Mendel's principles of heredity: A defence

by William Bateson; Gregor Mendel

III. The Facts in regard to Dominance of Characters in Peas.

Professor Weldon refers to no experiments of his own and presumably has made none. Had he done so he would have learnt many things about dominance in peas, whether of the yellow cotyledon-colour or of the round form, that might have pointed him to caution.

In the year 1900 Messrs Vilmorin-Andrieux & Co. were kind enough to send to the Cambridge Botanic Garden on my behalf a set of samples of the varieties of Pisum and Phaseolus, an exhibit of which had greatly interested me at the Paris Exhibition of that year. In the past summer I grew a number of these and made some preliminary cross-fertilizations among them (about 80 being available for these deductions) with a view to a future study of certain problems, Mendelian and others. In this work I had the benefit of the assistance of Miss Killby of Newnham College. Her cultivations and crosses were made independently of my own, but our results are almost identical. The experience showed me, what a naturalist would expect and practical men know already, that a great deal turns on the variety used; that some varieties are very sensitive to conditions while others maintain their type sturdily; that in using certain varieties Mendel’s experience as to dominance is regularly fulfilled, while in the case of other varieties irregularities and even some contradictions occur. That the dominance of yellow cotyledon-colour over green, and the dominance of the smooth form over the wrinkled, is a general truth for Pisum sativum appears at once; that it is a universal truth I cannot believe any competent naturalist would imagine, still less assert. Mendel certainly never did. When he speaks of the “law” or “laws” that he has established for Pisum he is referring to his own discovery of the purity of the germ-cells, that of the statistical distribution of characters among them, and the statistical grouping of the different germ-cells in fertilization, and not to the “Law of Dominance” which he never drafted and does not propound.

The issue will be clearer if I here state briefly what, as far as my experience goes, are the facts in regard to the characters cotyledon-colour and seed-shapes in peas. I have not opportunity for more than a passing consideration of the seed-coats of pure forms[63]; that is a maternal character, a fact I am not sure Professor Weldon fully appreciates. Though that may be incredible, it is evident from many passages that he has not, in quoting authorities, considered the consequences of this circumstance.

The normal characters: colour of cotyledons and seed-coats.

Culinary peas (P. sativum, omitting purple sorts) can primarily be classified on colour into two groups, yellow and green. In the green certain pigmentary matters persist in the ripe seed which disappear or are decomposed in the yellow as the seed ripens. But it may be observed that the “green” class itself is treated as of two divisions, green and blue. In the seedsmen’s lists the classification is made on the external appearance of the seed, without regard to whether the colour is due to the seed-coat, the cotyledons, or both. As a rule perhaps yellow coats contain yellow cotyledons, and green coats green cotyledons, though yellow cotyledons in green coats are common, e.g. Gradus, of which the cotyledons are yellow while the seed-coats are about as often green as yellow (or “white,” as it is called technically). Those called “blue” consist mostly of seeds which have green cotyledons seen through transparent skins, or yellow cotyledons combined with green skins. The skins may be roughly classified into thin and transparent, or thick and generally at some stage pigmented. In numerous varieties the colour of the cotyledon is wholly yellow, or wholly green. Next there are many varieties which are constant in habit and other properties but have seeds belonging to these two colour categories in various proportions. How far these proportions are known to be constant I cannot ascertain.

Of such varieties showing mixture of cotyledon-colours nearly all can be described as dimorphic in colour. For example in Sutton’s Nonpareil Marrowfat the cotyledons are almost always either yellow or green, with some piebalds, and the colours of the seed-coats are scarcely less distinctly dimorphic. In some varieties which exist in both colours intermediates are so common that one cannot assert any regular dimorphism[64].

There are some varieties which have cotyledons green and intermediate shading to greenish yellow, like Stratagem quoted by Professor Weldon. Others have yellow and intermediate shading to yellowish green, such as McLean’s Best of all[65]‍. I am quite disposed to think there may be truly monomorphic varieties with cotyledons permanently of intermediate colour only, but so far I have not seen one[66]. The variety with greatest irregularity (apart from regular dimorphism) in cotyledon-colour I have seen is a sample of “mange-tout à rames, à grain vert,” but it was a good deal injured by weevils (Bruchus), which always cause irregularity or change of colour.

Lastly in some varieties there are many piebalds or mosaics.

From what has been said it will be evident that the description of a pea in an old book as having been green, blue, white, and so forth, unless the cotyledon-colour is distinguished from seed-coat colour, needs careful consideration before inferences are drawn from it.

Shape.

In regard to shape, if we keep to ordinary shelling peas, the facts are somewhat similar, but as shape is probably more sensitive to conditions than cotyledon-colour (not than seed-coat colour) there are irregularities to be perhaps ascribed to this cause. Broadly, however, there are two main divisions, round and wrinkled. It is unquestioned that between these two types every intermediate occurs. Here again a vast number of varieties can be at once classified into round and wrinkled (the classification commonly used), others are intermediate normally. Here also I suspect some fairly clear sub-divisions might be made in the wrinkled group and in the round group too, but I would not assert this as a fact.

I cannot ascertain from botanists what is the nature of the difference between round and wrinkled peas, though no doubt it will be easily discovered. In maize the round seeds contain much unconverted starch, while in the wrinkled or sugar-maize this seems to be converted in great measure as the seed ripens; with the result that, on drying, the walls collapse. In such seeds we may perhaps suppose that the process of conversion, which in round seeds takes place on germination, is begun earlier, and perhaps the variation essentially consists in the premature appearance of the converting ferment. It would be most rash to suggest that such a process may be operating in the pea, for the phenomenon may have many causes; but however that may be, there is evidently a difference of such a nature that when the water dries out of the seed on ripening, its walls collapse[67]; and this collapse may occur in varying degrees.

In respect of shape the seeds of a variety otherwise stable are as a rule fairly uniform, the co-existence of both shapes and of intermediates between them in the same variety is not infrequent. As Professor Weldon has said, Telephone is a good example of an extreme case of mixture of both colours and shapes. William I. is another. It may be mentioned that regular dimorphism in respect of shape is not so common as dimorphism in respect of colour. Of great numbers of varieties seen at Messrs Suttons’ I saw none so distinctly dimorphic in shape as William I. which nevertheless contains all grades commonly.

So far I have spoken of the shapes of ordinary English culinary peas. But if we extend our observations to the shapes of large-seeded peas, which occur for the most part among the sugar-peas (mange-touts), of the “grey” peas with coloured flowers, etc., there are fresh complications to be considered.

Professor Weldon does not wholly avoid these (as Mendel did in regard to shape) and we will follow him through his difficulties hereafter. For the present let me say that the classes round and wrinkled are not readily applicable to those other varieties and are not so applied either by Mendel or other practical writers on these subjects. To use the terms indicated in the Introduction, seed-shape depends on more than one pair of allelomorphs—possibly on several.

Stability and Variability.

Generally speaking peas which when seen in bulk are monomorphic in colour and shape, will give fairly true and uniform offspring (but such strict monomorphism is rather exceptional). Instances to the contrary occur, and in my own brief experience I have seen some. In a row of Fill-basket grown from selected seed there were two plants of different habit, seed-shape, etc. Each bore pods with seeds few though large and round. Again Blue Peter (blue and round) and Laxton’s Alpha (blue and wrinkled), grown in my garden and left to nature uncovered, have each given a considerable proportion of seeds with yellow cotyledons, about 20% in the case of Laxton’s Alpha. The distribution of these on the plants I cannot state. The plants bearing them in each case sprang from green-cotyledoned seeds taken from samples containing presumably unselected green seeds only. A part of this exceptional result may be due to crossing, but heterogeneity of conditions[68] especially in or after ripening is a more likely cause, hypotheses I hope to investigate next season. Hitherto I had supposed the crossing, if any, to be done by Bruchus or Thrips, but Tschermak also suspects Megachile, the leaf-cutter bee, which abounds in my garden.

Whatever the cause, these irregularities may undoubtedly occur; and if they be proved to be largely independent of crossing and conditions, this will in nowise vitiate the truth of the Mendelian principle. For in that case it may simply be variability. Such true variation, or sporting, in the pea is referred to by many observers. Upon this subject I have received most valuable facts from Mr Arthur Sutton, who has very kindly interested himself in these inquiries. He tells me that several highly bred varieties, selected with every possible care, commonly throw a small but constant proportion of poor and almost vetch-like plants, with short pods and small round seeds, which are hoed out by experienced men each year before ripening. Other high-class varieties always, wherever grown, and when far from other sorts, produce a small percentage of some one or more definite “sports.” Of these peculiar sports he has sent me a collection of twelve, taken from as many standard varieties, each “sport” being represented by eight seeds, which though quite distinct from the type agree with each other in almost all cases.

In two cases, he tells me, these seed-sports sown separately have been found to give plants identical with the standard type and must therefore be regarded as sports in seed characters only; in other cases change of plant-type is associated with the change of seed-type.

In most standard varieties these definite sports are not very common, but in a few they are common enough to require continual removal by selection[69].

I hope before long to be able to give statistical details and experiments relating to this extraordinarily interesting subject. As de Vries writes in his fine work Die Mutationstheorie (I. p. 580), “a study of the seed-differences of inconstant, or as they are called, ‘still’ unfixed varieties, is a perfect treasure-house of new discoveries.”

Let us consider briefly the possible significance of these facts in the light of Mendelian teaching. First, then, it is clear that as regards most of such cases the hypothesis is not excluded that these recurring sports may be due to the fortuitous concurrence of certain scarcer hypallelomorphs, which may either have been free in the original parent varieties from which the modern standard forms were raised, or may have been freed in the crossing to which the latter owe their origin (see p. [28]). This possibility raises the question whether, if we could make “pure cultures” of the gametes, any variations of this nature would ever occur. This may be regarded as an unwarrantable speculation, but it is not wholly unamenable to the test of experiments.

But variability, in the sense of division of gonads into heterogeneous gametes, may surely be due to causes other than crossing. This we cannot doubt. Cross-fertilization of the zygote producing those gametes is one of the causes of such heterogeneity among them. We cannot suppose it to be the sole cause of this phenomenon.

When Mendel asserts the purity of the germ-cells of cross-breds he cannot be understood to mean that they are more pure than those of the original parental races. These must have varied in the past. The wrinkled seed arose from the round, the green from the yellow (or vice versâ, if preferred), and probably numerous intermediate forms from both.

The variations, or as I provisionally conceive it, that differentiant division among the gametes of which variation (neglecting environment) is the visible expression, has arisen and can arise at one or more points of time, and we have no difficulty in believing it to occur now. In many cases we have clear evidence that it does. Crossing,—dare we call it asymmetrical fertilization?—is one of the causes of the production of heterogeneous gametes—the result of divisions qualitatively differentiant and perhaps asymmetrical[70].

There are other causes and we have to find them. Some years ago I wrote that consideration of the causes of variation was in my judgment premature[71]. Now that through Mendel’s work we are clearing our minds as to the fundamental nature of “gametic” variation, the time is approaching when an investigation of such causes may be not unfruitful.

Of variation as distinct from transmission why does Professor Weldon take no heed? He writes (p. 244):

“If Mendel’s statements were universally valid, even among Peas, the characters of the seeds in the numerous hybrid races now existing should fall into one or other of a few definite categories, which should not be connected by intermediate forms.”

Now, as I have already pointed out, Mendel made no pretence of universal statement: but had he done so, the conclusion, which Professor Weldon here suggests should follow from such a universal statement, is incorrectly drawn. Mendel is concerned with the laws of transmission of existing characters, not with variation, which he does not discuss.

Nevertheless Professor Weldon has some acquaintance with the general fact of variability in certain peas, which he mentions (p. 236), but the bearing of this fact on the difficulty he enuntiates escapes him.

Results of crossing in regard to seed characters: normal and exceptional.

The conditions being the same, the question of the characters of the cross-bred zygotes which we will call AB’s depends primarily on the specific nature of the varieties which are crossed to produce them. It is unnecessary to point out that if all AB’s are to look alike, both the varieties A and B must be pure—not in the common sense of descended, as far as can be traced, through individuals identical with themselves, but pure in the Mendelian sense, that is to say that each must be at that moment producing only homogeneous gametes bearing the same characters A and B respectively. Purity of pedigree in the breeder’s sense is a distinct matter altogether. The length of time—or if preferred—the number of generations through which a character of a variety has remained pure, alters the probability of its dominance, i.e. its appearance when a gamete bearing it meets another bearing an antagonistic character, no more, so far as we are yet aware, than the length of time a stable element has been isolated alters the properties of the chemical compound which may be prepared from it.

Now when individuals (bearing contrary characters), pure in the sense indicated, are crossed together, the question arises, What will be the appearance of the first cross individuals? Here again, generally speaking, when thoroughly green cotyledons are crossed with thoroughly yellow cotyledons, the first-cross seeds will have yellow cotyledons; when fully round peas are crossed with fully wrinkled the first result will generally speaking be round, often with slight pitting as Mendel has stated. This has been the usual experience of Correns, Tschermak, Mendel, and myself[72] and, as we shall see, the amount of clear and substantial evidence to the contrary is still exceedingly small. But as any experienced naturalist would venture to predict, there is no universal rule in the matter. As Professor Weldon himself declares, had there been such a universal rule it would surely have been notorious. He might further have reflected that in Mendel’s day, when hybridisation was not the terra incognita it has since become, the assertion of such universal propositions would have been peculiarly foolish. Mendel does not make it; but Professor Weldon perceiving the inherent improbability of the assertion conceives at once that Mendel must have made it, and if Mendel doesn’t say so in words then he must have implied it. As a matter of fact Mendel never treats dominance as more than an incident in his results, merely using it as a means to an end, and I see no reason to suppose he troubled to consider to what extent the phenomenon is or is not universal—a matter with which he had no concern.

Of course there may be exceptions. As yet we cannot detect the causes which control them, though injury, impurity, accidental crossing, mistakes of various kinds, account for many. Mendel himself says, for instance, that unhealthy or badly grown plants give uncertain results. Nevertheless there seems to be a true residuum of exceptions not to be explained away. I will recite some that I have seen. In my own crosses I have seen green × green give yellow four times. This I incline to attribute to conditions or other disturbance, for the natural pods of these plants gave several yellows. At Messrs Suttons’ I saw second-generation seeds got by allowing a cross of Sutton’s Centenary (gr. wr.) × Eclipse (gr. rd.) to go to seed; the resulting seeds were both green and yellow, wrinkled and round. But in looking at a sample of Eclipse I found a few yellow seeds, say two per cent., which may perhaps be the explanation. Green wrinkled × green round may give all wrinkled, and again wrinkled × wrinkled may give round[73]. Of this I saw a clear case—supposing no mistake to have occurred—at Messrs Suttons’. Lastly we have the fact that in exceptional cases crossing two forms—apparently pure in the strict sense—may give a mixture in the first generation. There are doubtless examples also of unlikeness between reciprocals, and of this too I have seen one putative case[74].

Such facts thus set out for the first cross-bred generation may without doubt be predicated for subsequent generations.

What then is the significance of the facts?

Analysis of exceptions.

Assuming that all these “contradictory” phenomena happened truly as alleged, and were not pathological or due to error—an explanation which seems quite inadequate—there are at least four possible accounts of such diverse results—each valid, without any appeal to ancestry.

1. That dominance may exceptionally fail—or in other words be created on the side which is elsewhere recessive. For this exceptional failure we have to seek exceptional causes. The artificial creation of dominance (in a character usually recessive) has not yet to my knowledge been demonstrated experimentally, but experiments are begun by which such evidence may conceivably be obtained.

2. There may be what is known to practical students of evolution as the false hybridism of Millardet, or in other words, fertilisation with—from unknown causes—transmission of none or of only some of the characters of one pure parent. The applicability of this hypothesis to the colours and shapes of peas is perhaps remote, but we may notice that it is one possible account of those rare cases where two pure forms give a mixed result in the first generation, even assuming the gametes of each pure parent to be truly monomorphic as regards the character they bear. The applicability of this suggestion can of course be tested by study of the subsequent generations, self-fertilised or fertilised by similar forms produced in the same way. In the case of a genuine false-hybrid the lost characters will not reappear in the posterity.

3. The result may not be a case of transmission at all as it is at present conceived, but of the creation on crossing of something new. Our AB’s may have one or more characters peculiar to themselves. We may in fact have made a distinct “mule” or heterozygote form. Where this is the case, there are several subordinate possibilities we need not at present pursue.

4. There may be definite variation (distinct from that proper to the “mule”) consequent on causes we cannot yet surmise (see pp. [125] and 128).

The above possibilities are I believe at the present time the only ones that need to be considered in connexion with these exceptional cases[75]. They are all of them capable of experimental test and in certain instances we are beginning to expect the conclusion.

The “mule” or heterozygote.

There can be little doubt that in many cases it is to the third category that the phenomena belong. An indication of the applicability of this reasoning will generally be found in the fact that in such “mule” forms the colour or the shape of the seeds will be recognizably peculiar and proper to the specimens themselves, as distinct from their parents, and we may safely anticipate that when those seeds are grown the plants will show some character which is recognizable as novel. The proof that the reasoning may apply can as yet only be got by finding that the forms in question cannot breed true even after successive selections, but constantly break up into the same series of forms[76].

This conception of the “mule” form, or “hybrid-character” as Mendel called it, though undeveloped, is perfectly clear in his work. He says that the dominant character may have two significations, it may be either a parental character or a hybrid-character, and it must be differentiated according as it appears in the one capacity or the other. He does not regard the character displayed by the hybrid, whether dominant or other, as a thing inherited from or transmitted by the pure parent at all, but as the peculiar function or property of the hybrid. When this conception has been fully understood and appreciated in all its bearings it will be found to be hardly less fruitful than that of the purity of the germ-cells.

The two parents are two—let us say—substances[77] represented by corresponding gametes. These gametes unite to form a new “substance”—the cross-bred zygote. This has its own properties and structure, just as a chemical compound has, and the properties of this new “substance” are not more strictly traceable to, or “inherited” from, those of the two parents than are those of a new chemical compound “inherited” from those of the component elements. If the case be one in which the gametes are pure, the new “substance” is not represented by them, but the compound is again dissociated into its components, each of which is separately represented by gametes.

The character of the cross-bred zygote may be anything. It may be something we have seen before in one or other of the parents, it may be intermediate between the two, or it may be something new. All these possibilities were known to Mendel and he is perfectly aware that his principle is equally applicable to all. The first case is his “dominance.” That he is ready for the second is sufficiently shown by his brief reference to time of flowering considered as a character (p. [65]). The hybrids, he says, flower at a time almost exactly intermediate between the flowering times of the parents, and he remarks that the development of the hybrids in this case probably happens in the same way as it does in the case of the other characters[78].

That he was thoroughly prepared for the third possibility appears constantly through the paper, notably in the argument based on the Phaseolus hybrids, and in the statement that the hybrid between talls and dwarfs is generally taller than the tall parent, having increased height as its “hybrid-character.”

All this Professor Weldon has missed. In place of it he offers us the sententia that no one can expect to understand these phenomena if he neglect ancestry. This is the idle gloss of the scribe, which, if we erase it not thoroughly, may pass into the text.

Enough has been said to show how greatly Mendel’s conception of heredity was in advance of those which pass current at the present day; I have here attempted the barest outline of the nature of the “hybrid-character,” and I have not sought to indicate the conclusions that we reach when the reasoning so clear in the case of the hybrid is applied to the pure forms and their own characters.

In these considerations we reach the very base on which all conceptions of heredity and variation must henceforth rest, and that it is now possible for us to attempt any such analysis is one of the most far-reaching consequences of Mendel’s principle. Till two years ago no one had made more than random soundings of this abyss.

I have briefly discussed these possibilities to assist the reader in getting an insight into Mendel’s conceptions. But in dealing with Professor Weldon we need not make this excursion; for his objection arising from the absence of uniform regularity in dominance is not in point.

The soundness of Mendel’s work and conclusions would be just as complete if dominance be found to fail often instead of rarely. For it is perfectly certain that varieties can be chosen in such a way that the dominance of one character over its antagonist is so regular a phenomenon that it can be used in the way Mendel indicates. He chose varieties, in fact, in which a known character was regularly dominant and it is because he did so that he made his discovery[79]. When Professor Weldon speaks of the existence of fluctuation and diversity in regard to dominance as proof of a “grave discrepancy” between Mendel’s facts and those of other observers[80], he merely indicates the point at which his own misconceptions began.

From Mendel’s style it may be inferred that if he had meant to state universal dominance in peas he would have done so in unequivocal language. Let me point out further that of the 34 varieties he collected for study, he discarded 12 as not amenable to his purposes[81]. He tells us he would have nothing to do with characters which were not sharp, but of a “more or less” description. As the 34 varieties are said to have all come true from seed, we may fairly suppose that the reason he discarded twelve was that they were unsuitable for his calculations, having either ill-defined and intermediate characters, or possibly defective and irregular dominance.