What these factors are is still an open question. Recent
evidence of a chemical nature indicates that colour in a flower is due to
the interaction of two definitive substances: (1) a colourless "chromogen,"
or colour basis; and (2) a ferment which behaves as an activator of the
chromogen, and by inducing some process of oxidation, leads to the
formation of a coloured substance. But whether these two bodies exist as
such {49} in the gametes or whether in some other form we have as yet no
means of deciding.

Since the elucidation of the nature of colour in the sweet pea phenomena of
a similar kind have been witnessed in other plants, notably in stocks,
snapdragons, and orchids. Nor is this class of phenomena confined to
plants. In the course of a series of experiments upon the plumage colour of
poultry, indications were obtained that different white breeds did not
always owe their whiteness to the same cause. Crosses were accordingly made
between the white Silky fowl and a pure white strain derived from the white
Dorking. Each of these had been previously shown to behave as a simple
recessive to colour. When the two were crossed only fully coloured birds
resulted. From analogy with the case of the sweet pea it was anticipated
that such F_1 coloured birds when bred together would produce an F_2
generation consisting of coloured and white birds in the ratio 9 : 7, and
when the experiment was made this was actually shown to be the case. With
the growth of knowledge it is probable that further striking parallels of
this nature between the plant and animal worlds will be met with.

Before quitting the subject of these experiments attention may be drawn to
the fact that the 9 : 7 ratio is in reality a 9 : 3 : 3 : 1 ratio in which
the last three terms are indistinguishable owing to the special
circumstances that neither factor can produce a visible effect without {50}
the co-operation of the other. And we may further emphasise the fact that
although the two factors thus interact upon one another they are
nevertheless transmitted quite independently and in accordance with the
ordinary Mendelian scheme.

Agouti x Agouti
|
+---------------+
Agouti x Agouti
|
+---------+---------+
Agouti Black Albino
(9) (3) (4)

One of the earliest sets of experiments demonstrating the interaction of
separate factors was that made by the French zoologis