-------+------------+
Black Blue x Blue White
| | |
| +------+--+--+-----+ |
Black Black Blue Blue White White
| |
Black ------------- x ------------- White
|
Blue
(all)

Among birds a case of similar nature is that of the Blue Andalusian fowl.
Fanciers have long recognised the difficulty of getting this variety to
breed true. Of a slaty blue colour itself with darker hackles and with
black lacing on the feathers of the breast, it always throws "wasters" of
two kinds, viz. blacks, and whites splashed with black. Careful breeding
from the blues shows that the three sorts are always produced in the same
definite {71} proportions, viz., one black, two blues, one splashed white.
This at once suggests that the black and the splashed white are the two
homozygous forms, and that the blues are heterozygous, _i.e._, producing
equal numbers of "black" and "white splashed" gametes. The view was tested
by breeding the "wasters" together--black with black, and splashed white
with splashed white--and it was found that each bred true to its respective
type. But when the black and the splashed white were crossed they gave, as
was expected, nothing but blues. In other words, we have the seeming
paradox of the black and the splashed white producing twice as many blues
as do the blues when bred together. The black and the splashed white
"wasters" are in reality the pure breeds, while the "pure" Blue Andalusian
is a mongrel which no amount of selection will ever be able to fix.

In such cases as this it is obvious that we cannot speak of dominance. And
with the disappearance of this phenomenon we lose one criterion for
determining which of the two parent forms possesses the additional factor.
Are we, for example, to regard the black Andalusian as a splashed white to
which has been added a double dose of a colour-intensifying factor, or are
we to consider the white splashed bird as a black which is unable to show
its true pigmentation owing to the possession of some inhibiting factor
which prevents the manifestation of the black. Either interpretation fits
the facts equally well, {72} and until further experiments have been
devised and carried out it is not possible to decide which is the correct
view.

Besides these comparatively rare cases where the he