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