terozygote cannot be
said to bear a closer resemblance to one parent more than to the other,
there are cases in which it is often possible to draw a visible distinction
between the heterozygote and the pure dominant. There are certain white
breeds of poultry, notably the White Leghorn, in which the white behaves as
a dominant to colour. But the heterozygous whites made by crossing the
dominant white birds with a pure coloured form (such as the Brown Leghorn)
almost invariably show a few coloured feathers or "ticks" in their plumage.
The dominance of white is not quite complete, and renders it possible to
distinguish the pure from the impure dominant without recourse to breeding
experiments.

+------+------+------+------+
|CI |CI |CI |CI |
|CI |Ci |cI |ci |
| | | | |
| | | | |
+------+------+------+------+
|Ci |Ci....|Ci |Ci....|
|CI |Ci....|cI |ci....|
| |......| |......|
| |......| |......|
+------+------+------+------+
|cI |cI |cI |cI |
|CI |Ci |cI |ci |
| | | | |
| | | | |
+------+------+------+------+
|ci |ci....|ci |ci |
|CI |Ci....|cI |ci |
| |......| | |
| |......| | |
+------+------+------+------+

FIG. 13.

Diagram to illustrate the nature of the F_2 generation from the cross
between dominant white and recessive white fowls.

This case of the dominant white fowl opens up another interesting problem
in connection with dominance. By accepting the "Presence and Absence"
hypothesis we are committed to the view that the dominant form possesses an
extra factor as compared with the recessive. The natural way of looking at
this case of the fowl is to regard white as the absence of colour. But were
this so, colour should be dominant to white, which is not the case. We are
therefore forced to suppose that the absence of colour in this instance is
due to the presence of a factor whose {73} property is to inhibit the
production of colour in what would otherwise be a pure coloured bird. On
this view the dominant white fowl is a coloured bird plus a factor which
inhibits the development of the colour. The view can be put to the test of
experiment. We have already seen that there are other white fowls in which
white is recessive to colour, and that the wh