iteness of such birds is due
to the fact that they lack a factor for the development of colour. If we
denote this factor by C and our postulated inhibitor factor in the dominant
white bird by I, then we must write the constitution of the recessive white
as ccii, and the dominant white as CCII. We may now work out the results we
ought to obtain when a cross is made between these two pure white breeds.
The constitution of the F_1 bird must be CcIi. Such birds being
heterozygous for the inhibitor factor, should be whites showing some
coloured "ticks." Being heterozygous for both of the two factors C and I,
they will produce in equal numbers the four different sorts of gametes CI,
Ci, cI, ci. The result of bringing two such similar series of gametes
together is shown in Fig. 13. Out of the sixteen squares, twelve contain I;
these will be white birds either with or without a few coloured ticks.
Three contain C but not I: these must be coloured birds. One contains
neither C nor I; this must be a white. From such a mating we ought,
therefore, to obtain both white and coloured birds in the ratio 13 : 3. The
results thus theoretically {74} deduced were found to accord with the
actual facts of experiment. The F_1 birds were all "ticked" whites, and in
the F_2 generation came white and coloured birds in the expected ratio.
There seems, therefore, little reason to doubt that the dominant white is a
coloured bird in which the absence of colour is due to the action of a
colour-inhibiting factor, though as to the nature of that factor we can at
present make no surmise. It is probable that other facts, which at first
sight do not appear to be in agreement with the "Presence and Absence"
hypothesis, will eventually be brought into line through the action of
inhibitor factors. Such a case, for instance, is that of bearded and
beardless wheats. Though the beard is obviously the additional character,
the bearded condition is recessive to the beardless. Probably we ought to
regard the beardless as a bearded wheat in which there is an inhibitor that
stops the beard from growing. It is not unlikely that as time goes on we
shall {75} find many more such cases of the action of inhibitor factors,
and we must be prepared to find that the same visible effect may be
produced either by the addition or by the omission of a factor. The
dominant and recessive white poultry are indistinguishable in appearance.
Yet the one contains a factor more and the oth