ate the appearance of the reversionary blue pigeon in
F_2 from the cross of black with white.

* * * * *

{68}

CHAPTER VII

DOMINANCE

[Illustration: FIG. 12.

Primula flowers to illustrate the intermediate nature of the F_1 flower
when _sinensis_ is crossed with _stellata_.]

Sinensis x Stellata
|
Intermediate -------------------------- F_1
|
+---------+----+-------+-----------+
Sinensis Inter. Inter. Stellata --------- F_2
| | |
Sinensis | Stellata --------- F{3}
| +-----+-----+-----+ |
Sinensis Sin. Int. Int. Stell. Stellata --------- F{4}

In the cases which we have hitherto considered the presence of a factor
produces its full effect whether it is introduced by both of the gametes
which go to form the zygote, or by one of them alone. The heterozygous tall
pea or the heterozygous rose-combed fowl cannot be distinguished from the
homozygous form by mere inspection, however close. Breeding tests alone can
decide which is the heterozygous and which the homozygous form. Though this
is true for the majority of characters yet investigated, there are cases
known in which the heterozygous form differs in appearance from either
parent. Among plants such a case has been met with in the primula. The
ordinary Chinese primula (_P. sinensis_) (Fig. 12) has large rather wavy
petals much crenated at the edges. In the Star Primula (_P. stellata_) the
flowers are much smaller, while the petals are flat and present only a
terminal notch instead of the numerous crenations of _P. sinensis_. The
heterozygote produced by crossing these forms is intermediate in size and
appearance. When self-fertilised such plants behave in simple Mendelian
fashion, {69} giving a generation consisting of _sinensis_, intermediates,
and _stellata_ in the ratio 1 : 2 : 1. Subsequent breeding from these
plants showed that both the _sinensis_ and _stellata_ which appeared in the
F_2 generation bred true, while the intermediates always gave all three
forms again in the same proportion. But though there is no dominance of the
character of either parent in such a case as this, the Mendelian principle
of segregation could hardly have a better illustration.

{70}

Blue x Blue
|
+-------------+-