1 AA : 2 AB : 1 BB.
We have seen that Mendel makes no prediction as to the outward and visible characters of AB, but only as to the essential constitution and statistical condition of its gametes in regard to the characters A and B. Nevertheless in a large number of cases the character of AB is known to fall into one of three categories (omitting mosaics).
(1) The cross-bred may almost always resemble one of its pure parents so closely as to be practically indistinguishable from that pure form, as in the case of the yellow cotyledon-colour of certain varieties of peas when crossed with green-cotyledoned varieties; in which case the parental character, yellow, thus manifested by the cross-bred is called “dominant” and the parental character, green, not manifested, is called recessive.
(2) The cross-bred may present some condition intermediate between the two parental forms, in which case we may still retain the term “blend” as applied to the zygote.
Such an “intermediate” may be the apparent mean between the two parental forms or be nearer to one or other in any degree. Such a case is that of a cross between a rich crimson Magenta Chinese Primrose and a clear White, giving a flower of a colour appropriately described as a “washy” magenta.
(3) The cross-bred may present some form quite different from that of either pure parent. Though, as has been stated, nothing can be predicted of an unknown case, we already know a considerable number of examples of this nature in which the mule-form approaches sometimes with great accuracy to that of a putative ancestor, near or remote. It is scarcely possible to doubt that several—though perhaps not all—of Darwin’s “reversions on crossing” were of this nature.
Such a case is that of the “wild grey mouse” produced by the union of an albino tame mouse and a piebald Japanese mouse[19]. These “reversionary” mice bred together produce the parental tame types, some other types, and “reversionary” mice again.
From what has been said it will now be clear that the applicability of the Mendelian hypothesis has, intrinsically, nothing whatever to do with the question of the inheritance being blended or alternative. In fact, as soon as the relation of zygote characters to gamete characters is appreciated, it is difficult to see any reason for supposing that the manifestation of characters seen in the zygotes should give any indication as to their mode of allotment among the gametes.
On a previous occasion I pointed out that the terms “Heredity” and “Inheritance” are founded on a misapplication of metaphor, and in the light of our present knowledge it is becoming clearer that the ideas of “transmission” of a character by parent to offspring, or of there being any “contribution” made by an ancestor to its posterity, must only be admitted under the strictest reserve, and merely as descriptive terms.
We are now presented with some entirely new conceptions:—