CHAPTER V

INTERACTION OF FACTORS

We have now reached a point at which it is possible to formulate a definite conception of the living organism. A plant or animal is a living entity whose properties may in large measure be expressed in terms of unit-characters, and it is the possession of a greater or lesser number of such unit-characters renders it possible for us to draw sharp distinctions between one individual and another. These unit-characters are represented by definite factors in the gamete which in the process of heredity behave as indivisible entities, and are distributed according to a definite scheme. The factor for this or that unit-character is either present in the gamete or it is not present. It must be there in its entirety or completely absent. Such at any rate is the view to which recent experiment has led us. But as to the nature of these factors, the conditions under which they exist in the gamete, and the manner in which they produce their specific effects in the zygote, we are at present almost completely in the dark.

The case of the fowls' combs opens up the important question of the extent to which the various factors can influence one another in the zygote. The rose and the pea factors are separate entities, and each when present alone produces a perfectly distinct and characteristic effect upon the single comb, turning it into a rose or a pea as the case may be. But when both are present in the same zygote their combined effect is to produce the walnut comb, a comb which is quite distinct from either and in no sense intermediate between them. The question of the influence of factors upon one another did not present itself to Mendel because he worked with characters which affected different parts of the plant. It was unlikely that the factor which led to the production of colour in the flower would affect the shape of the pod, or that the height of the plant would be influenced by the presence or absence of the factor that determined the shape of the ripe seed. But when several factors can modify the same structure it is reasonable to suppose that they will influence one another in the effects which their simultaneous presence has upon the zygote. By themselves the pea and the rose factors each produce a definite modification of the single comb, but when both are present in the zygote, whether as a single or double dose, the modification that results is quite different to that produced by either when present alone. Thus we are led to the conception of characters which depend for their manifestation on more than one factor in the zygote, and in the present chapter we may consider a few of the phenomena which result from such interaction between separate and distinct factors.

One of the most interesting and instructive cases in which the interaction between separate factors has been demonstrated is a case in the sweet pea. All white sweet peas breed true to whiteness. And generally speaking the result of crossing different whites is to produce nothing but whites, whether in F₁ or in succeeding generations. But there are certain strains of white sweet peas which when crossed together produce only coloured flowers. The colour may be different in different cases, though for our present purpose we may take a case in which the colour is red. When such reds are allowed to self-fertilise themselves in the normal way and the seeds sown, the resulting F₂ generation consists of reds and whites, the former being rather more numerous than the latter in the proportion of 9 : 7. The raising of a further generation from the seeds of these F₂ plants shows that the whites always breed true to whiteness, but that different reds may behave differently. Some breed true, others give reds and whites in the ratio 3 : 1, while others, again, give reds and whites in the ratio 9 : 7. As in the case of the fowls' combs, this case may be interpreted in terms of the presence and absence of two factors. Factors in red and white sweet peas.Red in the sweet pea results from the interaction of two factors, and unless these are both present the red colour cannot appear. Each of the white parents carried one of the two factors whose interaction is necessary for the production of the red colour, and as a cross between them brings these two complementary factors together the F₁ plants must all be red. As this case is of considerable importance for the proper understanding of much that is to follow, and as it has been completely worked out, we shall consider it in some detail. Denoting these two colour factors by A and B respectively we may proceed to follow out the consequences of this cross. Since all the F₁ plants were red the constitution of the parental whites must have been AAbb and aaBB respectively, and their gametes consequently Ab and aB. The constitution of the F₁ plants must, therefore, be AaBb. Such a plant being heterozygous for two factors produces a series of gametes of the four kinds AB, Ab, aB, ab, and produces them in equal numbers (cf. p. 36). To obtain the various types of zygotes which are produced when such Fig. 7. Scheme of inheritance for red and white sweet peas.Fig. 7.
Diagram to illustrate the nature of the F₂ generation from the two white sweet peas which give a coloured F₁.a series of pollen grains meets a similar series of ovules we may make use of the same "chessboard" system which we have already adopted in the case of the fowls' combs. An examination of this figure (Fig. 7) shows that 9 out of the 16 squares contain both A and B, while 7 contain either A or B alone, or neither. In other words, on this view of the nature of the two white sweet peas we should in the F₂ generation look for the appearance of coloured and white flowers in the ratio 9 : 7. And this, as we have already seen, is what was actually found by experiment. Further examination of the figure shows that the coloured plants are not all of the same constitution, but are of four kinds with respect to their zygotic constitution, viz. AABB, AABb, AaBB, and AaBb. Since AABB is homozygous for both A and B, all the gametes which it produces must contain both of these factors, and such a plant must therefore breed true to the red colour. A plant of the constitution AABb is homozygous for the factor A, but heterozygous for B. All of its gametes will contain A, but only one-half of them will contain B, i.e. it produces equal numbers of gametes AB and Ab. Two such series of gametes coming together must give a generation consisting of x AABB, 2x AABb, and x AAbb, that is, reds and whites in the ratio 3 : 1. Lastly the red zygotes of the constitution AaBb have the same constitution as the original red made from the two whites, and must therefore when bred from give reds and whites in the ratio 9 : 7. The existence of all these three sorts of reds was demonstrated by experiment, and the proportions in which they were met with tallied with the theoretical explanation.

The theory was further tested by an examination into the properties of the various F₂ whites which come from a coloured plant that has itself been produced by the mating of two whites. As Fig. 7 shows, these are, in respect of their constitution, of five different kinds, viz. AAbb, Aabb, aaBB, aaBb, and aabb. Since none of them produce anything but whites on self-fertilisation it was found necessary to test their properties in another way, and the method adopted was that of crossing them together. It is obvious that when this is done we should expect different results in different cases. Thus the cross between two whites of the constitution AAbb and aaBB should give nothing but coloured plants; for these two whites are of the same constitution as the original two whites from which the experiment started. On the other hand, the cross between a white of the constitution aabb and any other white can never give anything but whites. For no white contains both A and B, or it would not be white, and a plant of the constitution aabb cannot supply the complementary factor necessary for the production of colour. Again, two whites of the constitution Aabb and aaBb when crossed should give both coloured and white flowers, the latter being three times as numerous as the former. Without going into further detail it may be stated that the results of a long series of crosses between the various F₂ whites accorded closely with the theoretical explanation.

From the evidence afforded by this exhaustive set of experiments it is impossible to resist the deduction that the appearance of colour in the sweet pea depends upon the interaction of two factors which are independently transmitted according to the ordinary scheme of Mendelian inheritance. What these factors are is still an open question. Recent evidence of a chemical nature indicates that colour in a flower is due to the interaction of two definitive substances: (1) a colourless "chromogen," or colour basis; and (2) a ferment which behaves as an activator of the chromogen, and by inducing some process of oxidation, leads to the formation of a coloured substance. But whether these two bodies exist as such in the gametes or whether in some other form we have as yet no means of deciding.

Since the elucidation of the nature of colour in the sweet pea phenomena of a similar kind have been witnessed in other plants, notably in stocks, snapdragons, and orchids. Nor is this class of phenomena confined to plants. In the course of a series of experiments upon the plumage colour of poultry, indications were obtained that different white breeds did not always owe their whiteness to the same cause. Crosses were accordingly made between the white Silky fowl and a pure white strain derived from the white Dorking. Each of these had been previously shown to behave as a simple recessive to colour. When the two were crossed only fully coloured birds resulted. From analogy with the case of the sweet pea it was anticipated that such F₁ coloured birds when bred together would produce an F₂ generation consisting of coloured and white birds in the ratio 9 : 7, and when the experiment was made this was actually shown to be the case. With the growth of knowledge it is probable that further striking parallels of this nature between the plant and animal worlds will be met with.

Before quitting the subject of these experiments attention may be drawn to the fact that the 9 : 7 ratio is in reality a 9 : 3 : 3 : 1 ratio in which the last three terms are indistinguishable owing to the special circumstances that neither factor can produce a visible effect without the co-operation of the other. And we may further emphasise the fact that although the two factors thus interact upon one another they are nevertheless transmitted quite independently and in accordance with the ordinary Mendelian scheme.

One of the earliest sets of experiments demonstrating the interaction of separate factors was that made by the French zoologist Cuénot on the coat colours of mice. It was shown that in certain cases agouti, which is the colour of the ordinary wild grey mouse, behaves as a dominant to the albino variety, i.e. the F₂ generation from such a cross consists of agoutis and albinos in the ratio 3 : 1. But in other cases the cross between albino and agouti gave a different result. In the F₁ generation appeared only agoutis as before, but the F₂ generation consisted of three distinct types, viz. agoutis, albinos, and blacks. Whence the sudden appearance of the new type? The answer is a simple one. The albino parent was really a black. But it lacked the factor without which the colour is unable to develop, and consequently it remained an albino. If we denote this factor by C, then the constitution of an albino must be cc, while that of a coloured animal may be CC or Cc, according as to whether it breeds true to colour or can throw albinos. Agouti was previously known to be a simple dominant to black, i.e. an agouti is a black rabbit plus an additional greying factor which modifies the black into agouti. This factor we will denote by G, and we will use B for the black factor. Our original agouti and albino parents we may therefore regard as in constitution GGCCBB and ggccBB respectively. Both of the parents are homozygous for black. The gametes produced by the two parents are GCB, and gcB, and the constitution of the F₁ animals must be GgCcBB. Being heterozygous for two factors they will produce four kinds of gametes in equal numbers, viz. GCB, GcB, gCB, and gcB. The results of the mating of two such similar series of gametes when the F₁ animals are bred together we may determine by the usual "chessboard" method (Fig. 8). Out of the 16 squares 9 contain both C and G in addition to B. Such animals must be agoutis. Three squares contain C but not G. Such animals must be coloured, but as they do not contain the modifying agouti factor their colour will be black. The remaining four squares do not contain C, and in the absence of this colour-developing factor they must all be albinos. Theory demands that the three classes agouti, black, and albino should appear in F₂ in the ratio 9 : 3 : 4; experiment has shown that these are the only classes that appear, and that the proportions in which they are produced accord closely with the theoretical expectation. Put briefly, then, the explanation Fig. 8. Scheme of inheritance for agouti and black mice.Fig. 8.
Diagram to illustrate the nature of the F₂ generation which may arise from the mating of agouti with albino in mice or rabbits.of this case is that all the animals are black, and that we are dealing with the presence and absence of two factors, a colour developer (C), and a colour modifier (G), both acting, as it were, upon a substratum of black. The F₂ generation really consists of the four classes agoutis, blacks, albino agoutis, and albino blacks in the ratio 9 : 3 : 3 : 1. But since in the absence of the colour developer C the colour modifier G can produce no visible result, the last two classes of the ratio are indistinguishable, and our F₂ generation comes to consist of three classes in the ratio 9 : 3 : 4, instead of four classes in the ratio 9 : 3 : 3 : 1. This explanation was further tested by experiments with the albinos. In an F₂ family of this nature there ought to be three kinds, viz. albinos homozygous for G (GGccBB), albinos heterozygous for G (GgccBB), and albinos without G (ggccBB). These albinos are, as it were, like photographic plates exposed but undeveloped. Their potentialities may be quite different, although they all look alike, but this can only be tested by treating them with a colour developer. In the case of the mice and rabbits the potentiality for which we wish to test is the presence or absence of the factor G, and in order to develop the colour we must introduce the factor C. Our developer, therefore, must contain C but not G. In other words, it must be a homozygous black mouse or rabbit, ggCCBB. Since such an animal is pure for C it must, when mated with any of the albinos, produce only coloured offspring. And since it does not contain G the appearance of agoutis among its offspring must be attributed to the presence of G in the albino. Tested in this way the F₂ albinos were proved, as was expected, to be of three kinds: (1) those which gave only agouti, i.e. which were homozygous for G; (2) those which gave agoutis and blacks in approximately equal numbers, i.e. which were heterozygous for G; and (3) those which gave only blacks, and therefore did not contain G.

Though albinos, whether mice, rabbits, rats, or other animals, breed true to albinism, and though albinism behaves as a simple recessive to colour, yet albinos may be of many different sorts. There are in fact just as many kinds of albinos as there are coloured forms—neither more nor less. And all these different kinds of albinos may breed together, transmitting the various colour factors according to the Mendelian scheme of inheritance, and yet the visible result will be nothing but albinos. Under the mask of albinism is all the while occurring that segregation of the different colour factors which would result in all the varieties of coloured forms, if only the essential factor for colour development were present. But put in the developer by crossing with a pure coloured form and their variety of constitution can then at last become manifest.

So far we have dealt with cases in which the production of a character is dependent upon the interaction of two factors. But it may be that some characters require the simultaneous presence of a greater number of factors for their manifestation, and the experiments of Miss Saunders have shown that there is a character in stocks which is unable to appear except through the interaction of three distinct factors. Coloured stocks may be either hoary, with the leaves and stem covered by small hairs, or they may lack the hairy covering, in which case they are termed glabrous. Hoariness is dominant to glabrousness; that is to say, there is a definite factor which can turn the glabrous into a hoary plant when it is present. But in families where coloured and white stocks occur the white are always glabrous, while the coloured plants may or may not be hoary. Now colour in the stock as in the sweet pea has been proved to be dependent upon the interaction of two separate factors. Hence hoariness depends upon three separate factors, and a stock cannot be hoary unless it contains the hoary factor in addition to the two colour factors. It requires the presence of all these three factors to produce the hoary character, though how this comes about we have not at present the least idea.

A somewhat different and less usual form of interaction between factors may be illustrated by a case in primulas recently worked out by Bateson and Gregory. Like the common primrose, the primula exhibits both pin-eyed and thrum-eyed varieties. In the former the style is long, and the centre of the eye is formed by the end of the stigma which more or less plugs up the opening of the corolla (cf. Fig. 9, A); in the latter the style is short and hidden by the four anthers which spring from higher up in the corolla and form the centre of the eye (cf. Fig. 9, B). The greater part of the "eye" is formed by the greenish-yellow patches on each petal just at the opening of the corolla. In most primulas the eye is small, but there are some in which it is large and extends as a flush over a considerable part of the petals (Fig. 10). Experiments showed that these two pairs of characters behave in simple Mendelian fashion, short style ( = "thrum") being dominant to long style (= "pin") and small eye dominant to large. Besides the normal long and short styled forms, there occurs a third form, which has been termed homostyle. In this form the anthers are placed low down in the corolla tube as they are in the long-styled form, but the style remains short instead of reaching up to the corolla opening (Fig. 9, C). In the course of their experiments Bateson and Gregory crossed a large-eyed homostyle plant with a small-eyed thrum ( = short style). The F₁ plants were all short styled with small eyes. Generations of primulas.On self-fertilisation these gave an F₂ generation consisting of four types, viz. short styled with small eyes, short styled with large eyes, long styled with small eyes, and homostyled with large eyes. The notable feature of this generation is the appearance of long-styled plants, which, however, occur only in association with the small eye. The proportions in which these four types appeared shows that the presence or absence of but two factors is concerned, and at the same time provides the key to the nature of the homostyled plants. These are potentially long styled, and the position of the anthers is that of normal long-styled plants, but owing to some interaction between the factors the style itself is unable to reach its full development unless the factor for the small eye is present. For this reason long-styled plants with the large eye are always of the homostyle form. What the connecting-link between these apparently unrelated structures may be we cannot yet picture to ourselves, any more than we can picture the relation between flower colour and hairiness in stocks. It is evident, however, that the conception of the interaction of factors, besides clearing up much that is paradoxical in heredity, promises to indicate lines of research which may lead to valuable extensions in our knowledge of the way in which the various parts of the living organism are related to one another.