|Previous Chapter : I. INTRODUCTORY. MENDEL'S DISCOVERY : page 1.
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THE MATERIAL INVESTIGATED.
List of Structural Characters in Plants and Animals - List of Types in which the inheritance of Colour has been studied - Preliminary Deductions - Dominance and heterozygous characters - Mendel's system distinguished from that of Galton.
Heredity following the general rules described in the last chapter has been witnessed in a great diversity of animals and plants. The characters already proved to follow such rules show an equal diversity. The following is a list of some of them. Adequately to represent the facts respecting each of these cases lengthy description would be needed. In regard to several of them occurrences which do not readily fall into line have been recorded. Of these some are probably due to errors of observation or mistakes of various kinds, but a few will doubtless prove to be genuine exceptions to rule and may constitute points of departure for fresh and more extended research. In the outline of the phenomena, which is all that this book can profess to offer, it seemed best to restrict as far as possible the enumeration of these details, which can only be thoroughly appreciated by reference to the original papers ; but such annotations as appeared necessary either in elucidation of the phenomena or by way of incentive to further work are briefly given with references to the original sources. These annotations will be better understood after the later chapters have been read.
In the following list when one character is conspicuously dominant it is put first, but in several cases the dominance is imperfect.
1. Tallness and dwarfness. Peas (Pisum) and Sweet Peas (Lathyrus odoratus). Runner and French Beans (Phaseolus).
As regards Peas the facts have been recorded by. Mendel (195) Tschermak (269, 270, &c.), R.E.C.* (20). When varieties differing
* R.E.C. stands throughout for Reports to the Evolution Committee of the Royal Society, giving an account of the experiments of W. Bateson, E. R. Saunders and R. C. Punnett. Other contributors to these Reports are mentioned by name.
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greatly in height are used, dominance is complete, and the two parent forms are represented as three to one in F2. No clear exception has yet been observed. Peas (Pisum) exist in a vast number of distinct horticultural varieties which can roughly be classified as tall (about 5-6 ft.), half-dwarfs (about 4 ft.), dwarfs (about 9 ins. to 3 .ft.). The genetic relations of the half-dwarfs to the others are not fully explored, and further investigation will probably lead to the discovery of important facts. The cross half-dwarf x tall giving tall as dominant has produced some extreme dwarfs in F2, doubtless by recombination (q.v.), R.E.C. 20, p. 69. The cross half-dwarf x dwarf has given intermediates in F1 (ibid.).
The cross between tall and dwarf "Cupid" Sweet Peas gives complete dominance of tallness and simple segregation in F2, "Cupids" indistinguishable from the original "Cupid" parent reappearing (Fig. 1).
Phaseolus has been investigated especially by Tschermak (278) who records some apparently anomalous results. de Vries (298, II. p. 76) states that he found that extracted F2 dwarf Antirrhinum did not breed true, but threw plants of various heights. The experiment should be repeated.
2. Branching habit and the unbranched habit. Sunflower (Helianthus, Shull, 241) and Cotton (Balls, 6). The branched form of Stock (Matthiola incana) is dominant to the unbranched Brompton type. In F2 the unbranched type reappears, but the ratio has not been determined (Fig. 4). E. R. Saunders (unpublished).
3. The straggling habit of both the tall and dwarf “Cupid” Sweet Peas, and the much-branched erect habit of the "Bush" Sweet Peas (R.E.C. 22)
The relation of these two types to each other is not altogether simple. As described (q.v.) F1 from Cupid x Bush is a reversionary farm exactly like the normal tall variety. Neither the tall varieties nor the Cupids show the profuse branching of the Bush Sweet Peas which gives them their peculiar appearance. This is evidently recessive to the unbranched condition, and the fact thus stands out in contrast to those observed in the case of Sunflower and Cotton. But in the Sweet Pea we have the additional complication that the factor which represses the excessive branching by its presence gives increase of height. The tall and the Bush differ from each other in respect of this factor only. It is present in the tall but absent from the Bush. In the cross between Bush and Cupid two pairs of factors are concerned as explained in the passage referred to.
4. Hairiness and glabrousness. Lychnis. Matthiola (Stocks). Wheat.
The case of Lychnis has been studied by de Vries (288) and R.E.C. (19) In crosses between fully hairy and glabrous strains the discontinuity is complete. Various forms intermediate in hairiness may nevertheless be found wild and are by no means rare. Silene inflata
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often exists in two forms, hairy and glabrous, growing side by side, and doubtless their genetic relations are the same as those found for the corresponding varieties of Lychnis In this species a third form is found with hairs on the edges only (12).
The case of Matthiola is important and presents features of special interest, R.E.C. (19, 20, 21, see also Correns, 61). Between thoroughly hoary and glabrous strains the discontinuity is absolute, and the glabrous
Fig. 4. Matthiola. Branched and unbranched forms in F2. A photograph of Miss Saunders' plants, the leaves removed. (Supplied by Miss Killby.)
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are entirely destitute of hairs. The dominance is complete and homozygotes cannot be distinguished from heterozygotes. A third, or "half hoary" form exists, which is glabrous or nearly so on the upper surface only. Its behaviour has not been fully investigated (19, p. 33.)
The genetics of hairiness in wheat have been studied by Spillman (247), Tschermak (270), Biffen (27). The heterozygotes are sometimes intermediate in hairiness.
The Peach and the Nectarine are'probably related to each other as hairy dominant and glabrous recessive.
Peculiar results are recorded in Cotton (Balls, 6).
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5. Prickliness and smoothness of fruits. Datura. (R.E.C. 19, 20.) Ranunculus arvensis (20) :
The case of Datura is interesting from the fact that it sometimes has mosaic fruits, one quarter or one half being prickly and the rest smooth. This is perhaps to be regarded as indicative of segregation occurring among zygotic cells (see Chap. XV.).
Ranunculus arvensis has three types, spiny, tuberculated, and smooth. The first is a simple dominant. Tuberculated x smooth gave F1 partially spiny (21, p. 55).
6. Absence of glands (Matthiola incana) on leaves was dominant to presence of glands (M. sinuata) (R.E.C. 20, p. 40).
Fig. 5. Cross between a normally awned Barley and a variety with "hooded" awns. P, P, the parents. F1 shows partial dominance of hoods. The increase in length of ear is noticeable. The case also illustrates the result of crossing a 2-row type with a 6-row type, showing dominance of the former. (From Professor Biffen's specimens.)
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7. Rough and smooth foliage. Wheat. Biffen (27).
8. Keeled glumes and rounded glumes : Wheat. Ibid.
9. Beardless and bearded ears. Wheat. Ibid. Also Spillman (247) and Tschermak (270).
Most, if not all, of the "beardless" varieties exhibit a slight and variable amount of awn especially on the uppermost spikelets (Fig. 4.).
10. The "hoods" or "Kapuzen" characteristic of certain Barleys show a partial dominance over the normal type. These hoods, Professor Biffen states, are, structurally, aborted florets (Fig. 5). Tschermak (270), Biffen (30).
11. Hollow and solid straw. Wheat. Biffen (27).
This is a structural character of an interesting kind, and one upon which the commercial value of straw very largely depends. It was shown that many factors were concerned in the production of the stem-characters ; and in F2 by the recombination of these factors a great variety of straws appeared.
12. Blunt and pointed pods. Pisum. Tschermak (271), R.E.C. (20). Phaseolus. Tschermak (272).
The dominance in this case is complete. Some varieties exist in both a blunt and a pointed type (e.g. Sutton's Continuity). The nature of these cases is discussed later.
13. Lax and dense ears of Wheat and Barley give different results according to the varieties used. Sometimes F1 is lax, sometimes it is intermediate (Spillman, 247 ; Biffen, 27, 28). See Fig. 6. In Barley an increase in earlength has been observed (Fig. 5).
14. Development of fibrous parchment-like lining to pods, and the absence of the same which constitutes the "sugar peas." Pisum. In Phaseolus (kidney-beans), where similar types occur, the evidence is that the dominance is reversed (Emerson, 120, 121).
This is one of the features originally investigated by Mendel. He regarded the parchmented type as a dominant. In our experiments F1 has always had some parchment but the quantity is so much reduced as to cause the heterozygote to have a very distinct appearance (R.E.C. 20).
15. Much serrated and little serrated edges of leaves. Urtica (cp. Phyteuma, Correns, 70, p. 197). This cross
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was described by Correns (77) who gives a striking diagram representing his results. The cross was made between two forms known as Dodartii and pilulifera, which were
Fig. 6. A cross between a beardless, lax-eared wheat and a bearded, dense-eared type. P, P, the parents. F1 is beardless and intermediate in lenght of ear. The six F2 types occur in the ratio indicated. (Photograph from specimens supplied by Professor Biffen.)
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regarded by Linnaeus as distinct species. The almost entire-leaved Dodartii has been treated by later authors as a variety of pilulifera.
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16. Palmatifid or "palm-leaf" and pinnatifid or "fern-leaf." Primula Sinensis (Fig. 7)
The fern-leaved form arose in English horticulture about 1860 as a variation from the normal type. I have had opportunities of seeing its genetic behaviour on a large scale at Messrs Sutton's, and many experiments have been made with it by Mr R. P. Gregory in conjunction with me. Dominance is usually complete, but at Messrs Sutton's I have seen on two occasions strains containing plants of intermediate leaf-shape, which were presumably heterozygous, for the two types occurred on sister-plants. The leaf-shape is entirely independent of the colours and other features of the plant, and can be transferred bodily from one colour-type to another. Messrs Sutton's varieties “Mont Blanc” and "Sirdar," for example, are sold both in the palm-leaved and in the fern-leaved forms.
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Fig. 7. The two types of leaf found in Primula Sinensis.
17. Leaves and petals normal or laciniated. Chelidonium majus. de Vries (290) and (298), I. p. 134.
This case is interesting in comparison with No. 15. In the Nettle, serration is a dominant, while here laciniation is a recessive. A careful study of the physiological distinction between the two processes would probably lead to important results (cp. Leake, 170, on leaves of Cotton).
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18. Certain leaf characters in Capsella bursa-pastoris. Shepherd's Purse. (Unpublished work of Shull ; about to appear as a publication of the Carnegie Institution.)
19. Various characters in the seed of Cotton. Balls (6).
Many of these are of great commercial importance. Balls (6) gives the following list :
Dominant: Long staple. Regular distribution. Coloured lint. Silky lint. More fuzz.
Recessive. Short staple. Irregular distribution. White lint. Harsh lint. Less fuzz.
He says that all the desirable characters are dominant, and that hence the chances of picking out a stable form by common selection are very small. Individual selection must be adopted.
20. Biennial habit and annual habit. Hyoscyamus. Correns (73).
More research on the relations of annuals to biennials is greatly to be desired. Points of the highest physiological interest are involved. In connection with root crops also some questions of commercial importance are raised. In R.E.C. (19, p. 135) I ventured to suggest that the persistence of “runners” which go to seed in such plants as Beet and Mangel may be due to want of individual selection of pure dominants, and in view of Correns' observarion the probability of this suggestion is increased.
21. Normal stem and fasciated stem associated with peculiar distribution of inflorescences. Peas (Pisum). Mendel (195), R.E.C. (20). In our experience various intermediates occur in F2.
22. Susceptibility to rust-disease (Puccinia glumarum) and resistance to the same. Wheat. Biffen (27, 29).
This is perhaps one of the most important instances to which Mendelian method has yet been applied. Using a variety very susceptible to rust and another practically immune to its attacks Biffen found that F1 was not perceptibly less attacked than the rusty type. F2 showed ordinary segregation, and the green, resistant plants, standing among the yellow rusty ones, formed a very striking spectacle. The recessives bred true and their progeny has remained rust-proof. It has not yet been shown to what the resistance is due. Working with Professor Biffen, Miss Marryat (193) found that the rust-hyphae are checked after entering the stomata of the resistant plants. If, as may be suspected, the resistance is due to the presence of some anti-toxin, the dominance of "susceptibility" must be taken to indicate that the formation of the anti-toxin is prevented by the presence of a factor in the dominant forms, a conclusion which may lead to definite progress in the physiology of disease-resistance.
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23. Flat standard and hooded standard in the flower of Sweet Pea. R. E. C. (22). See Plate V. Some very curious phenomena have been observed in this case, which are described in Chap. IX. The type known as “Snapdragon,” perhaps an extreme form of hood, is also a recessive to the flat type. R.E.C. (20, p. 83).
24. Imbricated petals and stellate or "star"-type. Primula Sinensis. Observations made at Messrs Sutton's and experiments of R. P. Gregory with W. Bateson. (See Fig. 14.)
25. The monstrous condition of the calyx in which it resembles the corolla, seen in “hose-in-hose” Campanula is an imperfect dominant to the normal. Associated with this homoeotic variation, the female organs are more or less completely sterile in certain strains. Correns (76). This subject is discussed in Chap. XI.
26. Abortion of the female organs in the lateral florets of Barley, as found in the 2-rowed types, and the complete or hermaphrodite development of the florets, as in the 6-rowed types (Fig. 8). Tschermak (270) and (275), p. 11. Biffen (30).
This case is somewhat complex. There are three types. (1) Six-row, in which 3 perfect hermaphrodite florets are developed in each spikelet. All set seed and the result is that the ear has 6 rows of seeds. (2) Types in which the lateral florets have anthers but no female organs. (3) The "Abyssinian" type in which the lateral florets contain neither male nor female organs. The types (2) and (3), being able to make seeds only in the central florets of each spikelet, alike develop two rows of seeds. When (1) is crossed with (3), F1 is like (2) ; and (2) crossed with (3) gives F1 also like (2). Some further complexities have been observed, but in general it appears that the dominant factor has the power of partially preventing the formation of the reproductive organs in the lateral florets. The facts may perhaps be interpreted as bearing on the phenomenon of Sex.
Tschermak (275) describes crosses between a 2-row and a “4-row” type. From Professor Biffen I understand that the latter is in reality a lax-eared 6-row type. F1 is 2-row, and in F2 the ratio is 12 2-row : 3 “4-row” : 1 6-row. This is a special case of the ratio 9 : 3 : 3 : 1, lax-ear and 2-row being dominants. Tschermak and Shull (242) regard it as an illustration of the effects of a latent factor.
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27. The two-celled type of fruit is dominant to the many-celled type in Tomato. Price and Drinkard (221).
The case is one of the few in which the genetic behaviour of a meristic or divisional feature has been investigated apart from any complexity introduced by differentiation.
Fig. 8. Cross between Abyssinian 2-row Barley and a club-headed 6-row type. The middle figure shows F1. The length of ear is increased. (From Professor Biffen's specimens.)
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28. Style short, associated with large pollen grains, constituting the "thrum" type, and style long, associated with small pollen grains constituting the "pin-eyed" type. Primula Sinensis and acaulis. Bateson and Gregory (17).
The short styled type has been found in the homozygous condition in P. Sinensis but not yet in P. acaulis. For the relations of these types to the "homostyled" form, see p. 68. Dominance is complete.
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29. Long style and short style in Oenothera. (This difference is probably quite distinct in nature from ordinary heterostylism as seen in Primula, &c.) de Vries (290). The same fact has been observed by Balls (6) in Cotton.
30. Normal long pollen grains with three pores, and rounded pollen grains usually with two pores. Sweet Pea (Lathyrus odoratus). R.E.C. (20, 21, 22). See Fig. 18.
31. Normal anthers, and sterile anthers. Sweet Pea. R.E.C. (20, 21, 22).
With regard to these two last features numerous corplilications occur, which are described in later chapters.
32. Roundness of seed connected with the presence of starch in large elongated simple grains, and wrinkledness of seed connected with the presence of peculiar compound starch-grains. Pisum.
This is one of the most familiar of Mendel's original examples (see Fig. 3). It has been re-investigated by many observers. Correns (60) ; Tschermak (269, &c.) ; R.E.C. (20) ; Hurst (155) ; Lock (172, 173). The F1 seeds made by fertilising an emasculated flower of a wrinkled variety with pollen from a round variety, or vice versa, are generally ordinary round seeds, and F2 shows the common ratio 3 round : 1 wrinkled, the two types being mixed in the pods of the F1 plants.
Among the multitude of varieties of peas now cultivated there is a great diversity both of rounds and of wrinkleds. The interrelations of these several types, even as regards seed-shape, have as yet been imperfectly explored. The degree to which the wrinkles are formed is fairly uniform for any one type, but the various types show different degrees of wrinkling. The differences obviously depend chiefly on the chemical and physical properties of the reserve-materials in the cotyledons, and an analysts of these peculiarities might lead to further discoveries.
Gregory (134) found that the starch in round peas occurs chiefly as large elongated simple grains, whereas in wrinkled peas it is in the form of small grains of irregular shape which are often compounded together (Fig. 9). Darbishire (94) added the interesting fact that in F1 the grains are intermediate, many being large and simple, but round instead of elongated, with an admixture of compound grains. He confirmed also Denaiffe's observations* that wrinkled take up more water than round, but he found that F1, is intermediate in this respect,, and he suggests that the size, the
* Denaiffe, Les Pois potagers, p. 9.
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shape of the grains, and their simple or compound nature, may be governed by distinct factors. He regards the absorptive power as again separable from these features.
Fig. 9. Outlines of starch grains in the different types of peas. The wrinkled contains many compound grains. (From Gregory.) Magnification the same in both figures.
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A third type of pea, of which the purple sugar-peas (sans parchemin) are a good instance, may be described as “indent.” These also have large, simple starch grains. Such seeds are of irregular flattened shape and may be confounded with true wrinkled peas. Their properties are entirely different, and the two sorts must be carefully distinguished. One of their special properties will be discussed in a later chapter, but here it must suffice to say that their genetic properties are essentially those of round peas. Much confusion has been introduced by want of care in distinguishing these types.
Intermediates, which on casual sorting, cannot be classed either as round or wrinkled, sometimes occur. Some of the round types (e.g. Victoria Marrow) contain a large proportion of such seeds. Their peculiarity is almost certainly due to environmental influence, though obviously the liability to this affection may be transmitted. When such seeds are found in F2 from a cross between thoroughly round and wrinkled varieties, the pitting, when it exists, generally affects all the round seeds of the pods in which it occurs. With experience such pitting can immediately be recognized as distinct from the true genetic wrinkling, and in our experiments the results of a further sowing have repeatedly confirmed the judgment made by inspection of the seeds.
A complete account of all the phenomena would run to great length. The interrelations of round and wrinkled seeds are to be recommended as offering perhaps the most favourable example for an investigation of the chemical nature of a genetic factor. The wrinkling is evidently the consequence of a particular method of drying, and this must depend on the nature of the reserve-materials. A first step would be to determine the relative amounts of sugar and starch in the two chief types. It is natural
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to suppose that the wrinkled peas are those in which the transformation of sugar into starch has gone less far than in the round peas ; but, as much starch is formed in the wrinkleds, one ferment having this transformative power must be present in them. Hence we are led to suppose that in the round pea a second ferment is present which can carry the process further. As offering an attractive problem in physiological chemistry the phenomena are recommended to those who have the requisite skill to investigate them.
33. Starchy endosperm giving a full, rounded seed, and sugary endosperm giving a shrivelled and wrinkled seed. Maize. Fig. 10. de Vries (290) ; Correns (63) ; Lock (172, 174).