V INTERACTION OF FACTORS 51 



throw albinos. Agouti was previously known, to be a sim- 

 ple 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 

 GGCCBB and ggccBB respectively. Both of the parents 

 are homozygous for black. The gametes produced by 

 the two parents are GCB, and gcB, and the constitution of 

 the Fi animals must be GgCcBB. Being heterozygous for 

 two factors they will produce four kinds of gametes in 

 equal numbers, viz. GCB, GcB, gCB, and gcB. The 

 results of the mating of two such similar series of gametes 

 when the Fi animals are bred together we may determine 

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

 1 6 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 agouti factor their colour will 

 be black. The remaining 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 agouti, black, and albino should appear in F2 in the 

 ratio 9:3:4; experiment has shown that these are the 

 only classes that appear, and that the proportions in 

 which they are produced accord closely with the theoret- 

 ical expectation. Put briefly, then, the explanation 



