 



agoutis, albinos, and blacks. Whence the sudden 

 appearance of the new type ? The answer is a simple 

 one. The albino parent was really a black. But it 

 lacked the factor without which the colour is unable to 

 develop, and consequently it remained" an albino. If 

 we denote this factor by C, then the constitution of an 

 albino must be cc, while that of a coloured animal may 

 be CC or Cc, according as to whether it breeds true to 

 colour or can throw albinos. Agouti was previously 

 known to be a 'simple dominant to black, i.e. an 

 agouti is a black rabbit plus an additional greying 

 factor which modifies the black into agouti. This 

 factor we will denote by G, and we will use B for 

 the black factor. Our original agouti and albino 

 parents we may therefore regard as in constitution 

 CCGGBB and ccggBB respectively. Both of the 

 parents are homozygous for black. The gametes 

 produced by the two parents are CG^B and cgB, and 

 the constitution of the F^ animals must be CcGgBB. 

 Being heterozygous for two factors they will produce 

 four kinds of gametes in equal numbers, viz. CGB, 

 CgB, cGB, and cgB. The results of the mating of 

 two such similar series of gametes when the Fj 

 animals are bred together we may determine by the 

 usual " chessboard " method (Fig. 8). Out of the 

 16 squares 9 contain both C and G in addition to 

 B. Such animals must be agoutis. Three squares 

 contain C but not G. Such animals must be 

 coloured, but as they do not contain the modifying 

 ag-outi factor their colour will be black. The remain- 

 ing four squares do not contain C, and in the absence 

 of this colour-developing factor they must all be 

 albinos. Theory demands that the three classes 



