Part 2
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 {46} 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.
+------+------+------+------+
AB....
AB....
AB....
AB....
AB....
Ab....
aB....
ab....
......
......
......
......
+------+------+------+------+
Ab....
Ab
Ab....
Ab
AB....
Ab
aB....
ab
......
......
+------+------+------+------+
aB....
aB....
aB
aB
AB....
Ab....
aB
ab
......
......
+------+------+------+------+
ab....
ab
ab
ab
AB....
Ab
aB
ab
......
+------+------+------+------+
FIG. 7.
Diagram to ill.u.s.trate the nature of the F_2 generation from the two white sweet peas which give a coloured F_1.
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 F_2 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 const.i.tution, but are of four kinds with respect to their zygotic const.i.tution, viz. AABB, AABb, AaBB, and AaBb. Since AABB is h.o.m.ozygous 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 {47} const.i.tution AABb is h.o.m.ozygous 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 const.i.tution AaBb have the same const.i.tution 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 F_2 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 const.i.tution, 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 const.i.tution AAbb and aaBB should give nothing but coloured plants; for these two whites are of {48} the same const.i.tution as the original two whites from which the experiment started. On the other hand, the cross between a white of the const.i.tution 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 const.i.tution aabb cannot supply the complementary factor necessary for the production of colour. Again, two whites of the const.i.tution 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 F_2 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 {49} 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 cla.s.s 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 a.n.a.logy with the case of the sweet pea it was antic.i.p.ated that such F_1 coloured birds when bred together would produce an F_2 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 circ.u.mstances that neither factor can produce a visible effect without {50} 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.
Agouti Agouti
+---------------+ Agouti Agouti
+---------+---------+ Agouti Black Albino (9) (3) (4)
One of the earliest sets of experiments demonstrating the interaction of separate factors was that made by the French zoologist Cuenot 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 F_2 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 F_1 generation appeared only agoutis as before, but the F_2 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 const.i.tution 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 {51} 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 const.i.tution GGCCBB and ggccBB respectively. Both of the parents are h.o.m.ozygous for black. The gametes produced by the two parents are GCB, and gcB, and the const.i.tution of the F_1 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 F_1 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 cla.s.ses agouti, black, and albino should appear in F_2 in the ratio 9 : 3 : 4; experiment has shown that these are the only cla.s.ses that appear, and that the proportions in which they are produced accord closely with the theoretical expectation. Put briefly, then, the explanation {52} 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 F_2 generation really consists of the four cla.s.ses 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 cla.s.ses of the ratio are indistinguishable, and our F_2 generation comes to consist of three cla.s.ses in the ratio 9 : 3 : 4, instead of four cla.s.ses in the ratio 9 : 3 : 3 : 1.
+-------+-------+-------+-------+
GCB....
GCB....
GCB....
GCB....
GCB....
GcB....
gCB....
gcB....
.......
.......
.......
.......
.Agouti
.Agouti
.Agouti
.Agouti
+-------+-------+-------+-------+
GcB....
GcB
GcB....
GcB
GCB....
GcB
gCB....
gcB
.......
.......
.Agouti
Albino
.Agouti
Albino
+-------+-------+-------+-------+
gCB....
gCB....
gCB####
gCB####
GCB....
GcB....
gCB####
gcB####
.......
.......
#######
#######
.Agouti
.Agouti
##BLACK
##BLACK
+-------+-------+-------+-------+
gcB....
gcB
gcB####
gcB
GCB....
GcB
gCB####
gcB
.......
#######
.Agouti
Albino
##BLACK
Albino
+-------+-------+-------+-------+
FIG. 8.
Diagram to ill.u.s.trate the nature of the F_2 generation which may arise from the mating of agouti with albino in mice or rabbits.
This explanation was further tested by experiments with the albinos. In an F_2 family of this nature there ought to be three kinds, viz. albinos h.o.m.ozygous 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. {53} 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 h.o.m.ozygous 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 F_2 albinos were proved, as was expected, to be of three kinds: (1) those which gave only agouti, _i.e._ which were h.o.m.ozygous 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, {54} 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 const.i.tution 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 h.o.a.ry, with the leaves and stem covered by small hairs, or they may lack the hairy covering, in which case they are termed glabrous. h.o.a.riness is dominant to glabrousness; that is to say, there is a definite factor which can turn the glabrous into a h.o.a.ry 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 h.o.a.ry. Now colour in the stock as in the sweet pea has been proved to be dependent upon the interaction of two separate factors. Hence h.o.a.riness depends upon three separate factors, and a stock cannot be h.o.a.ry unless {55} it contains the h.o.a.ry factor in addition to the two colour factors. It requires the presence of all these three factors to produce the h.o.a.ry character, though how this comes about we have not at present the least idea.
[Ill.u.s.tration: FIG. 9.
Sections of primula flowers. The anthers are shown as black. A, "pin" form with long style and anthers set low down; B, "thrum" form with short style and anthers set higher up; C, h.o.m.ostyle form with anthers set low down as in "pin," but with short style. This form only occurs with the large eye.]
[Ill.u.s.tration: FIG. 10.
Two primula flowers showing the extent of the small and of the large eye.]
A somewhat different and less usual form of interaction between factors may be ill.u.s.trated 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 {56} 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 h.o.m.ostyle. 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 h.o.m.ostyle plant with a small-eyed thrum ( = short style). The F_1 plants were all short styled with small eyes. {57} On self-fertilisation these gave an F_2 generation consisting of four types, viz. short styled with small eyes, short styled with large eyes, _long styled_ with small eyes, and _h.o.m.ostyled_ with large eyes. The notable feature of this generation is the appearance of long-styled plants, which, however, occur only in a.s.sociation 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 h.o.m.ostyled 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 h.o.m.ostyle 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 {58} 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.
Short style } { h.o.m.o style small eye } { large eye
Short style small eye
+-------------+----------+-----------+ Short style Short style Long style h.o.m.o style small eye large eye ("pin") large eye (9) (3) (3) (1)
{59}
CHAPTER VI
REVERSION
As soon as the idea was grasped that characters in plants and animals might be due to the interaction of complementary factors, it became evident that this threw clear light upon the hitherto puzzling phenomenon of reversion.
We have already seen that in certain cases the cross between a black mouse or rabbit and an albino, each belonging to true breeding strains, might produce nothing but agoutis. In other words, the cross between the black and the white in certain instances results in a complete reversion to the wild grey form. Expressed in Mendelian terms, the production of the agouti was the necessary consequence of the meeting of the factors C and G in the same zygote. As soon as they are brought together, no matter in what way, the reversion is bound to occur. Reversion, therefore, in such cases we may regard as the bringing together of complementary factors which had somehow in the course of evolution become separated from one another. In the simplest cases, such as that of the black and the white rabbit, only two factors are concerned, and one of them is brought in from each of the {60} two parents. But in other cases the nature of the reversion may be more complicated owing to a larger number of factors being concerned, though the general principle remains the same. Careful breeding from the reversions will enable us in each case to determine the number and nature of the factors concerned, and in ill.u.s.tration of this we may take another example from rabbits. The Himalayan rabbit is a well-known breed. In appearance it is a white rabbit with pink eyes, but the ears, paws, and nose are black (Pl. I., 2). The Dutch rabbit is another well-known breed. Generally speaking, the anterior portion of the body is white, and the posterior part coloured. Anteriorly, however, the eyes are surrounded by coloured patches extending up to the ears, which are entirely coloured. At the same time the hind paws are white (cf. Pl. I., 1). Dutch rabbits exist in many varieties of colour, though in each one of these the distribution of colour and white shows the same relations. In the experiments about to be described a yellow Dutch rabbit was crossed with a Himalaya. The result was a reversion to the wild agouti colour (Pl. I., 3). Some of the F_1 individuals showed white patches, while others were self-coloured. On breeding from the F_1 animals a series of coloured forms resulted in F_2. These were agoutis, blacks, yellows, and sooty yellows, the so-called tortoise sh.e.l.ls of the fancy (Pl.
I., 4-7).
[Ill.u.s.tration: PLATE I.
1, Yellow Dutch Rabbit; 2, Himalayan; 3, Agouti ( = grey) F_1 reversion; 4-8, F_2 types, viz.: 4, Agouti; 5, Yellow; 6, Black; 7, Tortoisesh.e.l.l; 8, Himalayan.]
{61}
Yellow Himalayan
+-------------+ Agouti Agouti
+--------+------+-------+----------+ Agouti Yellow Black Tortoise Himalayan Sh.e.l.l (27) (9) (9) (3) (16)
In addition to these appeared Himalayans with either black points or with lighter brownish ones, and the proportions in which they came showed the Himalayan character to be a simple recessive. A certain number of the coloured forms exhibited the Dutch marking to a greater or less extent, but as its inheritance in this set of experiments is complicated and has not yet been worked out, we may for the present neglect it and confine our attention to the coloured types and to the Himalayans. The proportion in which the four coloured types appeared in F_2 was very nearly 9 agoutis, 3 blacks, 3 yellows, and 1 tortoisesh.e.l.l. Evidently we are here dealing with two factors: (1) the grey factor (G), which modifies black into agouti, or tortoisesh.e.l.l into yellow; and (2) an intensifying factor (I), which intensifies yellow into agouti and tortoisesh.e.l.l into black. It may be mentioned here that other experiments confirmed the view that the yellow rabbit is a dilute agouti, and the tortoisesh.e.l.l a dilute black. The Himalayan pattern behaves as a recessive to self-colour. It is a self-coloured black rabbit lacking a factor that allows the colour to develop except in the points. That factor we may denote {62} by X, and as far as it is concerned the Himalayan is const.i.tutionally xx. The Himalayan contains the intensifying factor, for such pigment as it possesses in the points is full coloured. At the same time it is black, _i.e._ lacking in the factor G. With regard to these three factors, therefore, the const.i.tution of the Himalayan is ggIIxx. The last character which we have to consider in this cross is the Dutch character. This was found by Hurst to behave as a recessive to self-colour (S), and for our present purpose we will regard it as differing from a self-coloured rabbit in the lack of this factor.[3] The Himalayan is really a self-coloured animal, which, however, is unable to show itself as a full black owing to its not possessing the factor X. The results of breeding experiments then suggest that we may denote the Himalayan by the formula ggIIxxSS and the yellow Dutch by GGiiXXss. Each lacks two of the factors upon the full complement of which the agouti colour depends. By crossing them the complete series GIXS is brought into the same zygote, and the result is a reversion to the colour of the wild rabbit.
Bush Cupid
Tall -------------------------- F_1
+----------+---+------+----------+ Tall Bush Cupid Cupid -------- F_2 (proc.u.mbent) (erect)
Most of the instances of reversion yet worked out are those in which colour characters are concerned. The sweet pea, however, supplies us with a good example of reversion in structural characters. A dwarf variety known as the "Cupid" has been extensively grown for {63} some years. In these little plants the internodes are very short and the stems are few in number, and attain to a length of only 9-10 inches. In course of growth they diverge from one another, and come to lie prostrate on the ground (Pl. II., 2).
Curiously enough, although the whole plant is dwarfed in other respects, this does not seem to affect the size of the flower, which is that of a normal sweet pea. Another though less-known variety is the "Bush" sweet pea. Its name is derived from its habit of growth. The numerous stems do not diverge from one another, but all grow up side by side, giving the plant the appearance of a compact bush (Pl. II., 1). Under ordinary conditions it attains a height of 3-4 feet. A number of crosses were made between the Bush and Cupid varieties, with the somewhat unexpected result that in every instance the F_1 plants showed complete reversion to the size and habit of the ordinary tall sweet pea (Pl. II., 3), which is the form of the wild plant as it occurs in Sicily to-day. The F_2 generation from these reversionary talls consisted of four different types, viz. {64} talls, bushes, Cupids of the proc.u.mbent type like the original Cupid parent, and Cupids with the compact upright Bush habit (Pl. II., 4). These four types appeared in the ratio 9 : 3 : 3 : 1, and this, of course, provided the clue to the nature of the case. The characters concerned are (1) long internode of stem between the leaves which is dominant to short internode, and (2) the creeping proc.u.mbent habit which is dominant to the erect bush-like habit. Of these characters length of internode was carried by the Bush, and the proc.u.mbent habit by the original Cupid parent. The bringing of them together by the cross resulted in a proc.u.mbent plant with long internodes.
This is the ordinary tall sweet pea of the wild Sicilian type, reversion here, again, being due to the bringing together of two complementary factors which had somehow become separated in the course of evolution.
To this interpretation it may be objected that the ordinary sweet pea is a plant of upright habit. This, however, is not true. It only appears so because the conventional way of growing it is to train it up sticks. In reality it is of proc.u.mbent habit, with divergent stems like the ordinary Cupid, a fact which can easily be observed by anyone who will watch them grow without the artificial aid of prepared supports.
[Ill.u.s.tration: PLATE II.
1, Bush Sweet Pea; 2, Cupid Sweet Pea; 3, F_1 reversionary Tall; 4, Erect Cupid Sweet Pea; 5, Purple Invincible; 6, Painted Lady; 7, Duke of Westminster (hooded standard).]
{65}
The cases of reversion with which we have so far dealt have been cases in which the reversion occurs as an immediate result of a cross, _i.e._ in the F_1 generation. This is perhaps the commonest mode of reversion, but instances are known in which the reversion that occurs when two pure types are crossed does not appear until the F_2 generation. Such a case we have already met with in the fowls' combs. It will be remembered that the cross between pure pea and pure rose gave walnut combs in F_1, while in the F_2 generation a definite proportion, 1 in 16, of single combs appeared (cf. p.
32). Now the single comb is the form that is found in the wild jungle fowl, which is generally regarded as the ancestor of the domestic breeds. If this is so, we have a case of reversion in F_2; and this in the _absence_ of the two factors brought together by the rose-comb and pea-comb parents. Instead of the reversion being due to the bringing together of two complementary factors, we must regard it here as due to the a.s.sociation of two complementary absences. To this question, however, we shall revert later in discussing the origin of domesticated varieties.
Black Barb White Fantail Black Barb Spot[4]
Dark Dark Among the offspring one very similar to the wild blue rock.
Black White Barb Fantail
+------------------------+ Black Black (White Splashed)
(White Splashed)
+--------+--------+---------+-----------+ Black Black Blue Blue White (White Splashed) (White Splashed) --------------/ -------------/ (9) (3) (4)
There is one other instance of reversion to which we must allude. This is Darwin's famous case of the occasional appearance of pigeons reverting to the wild blue rock (_Columba livia_), when certain domesticated races are crossed together. As is well known, Darwin made use of this as an argument for regarding all the domesticated varieties as having arisen from the same wild species. The original experiment is somewhat complicated, and is shown in the accompanying scheme. Essentially it lay in {66} following the results flowing from crosses between blacks and whites. Experiments recently made by Staples-Browne have shown that this case of reversion also can be readily interpreted in Mendelian terms. In these experiments the cross was made between black barbs and white fantails. The F_1 birds were all black with some white splashes, evidently due to a separate factor introduced by the fantail. On breeding these blacks together they gave an F_2 generation, consisting of blacks (with or without white splashes), blues (with or without white splashes), and whites in the ratio 9 : 3 : 4.
The factors concerned are colour (C), in the absence of {67} which a bird is white, and a black modifier (B), in the absence of which a coloured bird is blue. The original black barb contained both of these factors, being in const.i.tution CCBB. The fantail, however, contained neither, and was const.i.tutionally ccbb. The F_1 birds produced by crossing were in const.i.tution CcBb, and being heterozygous for two factors produced in equal numbers the four sorts of gametes CB, Cb, cB, cb. The results of two such series of gametes being brought together are shown in the usual way in Fig.
11. A blue is a bird containing the colour factor but lacking the black modifier, _i.e._ of the const.i.tution CCbb, or Ccbb, and such birds as the figure shows appear in the F_2 generation on the average three times out of sixteen. Reversion here comes about in F_2, when the redistribution of the factors leads to the formation of zygotes containing one of the two factors but not the other.
