Previous Chapter : I. Introductory. Mendel's discovery
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| Seed-coats colourless or greenish in white-flowered plants. In plants with coloured flowers one or more of three distinct kinds of pigments always present : (1) a purple, occurring in spots, (2) a brown, distributed either generally over the surface, or in bands (as in Maple peas), (3) an insoluble greenish grey, distributed over the whole tests. Neither (1) nor (2) can be developed in the absence of (3), but traces of (2) may sometimes be seen in white-flowered plants. There are separate factors for (1), (2), and (3), of which (1) and (2) may be carried by the whites (Lock, 176).
Cotyledon-colours are yellow, and green. Yellow is a dominant, and heterozygotes are indistinguishable from homozygous dominants. In rare cases green has been seen as exceptions in F1, but these are probably due to abnormal conditions. Many modern varieties have cotyledons patched with green and yellow. Genetically these are greens which show a special liability to bleaching. If protected while ripening they remain green. Colours of Pisum have been chiefly studied by Mendel (195) ; Tschermak (269, 27I-3) ; Correns (60) ; R.E.C. (20) ; Lock (172, 173, 175-6) ; Hurst (155). Polemonium. Correns (70) found the white var. of P. coeruleum dominant to the yellow of P. flavum ; and the blue type of coeruleum x flavum gave F1 blue. It may be inferred that the yellow of flavum is a chromoplast colour, and that the blue anthocyan dominated as usual. Hybrids sterile. Primula. P. Sinensis exists in a long series of colour-types the relations of which are still being investigated by R. P. Gregory in con- [conjunction] < 041 > junction with me. Some of the more striking facts are referred to in later chapters. White flowers with green stem constitute an albino, recessive to all colours. The magenta shades have a factor epistatic to crimson and pink. Blue is hypostatic to all the red shades The whites which have red or reddish stems are dominant whites, showing only a pale shade or tinge of colour in F1. Deep colours cannot appear on stems that are not red except in the white-edged “Sirdar” (q.v.). Salvia Horminum. Purple, red, white, related as in Pisum, &c. Saunders, R.E.C. (20). Triticum (Wheat). Red chaff is dominant to white chaff. Grey chaff is epistatic to red and dominant to white. Tschermak (270) ; Biffen (27) ; Spillman (247). Verbascum blattaria. Yellow (a sap-colour) dominant to white. Shull (241). Viola. White is recessive to colour (de Vries, 290) in V. cornuta. The brown seed-colour of V. papilionacea is dominant to buff of V. hirsutula, and the purple of the capsule of hirsutula to the absence of purple in papilionacea (Brainerd, 41). Zea (Maize). Yellow endosperm dominant to white. Blue in aleurone layer an irregular dominant to absence of blue. (Definite exceptions are frequent.) Red pericarp, a plant-character, dominant to absence of red. The relations of the striped types have not been clearly determined. Correns (63) ; Lock (172, 174). Colours of Animals. Man. Eye-colour (q.v.), Hurst (161), Davenport (107). Albinism (q.v.) is doubtless recessive, but in man its descent is complex and has not yet been elucidated. Red hair is recessive to dark hair and perhaps to ordinary brown (see Hurst, 162). Cattle. Red-roan is a heterozygote of red and white (Wilson, 311) ; and blue roan is similarly related to black and white. Spillman (249) suggests that black is dominant to red. Cats. Red is dominant to black in males. Tortoiseshell is the corresponding form of the heterozygote in females. Doncaster (109). Dilution-types, blue, and cream, recessive to saturated colours. As to eye-colour see Przibram (224). Mice, Rats, Rabbits, Guinea-pigs. Colours fully discussed in later chapters. Chief papers by Allen (1) ; Bateson (10) ; Castle (48, 53) ; Crampe (83, a) ; Cuénot (84-9) ; Darbishire (90) ; Durham (116) ; Hurst (157) ; Mudge (204). Albino recessive in all cases. A piebald type dominant to self-colour exists in mice (Durham, 116). < 042 > Horse. Chestnut recessive to bays and browns. Relations of these two dominants to each other not clear. Hurst (158). Pigs. Several notes published by Spillman (249, 251-2). White is usually a dominant to colour in domesticated races* but piebalds are frequent in F1. The relation of black to red is not yet clear. The white belt, characteristic of certain breeds, is, according to Spillman, due to a complementary pair of factors which may be separately carried by self-blacks. He makes the interesting suggestion that the appearance of the belt may be a “reversion” to a condition like that of the Indian Tapir (251). The colour of the wild boar is dominant to the red of Tamworth and segregates normally from it (252). The wild colour is presumably due to an “agouti” factor like that of the rodents. Sheep. From such fragments of evidence as I can find it seems that the white of ordinary sheep is not, as in the pig, a dominant to colour, but a recessive. From Darwin's record (An. and Plts. II. p. 4) of the appearance of all black sheep from a cross between white Southdown ewes and a Spanish ram with two black spots, it may perhaps be inferred that the black colour is due to complementary factors. Black face and white face give a speckled face in the heterozygote. The dark ring round the eyes depends on a separable factor (Wood, 312). Fowls. Colours very complicated and genetics imperfectly understood. Whites are of various kinds, one being dominant and at least two recessive. Colour depends on complementary factors which may be borne by whites. Black is an imperfect dominant to black-red. Brown-red a dominant to black-red (Fig. 11). Blue is a heterozygous colour of black and a splashed white. The red and yellow pigments of the black-red cock may be replaced by white, thus giving the Silver Duckwing, but in the hen the red of the breast is not thus replaced, and the Duckwing hen differs from the blackred in having the yellow of hackle and mantle replaced by white. These replacements may occur as consequence of recombination in F2. from crosses between white and coloured breeds, whence it is to be inferred that the replaced reds and yellows depend on a special factor. Pencilling is a dominant to its absence, and various mottlings are also dominant. The descent of colour is influenced in some cases by sex in ways not yet clear, and in both sexes heterozygous types occur. The relations of the buff of Cochin (and of other breeds derived from it) to other colours are not yet known. As regards colour of the down, the brown striped condition is dominant to the pale brown down associated, for example, with Wheaten. Both types of down may occur in the same breed (e.g. Indian Game), and the fact seems to have no relation to the adult plumage. R.E.C. (21, 22). * Mr Staples-Browne has given me confirmatory evidence. < 043 > The black body-pigmentation of the Silky is a dominant, but may be inhibited by another factor, the descent of which is sex-limited. See Chap. X. Another sex-limited descent is to be found in the relations of Cuckoo to black, but the details have not been ascertained (Spillman, 249, 253, a). The daw-eye is recessive to red. The dark iris is usually a dominant to red. Red ear-lobe is an imperfect dominant to white. R.E.C. (19). Principal papers dealing with these features are R.E.C. (19-22), Hurst (156) ; Davenport (101). Pigeons. Black is dominant to blue. Relations of red and yellow not clear. Black and blue are dominant to white of Fantail ; heterozygotes generally, if not always, having some white. Chequering dominant to its absence. The white rump of the Rock-pigeon is dominant to blue rump (Staples-Browne, 255). Canaries. Presence of black, as in green and pied types, dominant to absence of black as in the various yellows and cinnamons. The pink eye of cinnamons is recessive to black eye, with a sex-limited inheritance. There are probably several heterozygous colours, but in order to determine these the genetic interrelationships of the various Yellows, Jonque, Mealy, must be worked out. The “cap” and lacing of the Lizard are dominants. (Noorduijn, 213-5 ; Davenport, 105 ; F. M. Durham, unpublished.) Axolotl. Crosses between normal and albino gave dominance of pigmentation. Subsequent generations showed remarkable and as yet unique features. In F2 dark larvae to white larvae were 3 : 1 ; but the white F2 larvae, though remaining red-eyed, acquired a certain amount of pigment, sometimes distributed as a metameric chequering. No thorough albino occurred in F2. When however these chequered albinos were bred with a true albino, the latter was found dominant and true albinos were produced (Hacker, 143-4). Hacker compares this case with the phenomena seen in Mice, &c., but there is an essential distinction in the fact that in all other instances true albinos come in F2 and in the dominance of the true albinism over the chequered character. It would be interesting to see whether the development of pigment in the F2 whites is in any way dependent on conditions. Lepidoptera. Bombyx mori (Silkworm). Brown colour seen in a dark variety of the moth was proved to bean imperfect dominant (Coutagne, 83, p. 122). The larvae have many colour-types. Coutagne used a dark “moricaud” variety, a variety with transverse stripes, and ordinary white larvae. Both the coloured types are dominant to white, but when the dark self-colour factor and the stripe-factor are present in the same larva the stripes show on the dark ground-colour (ibid. p. 142). Toyama (268) also made many experiments with the colours of the larvae. He found striping a dominant over plain white. In certain F2 families from striped x white a new pale < 044 > form was produced (p. 349). The ratio of striped : common marked white : pale unmarked white is as Shull points out (242) 12 : 3 : 1 (actually 1463, 363, 126). I incline to interpret this as signifying that the 12 striped were in reality of two kinds, in the ratio 9 : 3, but that the distinction between the common and the pale was not easy to detect in the striped class. On this view the striped parent was a “pale.” Shull regards the paleness as “latent” in both parents. As regards the colour of silk there is a complication. In Toyama's experiments yellow was always a dominant to white. Coutagne sometimes obtained this result, but (l. c. p. 123) the white of a race called “Blanc des Alpes” proved to be dominant to yellow. Abraxas grossulariata (Currant Moth). The type is dominant to var. lacticolor (q.v.), Doncaster (111, 114). See Plate 1, figs. 1, 2. The peculiarities of this case are discussed in connection with Sex. Mr L. W. Newman has been good enough to send me information as to a cross between A. grossulariata and the var. varleyata. This is a nearly black suffused form (see Porritt, Ent. Rec. XV. p. 10). F1 was typical grossulariata, and in F2 there were 24 typical and 7 varleyata (4 m, 3 f). Angerona prunaria. The dark-banded var. sordiata dominant to the normal, reticulated type. In the heterozygotes the lighter bands are more or less reticulated. Doncaster (111). See Plate 1, figs. 7-10. Xanthorhoe ferrugata. The form with purplish band is dominant to that with black band. Prout, L. B. (223). Hemerophila abruptaria : the dark var. may be inferred to be a dominant to the type, from the experiments of Harris (147). Plate I, figs. 5, 6. Amphidasys betularia (Peppered Moth). The normal is almost certainly recessive to the black, or doubledayaria form (see, for example, the records of Main and Harrison, 192). Plate I, figs. 3, 4. Triphaena comes : the reddish form is recessive to the melanic (see Bacot, 3, and Prout, 222). Callimorpha dominula : red of the hind wings is dominant to yellow. Standfuss (253, b, p. 222). Mr L. W. Newman has kindly given me information that he bred 34 reds and 10 yellows in F2. Aglia tau : type is recessive to the dark form lugens. Standfuss (253, b, p. 311), (see Chap. X, for the sex-distribution of these varieties). Lasiocampa quercus (Oak-egger). The heredity of colours of the hairs of the larvae has been investigated by Bacot (4) and Warburg (302). Several varieties were studied and their genetic interrelationships are not altogether certain, but it appeared that red fur of var. sicula was dominant to the white of the var. meridionalis. When English and French races were crossed, various blend-forms were produced in F1. Coleoptera. Most of the observations thus far made relate to Phytophaga. Complications were met with in all the cases investigated by Miss McCracken. I have not been able clearly to understand the exact procedure followed in the matings and must refer the reader to the original papers. < Plate I > 
1. Abraxas grossulariata. 2. Ditto var. lacticolor. 3. The var. Doubledayaria. 4. Amphidasys betularia. 5. Dark var. of 6. Hemerophila abruptaria. 7 and 8. Male and female var. sordiata of 9. and 10. Angerona prunaria, male and female. < 045 > Lina lapponica. Two forms occur, one having elytra spotted with black on a brown ground, while in the other the elytra are entirely black. The latter is recessive, and the formation of the brown pigment in the ground is thus due to a dominant factor. McCracken (189, 199). Melasoma (Lina) scripta. A totally black form was found to be recessive to the spotted type. Two intermediate conditions may occur, one of which may be a homozygous type (see original). McCracken (191). Gastroidea dissimilis. Two forms, either deep blue-black, or shiny bright green. The latter is recessive. Curious complication as regards numerical results. McCracken (190). Leptinotarsa decemlineata (Colorado potato beetle). Evidence obtained by Tower (266, pp. 275-9) indicates that a variety called by him pallida behaves as a recessive. The var. melanothorax of the species L. multitaeniata also proved to be a recessive to its type (ibid. pp. 284, 292, 293). Various other more complex phenomena are recorded (q. v.). Crioceris asparagi (Asparagus beetle). Each elytron has three yellow areas or spots on a blue-black ground. The upper spot is sometimes united to the middle one. This condition proved to be recessive to that in which the spots are separate, but all intermediate conditions occur [being presumably heterozygous]. Lutz (182). Mollusca. Helix hortensis and H. nemoralis. The unbanded variety is dominant to the banded types in both the species, sometimes completely, sometimes partially. Generally also red ground-colour is dominant to yellow or brown, but this effect may diminish with age of the hybrid individual. Lang (167-9). For details as to hybrids between the two species see (169). As regards dominance of colours very little in the way of general rule can yet be predicated, nor till the chemistry of pigments is much better understood is it likely that such general rules will be discovered. It may, however, be remarked that actual albinism, the total absence of pigmentation, is always, so far as we know, a recessive character in both animals and plants. Curious cases nevertheless are known both in animals and plants where a partial whiteness, which we should a priori imagine to be a kind of albinism, behaves as a dominant*. Another fact of a somewhat paradoxical nature is to be seen in the behaviour of some of the very deep colours, red and purples, characteristic of the flowers and other parts of some garden strains. These more intense colours both in Primula Sinensis, in the Stock, and in the Sweet Pea (and doubtless * See Chap. V. < 046 > Cyclamen) are recessive to the paler and more commonplace tints*. Though in the case of colours in plants which are due to the development of pigmented sap, albinos are recessive to the coloured types, the yellow or cream colour due to the presence of yellow chromoplasts is recessive to the colourless condition of the chromoplasts. Hence we find, what at first seems paradoxical, that white flowers are dominant over cream-coloured flowers. Yellow dependent on sap-colour is dominant to the corresponding white. With regard to the behaviour of black pigment, which might naturally be supposed to have similar genetic properties in the various animals, no quite satisfactory general rule can be laid down. The presence of black pigment is commonly dominant to the absence of black, as in the racehorse, where chestnut, namely the absence of black “points” is recessive to the presence of such “points” as in bays and browns. Most cases, however, such as that of the mouse, and other animals in which black pigment exists intimately mixed with other pigments are not so simple as this and involve special problems. In so far as the features of those cases can be expressed in the simple terminology hitherto used, these blacks must be classed as recessive to the normal colours. Further particulars will be given in the chapters on Colour. Preliminary Deductions from Mendelian Phenomena. It will be observed that animals and plants, as such, do not show any difference in their manner of heredity. Inheritance on simple Mendelian lines may be followed by characters of very diverse kinds, such as height, shape, chemical constitution, colour, and several structural features. In view of such a list the important question arises whether there is any distinct category or class of characters to which the Mendelian system does not apply. Various possible limitations may be discovered when the phenomena have been more fully examined, but it may be stated at once that no such class of characters has hitherto been identified. * In Antirrhinum Miss Wheldale finds that the deeper magentas are recessive to the ordinary magentas, but in the crimson-red series the paler are recessive to the deeper tints. < 047 > As yet only one example of a character which can at all readily be interpreted as meristic in nature has been shown to have a Mendelian inheritance. This is the case of the reduction in number of the human phalanges in brachydactyly. We speak of a character as “meristic” when it manifests itself in respect of the number of parts into which the body or one of its organs is divided. Meristic characters are in several ways distinguishable from other features of bodily organisation. The physiological occurrences which result in meristic variations are in all likelihood distinct from those which produce substantive changes, and exceptional interest would attach to any investigation of the genetic properties of such variations. Polydactylism is of course a meristic feature, but it may involve something more than a divisional change, pure and simple, since change in the number of digits is usually accompanied by change in the distribution of differentiation. A case in which the disturbance of differentiation is not so evident is provided by the cross between Oxalis tetraphylla, much cultivated in Germany as Glücksklee, and one of the forms with three leaflets. This cross was partially investigated by Hildebrand*, who used O. latifolia. He found that the 3-fold character was an imperfect dominant, the leaves being 3-fold with the exception of occasional 4-fold leaves which appeared for the most part at the flowering period. The hybrids were fully fertile, but their progeny has not been studied. Satisfactory meristic cases from which all confusing elements are eliminated must be rare, but it is greatly to be hoped that they will now be searched for. It is most desirable that cases of difference in the ground-plan numbers of some radial type will be found amenable to experimental tests. Here the problem may be seen in a somewhat simplified form on account of the elimination of serial differentiation†. * Hildebrand, Jenaische Ztsch. f. Naturw. 1889, XXIII. N. F. XVI. p. 56. † Since this paragraph was set up Price and Drinkard's (221) evidence has been published showing the dominance of two chambers in the fruit of the tomato over the many-chambered condition. More evidence as to such cases would be welcome. Drinkwater's recent discovery as to the bones of the brachydactylous fingers, showing that the middle phalanx is actually formed as a distinct bone which afterwards unites with the distal phalanx, raises considerable doubt whether the variation in that case is meristic after all. < 048 > Respecting the genetics of one most interesting class of variations evidence is scanty. This is right- and left-handedness. From Mayer 's* observations on Partula (Gastropod) we learn that parents of either twist may bear young of either twist. The numbers in the uteri were so small that the absolute numbers are insignificant ; and it may be an accident that no mixture of types was found in anyone uterus. Lang† bred numerous left-handed Helixpomatia with each other and obtained thousands of young, all right-handed, which in their turn again produced exclusively right-handed offspring. Direction of twist is a fundamental meristic phenomenon, being as Crampton and Conklin have proved, determined as early as the first cleavage-plane in the egg ; and great light on the problems of cell-division might perhaps be obtained if the inheritance of these differences could be determined. The only case we have attempted to study, that of Medicago, in which the fruits are right- or left-handed spirals according to species, proved unworkable, perhaps on account of the minute size of the flower and the roughness of the manipulations. Lutz (181) has collected facts as to the inheritance of the mode in which the hands are clasped, whether the right or left thumb is placed uppermost. No definite result was obtained, but effects of heredity were somewhat marked, though neither condition bred true. A fuller analysis should be attempted, taking families separately. When the Mendelian principles were first rediscovered the suggestion was made that though the system might apply to the unions of pure races, there was no certainty that such rules apply to the uncontrolled matings of natural forms. The objection was not one which was likely to have weight with those who had an acquaintance with genetic phenomena, but it had undoubtedly an effect in postponing general recognition of the importance of Mendel's discovery. Categorical proof of the invalidity of this objection is now provided by one of the cases referred to above-that which concerns the heterostylism of Primula. It is scarcely doubtful that in the Primrose nearly every plant arises by the “legitimate” union of long- and short- * Mayer, A. G., Hem. Mus. Comp. Zool. Harvard, XXVI. No. 2, 1902 |