Mendelism Part 4

On this notation our six coloured forms are:--

(1) Purple bicolor CRBLI.[6]

(2) Deep purple CRBlI.

(3) Picotee CRBLi or CRBli.

(4) Red bicolor ( = Painted Lady) CRbLI.

(5) Deep red ( = Miss Hunt) CRblI.

(6) Tinged white CRbLi or CRbli.

It will be noticed in this series that the various coloured {82} forms can be expressed by the omission of one or more factors from the purple bicolor of the wild type. With the complete omission of each factor a new colour type results, and it is difficult to resist the inference that the various cultivated forms of the sweet pea have arisen from the wild by some process of this kind. Such a view tallies with what we know of the behaviour of the wild form when crossed by any of the garden varieties. Wherever such crossing has been made the form of the hybrid has been that of the wild, thus supporting the view that the wild contains a complete set of all the differentiating factors which are to be found in the sweet pea.

Moreover, this view is in harmony with such historical evidence as is to be gleaned from botanical literature, and from old seedsmen's catalogues. The wild sweet pea first reached England in 1699, having been sent from Sicily by the monk Franciscus Cupani as a present to a certain Dr. Uvedale in the county of Middles.e.x. Somewhat later we hear of two new varieties, the red bicolor, or Painted Lady, and the white, each of which may be regarded as having "sported" from the wild purple by the omission of the purple factor, or of one of the two colour factors. In 1793 we find a seedsman offering also what he called black and scarlet varieties. It is probable that these were our deep purple and Miss Hunt varieties, and that somewhere about this time the factor for the {83} light wing (L) was dropped out in certain plants. In 1860 we have evidence that the pale purple or Picotee, and with it doubtless the Tinged White, had come into existence. This time it was the factor for intense colour which had dropped out. And so the story goes on until the present day, and it is now possible to express by the same simple method the relation of the modern shades, of purple and reds, of blues and pinks, of hooded and wavy standards, to one another and to the original wild form. The const.i.tution of many of these has now been worked out, and to-day it would be a simple though perhaps tedious task to denote all the different varieties by a series of letters indicating the factors which they contain, instead of by the present system of calling them after kings and queens, and famous generals, and ladies more or less well known.

From what we know of the history of the various strains of sweet peas one thing stands out clearly. The new character does not arise from a pre-existing variety by any process of gradual selection, conscious or otherwise. It turns up suddenly complete in itself, and thereafter it can be a.s.sociated by crossing with other existing characters to produce a gamut of new varieties. If, for example, the character of hooding in the standard (cf. Pl. II., 7) suddenly turned up in such a family as that shown on Plate IV. we should be able to get a hooded form corresponding to each of the forms with the erect {84} standard; in other words, the arrival of the new form would give us the possibility of fourteen varieties instead of seven.

As we know, the hooded character already exists. It is recessive to the erect standard, and we have reason to suppose that it arose as a sudden sport by the omission of the factor in whose presence the standard a.s.sumes the erect shape characteristic of the wild flower. It is largely by keeping his eyes open and seizing upon such sports for crossing purposes that the horticulturist "improves" the plants with which he deals. How these sports or _mutations_ come about we can now surmise. They must owe their origin to a disturbance in the processes of cell division through which the gametes originate. At some stage or other the normal equal distribution of the various factors is upset, and some of the gametes receive a factor less than others. From the union of two such gametes, provided that they are still capable of fertilisation, comes the zygote which in course of growth develops the new character.

Why these mutations arise: what leads to the surmised unequal division of the gametes: of this we know practically nothing. Nor until we can induce the production of mutations at will are we likely to understand the conditions which govern their formation. Nevertheless there are already hints scattered about the recent literature of experimental biology which lead us to hope that we may know more of these matters in the future. {85}

In respect of the evolution of its now mult.i.tudinous varieties, the story of the sweet pea is clear and straightforward. These have all arisen from the wild by a process of continuous loss. Everything was there in the beginning, and as the wild plant parted with factor after factor there came into being the long series of derived forms. Exquisite as are the results of civilization, it is by the degradation of the wild that they have been brought about. How far are we justified in regarding this as a picture of the manner in which evolution works?

There are certainly other species in which we must suppose that this is the way that the various domesticated forms have arisen. Such, for example, is the case in the rabbit, where most of the colour varieties are recessive to the wild agouti form. Such also is the case in the rat, where the black and albino varieties and the various pattern forms are also recessive to the wild agouti type. And with the exception of a certain yellow variety to which we shall refer later, such is also the case with the many fancy varieties of mice.

Nevertheless there are other cases in which we must suppose evolution to have proceeded by the interpolation of characters. In discussing reversion on crossing, we have already seen that this may not occur until the F_2 generation, as, for example, in the instance of the fowls' combs (cf. p.

65). The reversion to the single comb occurred as the result of the removal of the two factors {86} for rose and pea. These two domesticated varieties must be regarded as each possessing an additional factor in comparison with the wild single-combed bird. During the evolution of the fowl, these two factors must be conceived of as having been interpolated in some way. And the same holds good for the inhibitory factor on which, as we have seen, the dominant white character of certain poultry depends. In pigeons, too, if we regard the blue rock as the ancestor of the domesticated breeds, we must suppose that an additional melanic factor has arisen at some stage.

For we have already seen that black is dominant to blue, and the characters of F_1, together with the greater number of blacks than blues in F_2, negatives the possibility that we are here dealing with an inhibitory factor. The hornless or polled condition of cattle, again, is dominant to the horned condition, and if, as seems reasonable, we regard the original ancestors of domestic cattle as having been horned, we have here again the interpolation of an inhibitory factor somewhere in the course of evolution.

On the whole, therefore, we must be prepared to admit that the evolution of domestic varieties may come about by a process of addition of factors in some cases and of subtraction in others. It may be that what we term additional factors fall into distinct categories from the rest. So far, experiment seems to show that they are either of the nature of melanic factors, or of inhibitory {87} factors, or of reduplication factors as in the case of the fowls' combs. But while the data remain so scanty, speculation in these matters is too hazardous to be profitable.

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CHAPTER IX

REPULSION AND COUPLING OF FACTORS

Although different factors may act together to produce specific results in the zygote through their interaction, yet in all the cases we have hitherto considered the heredity of each of the different factors is entirely independent. The interaction of the factors affects the characters of the zygote, but makes no difference to the distribution of the separate factors, which is always in strict accordance with the ordinary Mendelian scheme. Each factor in this respect behaves as though the other were not present.

A few cases have been worked out in which the distribution of the different factors to the gametes is affected by their simultaneous presence in the zygote. And the influence which they are able to exert upon one another in such cases is of two kinds. They may repel one another, refusing, as it were, to enter into the same zygote, or they may attract one another, and, becoming linked together, pa.s.s into the same gamete, as it were by preference. For the moment we may consider these two sets of phenomena apart. {89}

One of the best ill.u.s.trations of repulsion between factors occurs in the sweet pea. We have already seen that the loss of the blue or purple factor (B) from the wild bicolor results in the formation of the red bicolor known as Painted Lady (Pl. IV., 7). Further, we have seen that the hooded standard is recessive to the ordinary erect standard. The omission of the factor for the erect standard (E) from the purple bicolor (Pl. II., 5) results in a hooded purple known as Duke of Westminster (Pl. II., 7). And here it should be mentioned that in the corresponding hooded forms the difference in colour between the wings and standard is not nearly so marked as in the forms with the erect standard, but the difference in structure appears to affect the colour, which becomes nearly uniform. This may be readily seen by comparing the picture of the purple bicolor on Plate II.

with that of the Duke of Westminster flower.

Now when a Duke of Westminster is mated with a Painted Lady the factor for erect standard (E) is brought in by the red, and that for blue (B) by the Duke, and the offspring are consequently all purple bicolors. Purples so formed are all heterozygous for these two factors, and were the case a simple one, such as those which have already been discussed, we should expect the F_2 generation to consist of the four forms: erect purple, hooded purple, erect red, and hooded red in the ratio 9 : 3 : 3 : 1. Such, however, is not the case. The F_2 generation {90} actually consists of only three forms, viz. erect red, erect purple, and hooded purple, and the ratio in which these three forms occur is 1 : 2 : 1. No hooded red has been known to occur in such a family. Moreover further breeding shows that while the erect reds and the hooded purples always breed true, the erect purples in such families _never_ breed true, but always behave like the original F_1 plant, giving the three forms again in the ratio 1 : 2 : 1. Yet we know that there is no difficulty in getting purple bicolors to breed true from other families; and we know also that hooded red sweet peas exist in other strains.

Painted Lady Duke of Westminster (erect red) (hooded purple) Purple Invincible (erect purple) +-------------+-----------------+ Painted Purple Invincible Duke of Lady Westminster (1) (2) (1)

EEbb eeBB Parents / / / / / / Eb Eb eB eB gametes ------------/ EeBb F_2 ____/ ____ / Fem. gametes of F_1 Eb ---> EEbb <--- eb="" male="" gametes="" of="" f_1="" eb="" ---=""> EeBb <--- eb="" eb="" ---=""> EeBb <--- eb="" eb="" ---=""> eeBB <--- eb="" ----/="" f_2="" generation="">

On the a.s.sumption that there exists a repulsion between the factors for erect standard and blue in a plant which is heterozygous for both, this peculiar case receives a simple explanation. The const.i.tutions of the erect red and the hooded purple are EEbb and eeBB respectively and that of the F_1 erect purple is EeBb. Now let us suppose that in such a zygote there exists a repulsion {91} between E and B, such that when the plant forms gametes these two factors will not go into the same gamete. On this view it can only form two kinds of gametes, viz. Eb and eB, and these, of course, will be formed in equal numbers. Such a plant on self-fertilisation must give the zygotic series EEbb + 2 EeBb + eeBB, _i.e._ 1 erect red, 2 erect purples, and 1 hooded purple. And because the erect reds and the hooded purples are respectively h.o.m.ozygous for E and B, they must thenceforward breed true. The erect purples, on the other hand, being always formed by the union of a gamete Eb with a gamete eB, are always heterozygous for both of these factors. They can, consequently, never breed true, but must always give erect reds, erect purples, and hooded purples in the ratio 1 : 2 : 1.

The experimental facts are readily explained on the a.s.sumption of repulsion between the two {92} factors B and E during the formation of the gametes in a plant which is heterozygous for both.

Other similar cases of factorial repulsion have been demonstrated in the sweet pea, and two of these are also concerned with the two factors with which we have just been dealing. Two distinct varieties of pollen grains occur in this species, viz. the ordinary oblong form and a rather smaller rounded grain. The former is dominant to the latter.[7] When a cross is made between a purple with round pollen and a red with long pollen the F_1 plant is a long pollened purple. But the F_2 generation consists of purples with round pollen, purples with long pollen, and reds with long pollen in the ratio 1 : 2 : 1. No red with round pollen appears in F_2 owing to repulsion between the factors for purple (B) and for long pollen (L).

Similarly plants produced by crossing a red hooded long with a red round having an erect standard give in F_1 long pollened reds with an erect standard, and these in F_2 produce the three types, round pollened erect, long pollened erect, and long pollened hooded, in the ratio 1 : 2 : 1. The repulsion here is between the long pollen factor (L) and the factor for the erect standard (E).

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Yet another similar case is known in which we are concerned with quite different factors. In some sweet peas the axils whence the leaves and flower stalks spring from the main stem are of a deep red colour. In others they are green. The dark pigmented axil is dominant to the light one.

Again, in some sweet peas the anthers are sterile, setting no pollen, and this condition is recessive to the ordinary fertile condition. When a sterile plant with a dark axil is crossed by a fertile plant with a light axil, the F_1 plants are all fertile with dark axils. But such plants in F_2 give fertiles with light axils, fertiles with dark axils, and steriles with dark axils in the ratio 1 : 2 : 1. No light axilled steriles appear from such a cross owing to the repulsion between the factor for dark axil (D) and that for the fertile anther (F).

These four cases have already been found in the sweet pea, and similar phenomena have been met with by Gregory in primulas. To certain seemingly a.n.a.logous cases in animals where s.e.x is concerned we shall refer later.

Now all of these four cases present a common feature which probably has not escaped the attention of the reader. In all of them _the original cross was such as to introduce one of the repelling factors with each of the two parents_. If we denote our two factors by A and B, the crosses have always been of the nature AAbb aaBB. Let us now consider what happens when both of the {94} factors, which in these cases repel one another, are introduced by one of the parents, and neither by the other parent. And in particular we will take the case in which we are concerned with purple and red flower colour, and with long and round pollen, _i.e._ with the factors B and L.

When a purple long (BBLL) is crossed with a red round (bbll) the F_1 (BbLl) is a purple with long pollen, identical in appearance with that produced by crossing the long pollened red with the round pollened purple. But the nature of the F_2 generation is in some respects very different. The ratio of purples to reds and of longs to rounds is in each case 3 : 1, as before.

But instead of an a.s.sociation between the red and the long pollen characters the reverse is the case. The long pollen character is now a.s.sociated with purple and the round pollen with red. The a.s.sociation, however, is not quite complete, and the examination of a large quant.i.ty of similarly bred material shows that the purple longs are about twelve times as numerous as the purple rounds, while the red rounds are rather more than three times as many as the red longs. Now this peculiar result could be brought about if the gametic series produced by the F_1 plant consisted of 7 BL + 1 Bl + 1 bL + 7 bl out of every 16 gametes. Fertilization between two such similar series of 16 gametes would result in 256 plants, of which 177 would be purple longs, 15 purple rounds, 15 red longs, and 49 red rounds--a proportion of the four different kinds very close to {95} that actually found by experiment. It will be noticed that in the whole family the purples are to the reds as 3 : 1, and the longs are also three times as numerous as the rounds. The peculiarity of the case lies in the distribution of these two characters with regard to one another. In some way or other the factors for blue and for long pollen become linked together in the cell divisions that give rise to the gametes, but the linking is not complete. This holds good for all the four cases in which repulsion between the factors occurs when one of the two factors is introduced by each of the parents. _When both of the factors are brought into the cross by the same parent we get coupling between them instead of repulsion._ The phenomena of repulsion and coupling between separate factors are intimately related, though hitherto we have not been able to suggest why this should be so.

Nor for the present can we suggest why certain factors should be linked together in the peculiar way that we have reason to suppose that they are during the process of the formation of the gametes. Nevertheless the phenomena are very definite, and it is not unlikely that a further study of them may throw important light on the architecture of the living cell.

APPENDIX TO CHAPTER IX

As it is possible that some readers may care, in spite of its complexity, to enter rather more fully into the peculiar phenomenon {96} of the coupling of characters, I have brought together some further data in this Appendix. In the case we have already considered, where the factors for blue colour and long pollen are concerned, we have been led to suppose that the gametes produced by the heterozygous plant are of the nature 7 BL : 1 Bl : 1 bL : 7 bl. Such a series of ovules fertilised by a similar series of pollen grains will give a generation of the following composition:--

49 BBLL + 7 BBLl + 7 BbLL + 49 BbLl + 7 BBLl + 7 BbLL + BbLl + BbLl + 49 BbLl ---------------------------------/ 177 purple, long

+ BBll + 7 Bbll + bbLL + 7 bbLl + 49 bbll + 7 Bbll + 7 bbLl -------------/ -----------/ -----/ 15 purple, 15 red, 49 red, round long round

and as this theoretical result fits closely with the actual figures obtained by experiment we have reason for supposing that the heterozygous plant produces a series of gametes in which the factors are coupled in this way. The intensity of the coupling, however, varies in different cases.

Where we are dealing with another, viz. fertility (F) and the dark axil (D), the experimental numbers accord with the view that the gametic series is here 15 FD : 1 Fd : 1 fD : 15 fd. The coupling is in this instance more intense. In the case of the erect standard (E) and blueness (B) the coupling is even more intense, and the experimental evidence available at present points to the gametic series here being 63 Eb : 1 EB : 1 eB : 63 eb. There is evidence also for supposing that the intensity of the coupling may vary in different families for the same pair of factors. The coupling between blue and long pollen is generally on the 7 : 1 : 1 : 7 {97} basis, but in some cases it may be on the 15 : 1 : 1 : 15 basis. But though the intensity of the coupling may vary it varies in an orderly way. If A and B are the two factors concerned, the results obtained in F_2 are explicable on the a.s.sumption that the ratio of the four sorts of gametes produced is a term of the series--

3 AB + Ab + aB + 3 ab 7 AB + Ab + aB + 7 ab 15 AB + Ab + aB + 15 ab, etc., etc.

In such a series the number of gametes containing A is equal to the number lacking A, and the same is true for B. Consequently the number of zygotes formed containing A is three times as great as the number of zygotes which do not contain A; and similarly for B. The proportion of dominants to recessives in each case is 3 : 1. It is only in the distribution of the characters with relation to one another that these cases differ from a simple Mendelian case.

As the study of these series presents another feature of some interest, we may consider it in a little more detail. In the accompanying table are set out the results produced by these different series of gametes. The series marked by an asterisk have already been demonstrated experimentally. The first term in the series, {98} in which all the four kinds of gametes are produced in equal numbers is, of course, that of a simple Mendelian case where no coupling occurs.

+-------+------------------+---------+---------------------------------+ No. of Distribution of No. of Gametes Factors in Gametic Zygotes Form of F_2 Generation. in Series produced. series. +-------+------------------+---------+---------------------------------+ AB. Ab. aB. ab. AB. Ab. aB. ab. 4 1: 1: 1: 1 16 9 3 3 1 8 3: 1: 1: 3 64 49 7 7 9 16 7: 1: 1: 7 256 177 15 15 49* 32 15: 1: 1: 15 1024 737 31 31 225* 64 31: 1: 1: 31 4096 3009 63 63 961 128 63: 1: 1: 63 16384 12161 127 127 3969* 2n (n-1): 1: 1:(n-1) 4n^2 3n^2-(2n-1) 2n-1 2n-1 n^2-(2n-1) +-------+------------------+---------+---------------------------------+

Now, as the table shows, it is possible to express the gametic series by a general formula (n + 1) AB + Ab + aB + (n - 1) ab, where 2n is the total number of the gametes in the series. A plant producing such a series of gametes gives rise to a family of zygotes in which 3n^2 - (2n - 1) show both of the dominant characters and n^2 - (2n - 1) show both of the recessive characters, while the number of the two cla.s.ses which each show one of the two dominants is (2n - 1). When in such a series the coupling becomes closer the value of n increases, but in comparison with n^2 its value becomes less and less. The larger n becomes the more negligible is its value relatively to n^2. If, therefore, the coupling were very close, the series 3n^2 - (2n - 1) : (2n - 1) : (2n - 1) : n^2 - (2n - 1) would approximate more and more to the series 3n^2 : n^2, _i.e._ to a simple 3 : 1 ratio. Though the point is probably of more theoretical than practical interest, it is not impossible that some of the cases which have hitherto been regarded as following a simple 3 : 1 ratio will turn out on further a.n.a.lysis to belong to this more complicated scheme.

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CHAPTER X