Chapter 7
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 illustration 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 shells of the fancy (Pl. I., 4-7).
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Yellow × Himalayan | +-------------+ Agouti × Agouti | +--------+------+-------+----------+ Agouti Yellow Black Tortoise Himalayan Shell (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 tortoiseshell. Evidently we are here dealing with two factors: (1) the grey factor (G), which modifies black into agouti, or tortoiseshell into yellow; and (2) an intensifying factor (I), which intensifies yellow into agouti and tortoiseshell into black. It may be mentioned here that other experiments confirmed the view that the yellow rabbit is a dilute agouti, and the tortoiseshell 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 constitutionally 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 constitution 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 (procumbent) (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 3½-4 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 procumbent 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 procumbent habit which is dominant to the erect bush-like habit. Of these characters length of internode was carried by the Bush, and the procumbent habit by the original Cupid parent. The bringing of them together by the cross resulted in a procumbent 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 procumbent 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.
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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 association 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 constitution CCBB. The fantail, however, contained neither, and was constitutionally ccbb. The F_1 birds produced by crossing were in constitution 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 constitution 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.
+-------+-------+-------+-------+ |CB#####|CB#####|CB#####|CB#####| |CB#####|Cb#####|cB#####|cb#####| |#######|#######|#######|#######| |##BLACK|##BLACK|##BLACK|##BLACK| +-------+-------+-------+-------+ |Cb#####|Cb.....|Cb#####|Cb.....| |CB#####|Cb.....|cB#####|cb.....| |#######|.......|#######|.......| |##BLACK|...Blue|##BLACK|...Blue| +-------+-------+-------+-------+ |cB#####|cB#####|cB |cB | |CB#####|Cb#####|cB |cb | |#######|#######| | | |##BLACK|##BLACK| | | +-------+-------+-------+-------+ |cb#####|cb.....|cb |cb | |CB#####|Cb.....|cB |cb | |#######|.......| | | |##BLACK|...Blue| | | +-------+-------+-------+-------+
FIG. 11.
Diagram to illustrate the appearance of the reversionary blue pigeon in F_2 from the cross of black with white.
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