|Previous Chapter : VIII. HEREDITY OF COLOUR (continued) : page 132.
< 148 >
GAMETIC COUPLING AND SPURIOUS ALLELOMORPHISM.
Pollen-Shape and Flower-Colour. Axil-Colour and Sterile Anthers - Hooded Standard and Flower-Colour in Sweet Peas.
We have now to consider one of the most curious and interesting developments of Mendelian research. In all the examples hitherto described the F2 numbers have shown that when allelomorphs belonging to various pairs are simultaneously distributed among the gametes in the process of gametogenesis, the distribution is random, so that all possible combinations are represented by equal numbers of gametes. For example, in the case of the double heterozygote formed by crossing a yellow, round pea with a green, wrinkled variety, the gametes produced by the F1 plant are in equal numbers bearers of
(1) yellow, round,
(2) yellow, wrinkled,
(3) green, round,
(4) green, wrinkled.
This fact is proved by the numbers 9 : 3 : 3 : 1 in which the several types of zygotes appear in F2. The phenomena now to be described indicate a system of segregation taking place in such a way that gametes presenting certain such combinations occur with greater frequency than the others.
The example in which this state of things was first detected is that of the pollen-shapes of the Sweet Pea (Lathyrus odoratus). The experiments in that case were begun, as has already been described (p. 89), by crossing a white Emily Henderson having long pollen, with a white
< 149 >
Emily Henderson having round pollen. The F1 generation was a reversionary purple bicolor, with long pollen -for the long pollen-shape is dominant- and from this the F2 generation was raised which consisted of
27 purples : 9 reds : 28 whites.
36 : 28
It has been shown that the interrelations of these several types proved that the colour in both purples and reds is due to the simultaneous presence in the zygote of two factors which we have called C and R, and that the white flowered plants are those in which one or both of these factors are absent. The point, however, which now concerns us more immediately is the distinction between the purples and the reds. This, we have seen, is due to the presence or absence of a factor B, blue. Those coloured plants which have the factor B are purple, those devoid of this factor B are red-flowered.
We saw also that there were various subordinate classes among the purples with corresponding subordinate classes among the reds, each due to the possession or to the want of some special factor. These differences are similarly distributed among the purples and among the reds. For example if the family is one which contains picotee types, there are picotees among the purples and corresponding types among the reds, and in each colour-group the proportions are the same, averaging 3 fully-coloured to 1 picotee in the case of the purples and the same in the case of the reds.
When however the distribution of the pollen-characters, long and round, in a family containing purple, red, and white members is examined, it is found that taking the family as a whole the long-pollened plants are to the round-pollened as 3 to 1 in the usual way. Among the white-flowered also there are 3 longs to 1 round. But when the purples and the reds are separately studied, the numbers 3 long : 1 round are not found. On the contrary, in the purples there is a great excess of longs, which are to the rounds as about 12 to 1, while among the reds there is an excess of rounds, which are to the longs as about 32 to 1.
< 150 >
The result of extensive counting shows that an approximation to the observed numbers would be produced by a gametic system of such a kind that the combinations of long pollen with blue factor, and round pollen with no blue factor
Fig. 19. Pollen-grains of Sweet Pea. The normal long (dominant) type is shown in II and IV : the peculiar round type (recessive) in I and III. The upper figures represent the dry condition. The lower figures show the appearance in sulphuric acid, which makes the pores visible. Three pored and one-pored grains sometimes occur among the rounds, but they are usually two-pored. In IV one of the grains is seen end-on, and in I three of the disc-like grains are seen edge-wise.
[Figure not reproduced in this version]
< 151 >
occur seven times as often as the other two possible combinations. We speak of this phenomenon as Gametic Coupling.
The term "coupling" is strictly applicable, because the association is between two dominant or "present" factors, here those for blue colour and long pollen. Abnormal distributions due to such coupling are to be carefully distinguished from those described later under the name "Spurious Allelomorphism," to which the term "coupling" should not be applied.
If the two pairs of factors are expressed thus :
Blue colour B
Long pollen L
Red colour b
Round pollen l
the gametic series is not
1 BL + 1 Bl + r bL + 1 bl,
7 BL + 1 Bl + 1 bL + 7 bl,
or very nearly so.
Such a gametic series would give an F2 family composed thus :
177 blue-long : 15 blue-round : 15 red-long : 49 red-round.
Reference to Fig. 17 will show how these numbers are arrived at.
These ratios agree very nearly with those observed in actual experiments. For example the following series has been produced.
[Table not reproduced in this version]
The correspondence between calculation and observation is very close, and in the case of the one figure which departs sensibly from expectation it may be suggested with great probability that the observed excess of purple longs is due to some amount of cross-fertilisation effected by the leaf-cutter bee, Megachile. Such crossing would obviously tend to increase this class at the expense of the rest.
Study of the F3 families has proved conclusively that the abnormal distribution occurs only among the gametes of plants which are heterozygous both m the pollen characters
< 152 >
and also as regards the factor B. Plants that are homozygous in either of these allelomorphs have the normal distribution of characters among their gametes, and they may be heterozygous in C, R, or in any of the other factors recognized in the Sweet Pea without any departure from the ordinary ratios being produced.
The gametic series has been spoken of as 7 : 1 : 1 : 7 and these are the numbers which fit the observed result most closely, but attention should at once be called to the possibility that the series may in reality be 8 : 1 : 1 : 8. The observed numbers are too small to enable us as yet to discriminate between these two possibilities, though, as will be seen when the nature of coupling is discussed, the significance of the two series must be entirely different. It is however to be noticed that the series of gametes necessary to complete the whole system is thus either 16, or 16 + 2.
In the next two examples of such partial coupling the association is in groups of 32, or 32 + 2. Both these also occur in the Sweet Pea. The first concerns the peculiar sterility or contabescence of the anthers which has already been mentioned as a recessive character. The second factor is again a colour-factor. Among the various factors which control colour in the Sweet Pea is one which causes the appearance of a reddish purple spot in the axils of the leaves, referred to already as the dark-axil factor. When this factor is present (and the flowers are coloured) the axils are dark ; when it is absent the axils are simply green as they always are in white-flowered plants*. At an early stage in the Sweet Pea investigation it was noticed that when a family contained plants differing in respect of sterility and fertility of anthers as well as in respect of dark and light axils, the plants with sterility in the anthers (having coloured flowers) were almost always light-axilled, and conversely the dark-axilled plants were almost always fertile in the anthers. In such families, among the white-flowered
* Dark axils sometimes exist in plants which have the flowers so nearly white as to pass for real whites. Probably in all such flowers a trace of colour is developed, and certainly in them the seed-coat is always black as it is in all the Sweet Peas with coloured flowers.
Plants raised from wild Sicilian seed were all purple bicolour in flower-colour, and nearly all had dark axils, but a few had light axils.
< 153 >
plants, the sterile to the fertile were 3 to 1 without complication.
Statistical examination of these families on a large scale has shown that among the plants with coloured flowers the ratio of fertile with dark axil : fertile with light axil sterile with dark axil : sterile with light axil, approaches closely to that which would be produced if the series of gametes bearing these four respective combinations were
15 : 1 : 1 : 15.
The actual numbers observed were
Dark axil : Fertile : 627 (Expectation : 637), sterile : 27 (Expectation : 27)
Light axil : Fertile : 17 (Expectation : 27), Sterile : 214 (Expectation : 194)
A third example of partial gametic coupling relates again primarily to the blue factor (B) and the pollen-shapes, but in order to make clear the circumstances in which it occurs, another set of phenomena must first be described.
In the old types of Sweet Pea the standard is erect and has a small notch in the middle of its upper border. This is the natural shape of the wild flower. Of the modern types many are what is called "hooded." The standard in hooded forms turns forward and downward in various degrees, the amount varying with the type and also to some extent with the weather and the condition of the flower. The hooded standard differs also from the erect one in having little or no trace of the central notch. This difference causes the buds of the two types to be recognizably distinct before the flower opens, for in the hooded type the point of the folded standard projects sharply forward in front of the wings, while in the erect type this tip is rounded off by reason of the notch. (Plate V.)
The hooded standard also is sometimes distinguished by the existence of a sinus of variable size on each side of the standard, which thus has lateral lobes more or less well developed. These differences obviously point to a different distribution of the strains produced by the growth in the two types. The lateral sinus is not represented in the hooded flowers shown in Plate V.
< 154 >
The hooded types may have a great diversity of colours, and fixed hooded varieties now exist in the purple, blue, red, pink, cream and other classes. It is nevertheless a remarkable fact that, so far as I am aware, none of the regular bicolour varieties ever have a really hooded standard. There is for instance no hooded type having the colour of the original purple, with its chocolate-purple standard and blue wings, nor can Painted Lady with standard red and wings nearly white be produced in a hooded shape. On the contrary the hooded types always have the standard and wings more nearly alike in colour, and there is the clearest evidence that in families (F2 and later generations) which contain original bicolour purples as well as hooded types, the hooded types corresponding to them are of the unicolorous kind known as "Duke of Westminster*."
From these facts it is evident that there is here some interdependence between the colour of the flower and its form. This interdependence is of course somatic, but as will be seen there is also a gametic connection between the phenomena of shape and colour.
The experiments bearing on these questions originated in a cross between the white, round-pollened Emily Henderson and a white, long-pollened hooded type known as Blanche Burpee. The Emily Henderson has an ordinary erect standard with the central notch.
F1 produced from these two is a bicolour purple, with erect standard and long pollen, indistinguishable from the reversionary F1 previously described as the offspring of the long and round whites. F2 from such plants consists of the following types :
[Table not reproduced in this version]
* Similarly if the bicolour purples with dark wings are present in the class with an erect standard, they are represented by "Duke of Sutherland" in the hooded class, viz. a deep unicolorous purple.
< Plate V >
Parent F1 Parent
White : flat standard White : hooded standard
1. Emily Henderson. 2. Blanche Burpee. 3. Purple Invicible, F1. 4-11. The various F2 types obtained by self-fertilising F1. 4. Purple Invisible. 5. Duke of Westminster. 6. Painted Lady. 7-9. Corresponding dark winged types. 7. Purple, with purple wings. 8. Duke of Sutherland. 9. Miss Hunt. 10 and 11. F2 whites. Notice that there is non hooded red.
< 155 >
From these results it appears that the erect standard is dominant to the hooded. Next we have the remarkable feature that whereas the purples and the whites are both represented in the two classes erect and hooded, the reds are all erect. This fact indicates that those gametes which bear the factor for erect standard do not bear the factor B which causes the purple or blue colour ; and conversely the gametes which do not bear E, the erectness-factor, bear B. The gametes in respect of these two allelomorphic pairs have thus the composition Be, or bE, and as regards transmission of characters the effect is that which would be produced if B were allelomorphic to E. In view of this curious fact it seems not impossible that we may be obliged hereafter to extend the conception of allelomorphism, and to recognize that factors concerned with features of organisation which seem to have no special physiological association, may be allelomorphic to each other.
Another curious result follows from the existence of this unexpected or spurious allelomorphism. If the hooded purples in F1 be bred from, the F3 families will be either all hooded purples again or they will be such plants together with hooded whites. Similarly the red bicolours will give F3 consisting either of red bicolours, or of red bicolours and erect whites. But the purple bicolours, must all by their constitution be heterozygous in both the factor for erectness and for blueness, and consequently their F3 families contain hooded purples, erect bicolour purples, and erect bicolour reds in the ratio 1 : 2 : 1 (together with whites if the particular F2 happens to be heterozygous in either C or R, the fundamental colour-factors).
Such bicolour purples thus present the anomaly of being permanent heterozygotes, though in appearance they are actually the original type of Sweet Pea. Families may thus consist entirely of
1 Hooded unicolorous purples : 2 Erect bicolour purples : 1 Erect bicolour reds
and the bicolour purples in such families will give similar families again indefinitely.
On determining the pollen-characters of those families
< 156 >
which contain erect and hooded, coloured and white, long and round pollens, it was found that in F2 the distribution followed the system derived from the gametic series 7 : 1 : 1 : 7, but among the F3 families derived from them several were found to exhibit coupling between the blue factor and long pollen according to the system 15 : 1 : 1 : 15. For instance, to take the group of plants in which this was most evident, a certain F3 plant with purple flower and erect standard together with eight similar plants (its offspring, in F4) gave collectively
Purple : long : 583 (expectation on 15 : 1 : 1 : 15 basis : 578), round : 26 (expectation on 15 : 1 : 1 : 15 basis : 24)
Red : long : 24 (expectation on 15 : 1 : 1 : 15 basis : 24), round : 170 (expectation on 15 : 1 : 1 : 15 basis : 177)
Here it is practically certain that the 15 : 1 formula correctly expresses the gametic distribution. Nevertheless, though some of the offspring of F2 gave such definite indications of the 15 : 1 system others no less definitely followed the 7 : 1 plan, and others again gave results so uncertain that it was impossible to assign them to either group with any confidence.
The facts are thus exceedingly complex, and all that can be stated is that coupling between the blue factor and the long pollen does certainly exist in certain families derived from a cross which involved the factors for erect and hooded standard ; but that inasmuch as the F2 distribution followed the 7 : 1 plan*, the heterozygosis between erectness and hood cannot be the direct cause of this change in the coupling. Pending further analysis the distinction which decides whether the coupling shall follow the 7 : 1 system or the 15 : 1 system must be regarded as quite unknown, for examination of the various families has not revealed any consistent difference between them (see 22, pp. 10-13).
Since the B factor is alternative to E, the erect standard, in the gametic composition, it follows that the zygotic com- [combination]
* F2 here was
Purple : long : 296 (expectation on 7 : 1 : 1 : 7 basis : 295), round : 19 (expectation on 7 : 1 : 1 : 7 basis : 25)
Red : long : 27 (expectation on 7 : 1 : 1 : 7 basis : 25), round : 85 (expectation on 7 : 1 : 1 : 7 basis : 82)
< 157 >
bination of hooded standard with round pollen must be exceedingly rare in any of these families. If the gametic coupling is
7 Blue long + 1 Blue round + 1 Red long + 7 Red round,
the zygotic expectation is, in the simpler case where all the standards are erect,
177 Blue long + 15 Blue round + 15 Red long + 49 Red round.
But when the standards may be either erect or hooded, all the hooded plants are homozygous in B, and the expectation of a round-pollened plant occurring among the BB class is only 1 in 64. Observation agreed with this expectation, for in the F2 families which certainly all followed the 7 : 1 system, there were 83 hooded purple plants and of them one was round-pollened. The same expectation holds in regard to the hooded plants with white flowers, which also must all be BB. Unfortunately most of these were recorded before the importance of the question was appreciated, and in them the hoods were not noted. Of 17 plants of this class which were examined 16 were, long and 1 was round.
Similarly, when the coupling is on the 15 : 1 : 1 : 15 system the hooded purples will be still rarer, and should occur only as 1 in 256 of their class. Among the 9 families which definitely followed the 15 : 1 plan, one hooded round occurred among 209 plants of the hooded class.
Discussion of the Physiological Significance of Gametic Coupling and Spurious Allelomorphism.
The significance of the phenomena just described lies in the fact that they demonstrate the existence of a complex interrelation between the factorial units. This interrelation is such that certain combinations between factors may be more frequent than others. The circumstances in which this interrelation is developed and takes effect we cannot as yet distinguish ; still less can we offer with confidence any positive conception as to the mode in which it is exerted.
< 158 >
The time has not yet come for such an analysis to be attempted. Nevertheless we can scarcely forbear from considering some of the possibilities which suggest themselves.
In spurious allelomorphism the outward facts are comparatively simple. Two dominant, or "present" factors, behave as if in the cell-divisions of gametogenesis they repelled each other, and we must suppose that this repulsion is exerted at some definite cell-division, such that one factor passes into one daughter-cell and the other factor into the other. The dividing cell being AaBb, the daughter-cells are respectively Ab and aB. Though as yet only one case has been definitely proved to follow this system, the evidence in that case is very positive. Moreover when the facts of sexual inheritance come to be related, a group of cases will be described which conform so precisely with this type-example of spurious allelomorphism that it is practically certain that this case is not a solitary example, but one which typifies a category of genetic phenomena. It may therefore be taken that repulsion - or, more strictly, a relation which can be represented as repulsion - may exist between factors belonging to distinct allelomorphic pairs.
The state of things which results in gametic coupling is much more obscure. The association of characters here is quite distinct from the association of characters produced by spurious allelomorphism. In gametic coupling the dominant factors are associated together, while in spurious allelomorphism the dominant factors are dissociated from each other. If the coupling were total, so that all the gametes were either Ab or ab just as in spurious allelomorphism they are all either Ab or aB we might naturally suppose the one phenomenon to be the converse of the other. The one might then be represented as an effect of attraction just as the other may be represented as the result of repulsion between the two dominant factors. So far, however, as experiment has yet gone, we have no certain case in which the coupling is complete. There are no doubt instances of features apparently distinct which are nevertheless transmitted in collocation. In the Sweet Pea, for instance, the deep brown or blackish pigmentation of the seed-coat occurs only in plants with some colour in the flower, but these two features may thus be supposed to
< 159 >
depend on one allelomorph, not on two. To prove the existence of complete coupling it would be necessary to show that features elsewhere known to depend on separate allelomorphs, could on occasion be linked in a complete union. Whether such a state of things is possible we do not know. There is no reason for supposing that it is impossible.
The arithmetical series in which the numbers occur is the only guide as to the nature of the process, and obviously this is quite insufficient. The existence of the 7 : 1 systems and of the 15 : 1 systems naturally suggests the possibility that a system based on 3 : 1 may exist. We might then arrange the systems in a series thus* :
[Table not reproduced in this version]
Hitherto, though some dubious indications of such a series have been seen, there is no clear case of coupling on the system 3 : 1.
It is not easy to conceive any probable system of symmetrical cell-divisions or dichotomies which would produce the series 7 : 1 and 15 : 1. If the segregation of characters were not all completed at one cell-division we might of course imagine a scheme which would give the system 8 + 1 + 1 + 8, thus :
after which, if the cells AB and ab each divided again
[Table not reproduced in this version]
after which, if the cells AB and ab each divided agin
* The F2 numbers resulting from these couplings are as follows :
3 : 1 : 1 : 3, AB. Ab. aB. ab : 41 : 7 : 7 : 9
7 : 1 : 1 : 7, AB. Ab. aB. ab : 177 : 15 : 15 : 49
15 : 1 : 1 : 15, AB. Ab. aB. ab : 737 : 31 : 31 : 225.
If n be half the number of gametes needed to express the whole series of couplings in a given case, then the four F2 numbers are given by the formula
3n2 - (2n - 1) : 2n - 1 : 2n - 1 : n2 - (2n - 1).
< 160 >
three times, the series 8AB + 1Ab + 1aB + 8ab would result. We cannot, on the observed numbers, assert quite positively that the system is not 8 : 1, but as observations accumulate, this supposition becomes increasingly improbable, for the numbers all point rather to 7 : 1 than 8 : 1.
If, as many suppose, the whole process of segregation is completed in the reduction-division, it is obvious that any suggestion involving successive segregations fails. Still it is worth noting that nothing yet limits us to the conception of segregation as occurring all at once. We know very little yet as to the cytological processes antecedent to the reduction-division. Moreover it cannot yet be asserted that all the gametes, or even all the gametes of one sex (in hermaphrodite forms) are in the same cell-generation, counting from the first cleavage-plane of the zygote.
It is to be noted also that where the germ-cells are many, as in the testes of animals and the anthers of most plants, it is not difficult to imagine the formation of even very long series of couplings. The egg-cells, on the contrary, are few, and in plants they are very often definitely grouped in special organs which again are arranged on a definite geometrical plan relatively to the gross anatomy of the plant. Even if the various accessory cells of the plant ovary are reckoned as belonging to the gametic series, the number still seems insufficient to allow for the development of a coupling which demands a long series for its expression. The question may naturally be asked whether there is any organised system of differentiation connecting the several ovaries into a common plan. The differentiation among the egg-cells might conceivably be distributed on a geometrical plan like the differentiation among the somatic organs of the plant. All the available evidence is however against this suggestion, for in maize and peas, where indications of this system might be found if they existed, all the evidence is entirely negative.
There is still another direction in which we may look for an elucidation of the nature of gametic coupling. If the factors can act upon each other in such a way that certain combinations do not occur, as we have already seen actually happening in the case of Spurious Allelomorphism, it seems
< 161 >
possible that such a system as the 7 : 1 : 1 : 7 may be the result of a complex series of repulsions exerted among a number of factors. At present this suggestion is quite unfounded. It could however be tested if breeding on a really large scale could be undertaken, and supposing it to be true, the evidence for its truth would appear in the relative infrequency with which some types appeared. From that evidence the missing gametic combinations could be identified. As yet however it is quite premature to pursue such an analysis, and we must be content to note that when, as in these Sweet Peas, there is heterozygosis between a number of distinct allelomorphic pairs, the numerical proportions in which the various combinations occur may certainly be affected by the interactions exerted by allelomorphs of different pairs upon each other.
The Possibility of Selective Mating between Gametes.
It has naturally occurred to many minds that as gametes are now known to possess differentiating qualities, these differentiations may affect the readiness with which various classes of gametes may unite. We recognize that the simple Mendelian numbers are produced when every kind of female gamete has an equal probability of uniting with every kind of gamete produced by the male. Conversely, when irregular and unexpected numbers appear as the result of experiment, the question may have to be considered whether the irregularity is not due to a selective assortment taking place among the gametes, such that certain types of unions occur in fertilisation with greater readiness than others. Hitherto it is doubtful whether any instance has been discovered in which abnormal numbers can be proved to occur with such regularity as to warrant a recourse to this hypothesis. Correns observed several families of maize where F2 from F1 round seed x wrinkled seed, self-fertilised, contained a great excess of round seeds. The totals were 8975 round : 1711 wrinkled where the expectation is 8014 round : 2671 wrinkled. Thirty-five F1 plants contributed to this total and the discrepancy between observed result and expectation was fairly constant throughout.
< 162 >
To test whether the numerical output of gametes was abnormal, reciprocal crosses were made between F1 plants of the same breeding and recessives. In both cases the normal equality between round and wrinkled seeds was produced. Correns therefore concludes that some process of selective mating was responsible for the aberrant F2 numbers (65).
So far as I am aware, no case altogether similar to this one has been observed, certainly none in which the numbers available are so large. The proportions for maize seeds are usually very regular in regard to the round and wrinkled characters, as the records of both Correns and Lock testify.
Pending further acquaintance with phenomena of this class there is no more to be said. The possibility of disturbance by selective attraction between particular kinds of gametes must be recognized, though without much more definite evidence its occurrence can scarcely be regarded as demonstrated.
In another instance of a different kind the same suggestion was made by Cuénot. Of this case I have already spoken* in describing the inheritance of yellow colour in animals. Experimenting with mice he found it impossible to find a yellow mouse pure to yellowness. Among mice yellow behaves as a dominant, in the sense that agoutis or blacks may be bred from two yellows. If the case were an ordinary one, some of the yellows produced by the mating of two yellows should be pure, and on breeding to blacks or agoutis they would be expected to give all yellows. Cuénot's experience is that this is never realized, and all the yellows he has ever tested, amounting to 81 individuals, also show, in such matings, some colours other than yellow (cp. Basset Hounds, p. 128). Miss Durham has made similar experiments with the same result. Yellows were always found to give off either agoutis, or blacks, chocolates.
Cuénot interprets the peculiar result as two gametes both bearing the determiner are incapable of uniting in fertilisation. The numbers were
* The discussion of this remarkable case was given in another connection at p. 119, but in view of its special importance the facts and argument are repeated here.
< 163 >
232 yellows and 86 agoutis, which is a near approach to the normal 3 : 1 (228.5 : 79.5). Cuénot comments on this as a difficulty in the way of his view, saying that he would have expected the ratio 2 : 1 ; but as Mr Punnett pointed out to me, if all the ova bearing the yellow factor were fertilised by agouti spermatozoa, the number of these being indefinite, the chances of the non-yellow ova being fertilised by a spermatozoon bearing yellow or non-yellow would remain sensibly equal. Thus the ratio 3 yellow : 1 agouti would result.
Nevertheless the impression left on my mind by these observations, and indeed by other strange phenomena which yellows exhibit, is that the genetics of yellow mice are very imperfectly investigated and that it is premature to formulate definite views as to their behaviour.
One of the peculiarities of yellow mice, well known to fanciers, is their frequent tendency to excessive fatness. Miss Durham, who has had considerable experience with yellows, finds that this condition is not universal among them, but shows itself in frequent individuals. She has also found the genetic investigation of yellows very difficult on account of the fact that they are often sterile, and the suggestion is perhaps worth considering that this sterility may be responsible for some of the complications.
Next Chapter : X. HEREDITY AND SEX : page 164.
TABLE OF CONTENTS