Part 8

See von Guaita, Ber. naturf. Ges. Freiburg X. 1898 and XI. 1899, quoted by Professor Weldon (see later).

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.

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We are now presented with some entirely new conceptions:--

(1) The purity of the gametes in regard to certain characters.

(2) The distinction of all zygotes according as they are or are not formed by the union of like or unlike gametes. In the former case, apart from Variation, they breed true when mated with their like; in the latter case their offspring, collectively, will be heterogeneous.

(3) If the zygote be formed by the union of dissimilar gametes, we may meet the phenomenon of (a) dominant and recessive characters; (b) a blend form; (c) a form distinct from either parent, often reversionary.

This fact sufficiently indicates the difficulties involved in a superficial treatment of the phenomenon of reversion. To call such reversions as those named above “returns to ancestral type” would be, if more than a descriptive phrase were intended, quite misleading. It is not the ancestral type that has come back, but something else has come in its guise, as the offspring presently prove. For the first time we thus begin to get a rationale of “reversion.”

But there are additional and even more significant deductions from the facts. We have seen that the gametes are differentiated in respect of pure characters. Of these pure characters there may conceivably be any number associated together in one organism. In the pea Mendel detected at least seven--not all seen by him combined in the same plant, but there is every likelihood that they are all capable of being thus combined.

Each such character, which is capable of being dissociated or replaced by its contrary, must henceforth be conceived of as a distinct unit-character; and as we know that the several unit-characters are of such a nature that any one of them is capable of independently displacing or being displaced by one or more alternative characters taken singly, we may recognize this fact by naming such unit-characters allelomorphs. So far, we know very little of any allelomorphs existing otherwise than as pairs of contraries, but this is probably merely due to experimental limitations and the rudimentary state of our knowledge.

In one case (combs of fowls) we know three characters, pea comb, rose comb and single comb; of which pea and single, or rose and single, behave towards each other as a pair of allelomorphs, but of the behaviour of pea and rose towards each other we know as yet nothing.

We have no reason as yet for affirming that any phenomenon properly described as displacement of one allelomorph by another occurs, though the metaphor may be a useful one. In all cases where dominance has been perceived, we can affirm that the members of the allelomorphic pair stand to each other in a relation the nature of which we are as yet wholly unable to apprehend or illustrate.

To the new conceptions already enumerated we may therefore add

(4) Unit-characters of which some, when once arisen by Variation, are alternative to each other in the constitution of the gametes, according to a definite system.

From the relations subsisting between these characters, it follows that as each zygotic union of allelomorphs is resolved on the formation of the gametes, no zygote can give rise to gametes collectively representing more than two characters allelomorphic to each other, apart from new variation.

From the fact of the existence of the interchangeable characters we must, for purposes of treatment, and to complete the possibilities, necessarily form the conception of an irresoluble base, though whether such a conception has any objective reality we have no means as yet of determining.

We have now seen that when the varieties A and B are crossed together, the heterozygote, AB, produces gametes bearing the pure A character and the pure B character. In such a case we speak of such characters as simple allelomorphs. In many cases however a more complex phenomenon happens. The character brought in on fertilisation by one or other parent may be of such a nature that when the zygote, AB, forms its gametes, these are not individually bearers merely of A and B, but of a number of characters themselves again integral, which in, say A, behaved as one character so long as its gametes united in fertilisation with others like themselves, but on cross-fertilisation are resolved and redistributed among the gametes produced by the cross-bred zygote.

In such a case we call the character A a compound allelomorph, and we can speak of the integral characters which constitute it as hypallelomorphs. We ought to write the heterozygote (A A′ A″ ...) B and the gametes produced by it may be of the form A, A′, A″, A‴,... B. Or the resolution may be incomplete in various degrees, as we already suspect from certain instances; in which case we may have gametes A, A′ A″, A‴ A″″, A′ A″ A^v,... B, and so on. Each of these may meet a similar or a dissimilar gamete in fertilisation, forming either a homozygote, or a heterozygote with its distinct properties.

In the case of compound allelomorphs we know as yet nothing of the statistical relations of the several gametes.

Thus we have the conception

(5) of a Compound character, borne by one gamete, transmitted entire as a single character so long as fertilisation only occurs between like gametes, or is, in other words, “symmetrical,” but if fertilisation take place with a dissimilar gamete (or possibly by other causes), resolved into integral constituent characters, each separately transmissible.

Next, as, by the union of the gametes bearing the various hypallelomorphs with other such gametes, or with gametes bearing simple allelomorphs, in fertilisation, a number of new zygotes will be formed, such as may not have been seen before in the breed: these will inevitably be spoken of as varieties; and it is difficult not to extend the idea of variation to them. To distinguish these from other variations--which there must surely be--we may call them

(6) Analytical variations in contradistinction to

(7) Synthetical variations, occurring not by the separation of pre-existing constituent-characters but by the addition of new characters.

Lastly, it is impossible to be presented with the fact that in Mendelian cases the cross-bred produces on an average equal numbers of gametes of each kind, that is to say, a symmetrical result, without suspecting that this fact must correspond with some symmetrical figure of distribution of those gametes in the cell-divisions by which they are produced.

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At the present time these are the main conceptions--though by no means all--arising directly from Mendel’s work. The first six are all more or less clearly embodied by him, though not in every case developed in accordance with modern knowledge. The seventh is not a Mendelian conception, but the facts before us justify its inclusion in the above list though for the present it is little more than a mere surmise.