t Cuenot on the coat
colours of mice. It was shown that in certain cases agouti, which is the
colour of the ordinary wild grey mouse, behaves as a dominant to the albino
variety, _i.e._ the F_2 generation from such a cross consists of agoutis
and albinos in the ratio 3 : 1. But in other cases the cross between albino
and agouti gave a different result. In the F_1 generation appeared only
agoutis as before, but the F_2 generation consisted of three distinct
types, viz. 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 {51} 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 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 F_1 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 F_1 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 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 F_2 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
theoretical expectation. Put briefly, then, the explanation {52} of this
case is that all the animals are black, and