Genetics - Encyclopedia




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"==GENETICS==This term was proposed at the third conference on hybridization 1906 to denote the study of heredity and variation. In that sense it has been generally adopted, and by extension is understood to include the physiology of reproduction and the art of breeding. Though such inquiries have been pursued from the earliest times, the development of a special branch of science relating to them is recent. The primary incentive was the hope that by applying accurate methods of observation and experiment to the course of heredity and variation a more precise knowledge of evolutionary processes might be acquired. Modern theories of evolution are based on the assumption that species have arisen by descent with modification, and that the constancy and diversity which living things manifest in their reproduction provide a sufficient basis for that conception. It is significant that as a result of the preliminary work done under the new inspiration attention has been largely diverted from these more philosophical aims. Beliefs current among naturalists, especially as to the nature and incidence of variability, were at once found to be widely incorrect. The scope and character of these discoveries are referred to below. Their immediate consequence has been that the development of evolutionary theory is tacitly suspended or postponed, and activity is concentrated on the exploration of genetical physiology, the theoretical evaluation of the knowledge thus gained being relegated to the future.

In these researches several methods of investigation are available. Modern genetics began with an attempt to observe empirically the course of contemporary variation from type; but though observations of this class have proved valuable in a preliminary survey, and have often been of use as indicating material for more prolonged investigation, the main advances have been accomplished by either (i) experimental breeding or (2) cytology. Important sidelights on genetical problems have also been obtained through the study of developmental mechanics (Entwicklungsmechanik) by experimental methods.

(1) Experimental Breeding. - The great stimulus to this method of research was given by the rediscovery in 1900 of Mendel's paper (see 18.115). Heredity, long regarded as a fortuitous and seemingly lawless phenomenon, was proved to follow regular principles which could in great measure be ascertained by experiments properly planned. A vast field was at once thrown open for investigation. Mendel's success was made possible by his genius for simplification. Working with peas he made crosses between distinct varieties and watched the descent of their numerous characteristics, fixing his attention on each separately, and disregarding other differences. He then found that numerous distinctive features behaved in descent as if they were transmitted as units. These determining elements or units are referred to as factors or " genes " (a term especially used by American writers, the equivalent of Johannsen's Genen). The differences determined by these factors can commonly be shown to be treated in heredity as pairs of alternatives or opposites, such as tall and short, coloured and colourless, hairy and smooth, each germ-cell being usually pure in respect of one or other of the contrasted characteristics. This is the principle of allelomorphism, and the members of such pairs are called allelomorphs. The zygote, formed by two germ-cells united in fertilization, may be made up of two germ-cells alike in respect of any given pair, in which case it is said to be homozygous in that respect, or it may be a heterozygote if the uniting pair of cells are unlike. Before the germ-cells of the heterozygote are formed a process of segregation occurs, and there is a dissociation between the opposing elements introduced at fertilization, such that the resulting germ-cells are again in normal cases pure in regard to each allelomorph.

After the rediscovery of Mendel's work, progress was rapid, and it was soon found that similar principles of descent apply to a great range of characteristics by which living things are distinguished. The number of forms of life studied is now very large, and includes most of the kinds of plants and animals which are readily amenable to experiment or observation. Man is evidently no exception, and we already know that certain features of human coloration, especially of hair and eyes, and several congenital abnormalities are transmitted according to the Mendelian scheme, some being dominant, others recessive.

Scarcely any satisfactory opportunities for studying the genetics of the lower plants (ferns, mosses, Algae, etc.) have yet occurred, but one example has been described in a unicellular Alga (Pascher). Of the features by which animals and plants are distinguished most have now been shown to be dependent on segregable elements. Reservation must be made in regard to differences which are simply quantitative, for there is a good deal of evidence suggesting that the elements by which size and weight are determined do not often form themselves into simple allelomorphic pairs. A similar doubt exists in regard to numerical or meristic distinctions.

Differences in instinct and other characters dependent on nervous mechanism are not, as such, distinct in their genetical behaviour, and some have been proved to depend on segregable factors or elements. In several breeds of fowls the hens are devoid of maternal instincts, and do not sit on eggs. This characteristic is recessive to the normal instinct, and segregation takes place in regard to it. The same is true of the pacing habit in horses as opposed to the trotting habit. The " waltzing " habit of certain Japanese mice is recessive to the normal, segregates from it and breeds true when it reappears. This example is interesting, since the abnormality is almost certainly a consequence of deformity in the semicircular canals of the ear.

As to the descent of the normal mental attributes of man little is known with accuracy, but several abnormalities of the nervous system are known to follow modes of descent which prove them to be subject to segregation. Feeble-mindedness is a recessive condition which breeds true. Paralysis agitans is also a recessive. Hereditary chorea descends as a dominant; colour-blindness and a form of night-blindness may also be termed recessive (see SEX). In heterozygous combination with the normal there is segregation, but the descent of these conditions is complicated by sex.

It will readily be understood that though the determining factors may be transmitted as units, the distinguishing characters of animals and plants must be often due to the association of many independent units. Of these some produce their effects separately; but not rarely, though independently transmitted, two or more unit-factors may be complementary to each other and combine to produce a joint effect, or "compound character" as it is sometimes called. Such complementary factors if separately present in the organism without their complement need not manifest their presence at all, and it is then only by breeding tests that their existence can be demonstrated.

Organisms may now be represented as aggregates of units which confer upon them their various attributes. The degree to which an organism may be thus resolved is as yet undetermined, but there is presumably a limit to the process, and it is natural to suppose that the detachable elements are implanted on a basis which for a given type is irreducible.

Table of contents

Reversion

Conceptions, formerly vague, now acquire an exact meaning. For example, reversion or " throwing-back " to an ancestral form, previously regarded as a mere caprice of nature, can at once be perceived to be due to one of two definite causes which operate with regularity. The reversion is either (a) the reappearance of a recessive characteristic, or (b) it is the consequence of the reunion of complementary factors which, though both present together in the ancestor, had been separated by variation and transmitted in distinct strains. For example, when a red-haired child is born to dark-haired parents the fact proves that the two parents are heterozygous in respect of the recessive red, which reappears when two germ-cells carrying it unite in fertilization. Moreover, if the statistics of a considerable number of such families of children were collected and added together it would be found that the proportion of red-haired was approximately a quarter of the whole. The mere fact that one or both of the parents traces descent from a red-haired ancestor is not the cause of the reversion - for if either of the parents were homozygous in dark hair the red would not have reappeared.

The reversion to an actual or supposed ancestral form consequent on the meeting of complementary factors is less common in the ordinary practice of breeders, but is frequently seen in experimental crossing. When two white orchids crossed together give a coloured flower in F 1 , or when a rose-combed fowl bred with a pea-combed bird gives chickens with the walnut comb of the Malay fowl, the production of the unexpected colour or structure is due to complementary action of two independent factors. But the old interpretation of the phenomenon as a consequence of such an ancestor having occurred in the pedigree is illogical and misleading. In the case of the walnut comb, for instance, it is quite possible that either or both of the parent breeds never had a Malay ancestor. The production of a new form by the meeting of complements should be regarded, like the properties of a chemical compound, simply as the empirical consequence of a certain combination of units, without reference to the previous history of those units.

Purity of Type

Of greater importance, both theoretical and practical, is the fact that it is now possible to assign a precise meaning to this expression. To the pre-Mendelian evolutionist purity was always a matter of degree, which might be gradually and, as it were, asymptotically approached in successive generations of selection, but never actually attained. The practical breeder also has always regarded purity as a property necessarily dependent on a long course of selection. Purity is now seen to be the condition of the animal or plant which is formed by the union of gametes bearing identical units. In respect of any allelomorphic pair purity may thus be conferred, though in respect of other pairs of units the same organism may be impure, i.e. heterozygous, or, in ordinary parlance, cross-bred. This is the central fact of Mendelism, and on it Genetics is based.

The question of purity must therefore be considered separately for each pair of units. A thoroughbred horse, for example, may be pure in a number of characteristics which go to the making of the breed, but it may be impure in, say, colour. A chestnut horse, however, of whatever parentage, is pure-bred in colour, since that colour is the lowest of the series of horse colours, and chestnuts bred together give chestnuts only. By selection the likelihood of producing purity is increased, but, as will subsequently appear, no amount of selection can ensure purity. On the other hand, purity in respect of any character may be attained at once in any mating by which gametes of similar factorial composition happen to be brought together in fertilization. From this proposition the corollary follows that the combination of two strains pure in any given respect will give a family uniform as regards the character considered, and the uniformity of such cross-bred families, especially when one of the parents contains few dominant factors, is in practice one of the simplest and most convincing tests of purity.

Genetic Analysis

By the institution of a series of crosses with varieties and study of the composition of the succeeding generations an analysis of the factorial constitution of a given type can be made. The numerical proportions or ratios in which the several combinations of characters are represented, the number of these terms in the series, and their respective genetical powers of transmission furnish the data from which the nature and number of the factors comprising the parental type may be determined. In the earlier article on Mendelism (see 18.115) some of the simpler ratios and their significance are explained, but examples of a much higher order of complexity are often encountered. The unravelling of these complications has led to some important discoveries. The many ways in which it may come to pass that two or more terms in a series of factorial combinations may be indistinguishable from each other cannot be enumerated here, but a knowledge of some of the more significant causes of disturbance of what may be called the normal ratios (9: 3: 3: 1; 9: 3: 4; 27:9:9:9:3:3:3:1, etc.) is essential to a proper comprehension of Genetics.

Cumulative Factors

From certain crosses (especially of cereals) into which only one pair of differences had apparently been introduced it was observed (Nilsson-Ehle; East) that the recessives reappearing in F2 were only I :15 instead of the usual I :3. Investigation proved that from the dominant side two factors with identical functions, though belonging to distinct pairs, had been introduced. Consequently, among the dominants in F2 were some containing both these factors and others having one only. Various results suggest that this multiplication, or better, accumulation, of similar factors is a phenomenon of common occurrence, and that the process may be extended in special cases.

Inhibiting and Lethal Factors

Many factors act by producing a negative result, inhibiting the development of some character, the determining elements of which are present though their action is not perceptible or largely diminished. Of these the most easily demonstrable operate by inhibiting the formation of colour. The white pigment of the coats of animals and the feathers of birds, or of flowers, for example, is commonly due to the absence of the elements necessary for the formation of colour, but both in animals and in plants varieties have been found which are white, or nearly so, not through absence of pigment, but through the presence of factors which, in some way not yet defined, inhibit the production of the coloured pigments. From some matings a mixture of white individuals may be obtained, which to the eye look alike, or nearly so, though they represent various factorial terms and are genetically dissimilar. The process of inhibition may be carried much further, and there are well-established instances in which the animal or the plant cannot live if it is homozygous (containing two " doses," in popular terms) for a given factor. The classical instance of such lethal factors, as Morgan has called them, was met with in the breeding of yellow mice (Cuenot; F. M. Durham). Mice with yellow coats, bred together, give a majority of yellows, but always throw a proportion of some other colour - for example, chocolate or black. Since in mice yellow is a dominant, it is clearly caused by a factor which the gametes can carry. But the union of two gametes, both carrying this factor, does not give rise to a viable animal. It was suggested that two such gametes could not unite in fertilization, but later work has practically proved that these fertilizations occur and that the resulting embryo perishes at an early stage (Ibsen). The physiological action of the yellow factor in causing death is not known. In plants the " golden "-leaved varieties are comparable. They cannot breed true, but throw 2 yellow: i green. The purely yellow term is missing, and is clearly not viable (Baur). The suggestion has been made that the yellow factor acts not merely negatively by diluting the amount of chlorophyll, but by inhibiting its formation, probably producing a body with this specific power. This is the more likely since golden varieties in dull weather turn almost a full green, whereas in sunlight they bleach to a full yellow, the fact indicating that the production of the inhibiting body is promoted by sunlight. Two doses of this factor kill the plant altogether, probably during embryonic life.

Linkage

At an early stage in these inquiries it was observed that factorial units belonging to separate allelomorphic pairs are not always distributed independently among the gametes of a heterozygote, but that some combinations occur regularly with a greater frequency than others. The next step was the discovery that this linkage depends on the association of the linked factors in the parent from which the heterozygote was formed. For example, if a form AB is crossed with ab the normal expectation is that the double heterozygote AaBb will form gametes AB, Ab, aB, ab in equal numbers; but if there is linkage between A and B, then the parental combinations AB and ab will be more frequently represented in the gametic series than the other, or " cross-over " combinations, Ab and aB. But if the original cross were in the form Ab x aB, then the most frequent gametes will be Ab and aB, the cross-overs, AB and ab being the rarer. This observation forms the starting-point from which modern genetical theory has been very largely developed. The terminology followed above is that introduced by T. H. Morgan, to whom progress has been especially due. It is sometimes convenient to distinguish the case in which the two dominants (AB x ab) are introduced together by the parent as coupling, and the converse (Ab x aB) as repulsion, but the physiological process is now recog nized as being clearly the same in both cases, and there is no difference in the numerical proportions in which the parental combinations respectively reappear. It should be observed that the factors thus linked have plainly no connexion with each other as regards the effects which they produce in the zygote, but may concern the most dissimilar characters. For instance, in the example first observed the linkage was that between the factor which makes the flower of the sweet pea blue or purple (as distinguished from red) and that which makes the pollen grains long (as distinguished from round). According as the proportion of cross-overs is small or large the linkage is more or less complete. If both parental and cross-over terms are equally common there is no linkage. The most satisfactory test of the linkage-ratio is obviously provided by breeding the double heterozygote (Aa-Bb) with the double recessive (aabb), and this mating should be carried out reciprocally since it is known that in plants (e.g., Primula sinensis) the male and female sides of the same plant may show different degrees of linkage (R. P. Gregory), and that in animals (e.g., Drosophila and the silkworm) crossing-over may be entirely absent in one sex though occurring in the other.

Allelomorphism: Multiple Allelomorphs

Apart from linkage, segregation is always a separation of units affecting the same character, and from a very large range of observations it is possible to represent the distinction between the allelomorphic pair as one in which a positive element separates from a negative. In other words, allelomorphism may commonly be conceived as a difference which consists in the presence of something on the one side and its absence on the other. This conception is applicable whenever there is definitely pronounced dominance. It is natural that the characteristic which possesses dominance should be looked upon as due to the positive or present element, the recessive being the consequence of its absence. Nevertheless there is as yet no strict proof that this representation is physiologically correct. For since we know that many factors may operate by inhibition it is always possible to invert the conventional representation and, by putting negative for positive, to make a factorial scheme which equally agrees with the observed results. Conventionally, for instance, the tall pea is represented as either TT (homozygous) or Tt (heterozygous), the dwarf being tt, from which the positive element T tallness is absent. But we cannot positively declare that the dwarfs may not be TT homozygous in the presence of an inhibitor T, whereas the tall plants might be either Tt heterozygous or tt homozygous in respect of the absence of this inhibitor. The significance of this alternative mode of representation will be apparent when the application of factorial systems to evolutionary theory is attempted (see MENDELisM). But when the heterozygote is intermediate between the two homozygous forms the " presence-andabsence" method of representation cannot be applied with any confidence. From the existence of such cases and from certain other considerations it has been urged, especially by American geneticists, that the method of representation by presence-and-absence is incorrect, and that a negative allelomorph should be treated as a real entity. There is no valid means of deciding this question as yet. The probability is perhaps that the absence should always be regarded as relative only. As a mode of symbolic expression the representation of the two allelomorphs as differing quantitatively is often convenient, though certainly not universally applicable.

Allelomorphism is, as the term implies, a relation between two alternatives, and in any one zygote there can be no more than two. Nevertheless there are instances in which the same unit-factor enters into heterozygous combination with various alternatives in different zygotes, and each of these may thus be in allelomorphic relation with it. Alternatives composing such a group of possibilities have been termed by Morgan multiple allelomorphs, and this expression is commonly adopted. Its use, however, makes the application of the term multiple " to " factors " in a totally different sense a probable source of confusion, and for this reason the word cumulative or some equivalent is there to be preferred, as suggested above. The distinctions which together make up a set of multiple allelomorphs may commonly be recognized as a series of quantitative differences, the character affected being throughout the series the same. One of the most familiar illustrations is provided by the degree of albinism in rabbits. The fully albino form is white with pink eyes, but there is a variety called Himalayan, which, though born white with pink eyes, acquires some chocolate pigment in certain parts. Himalayan is dominant to albino but recessive to the ordinary coloured types. If a coloured type is bred with Himalayan the heterozygotes so raised cannot, when interbred, throw albinos, nor can heterozygotes raised from coloured X albino throw Himalayans, even though the albino used as their parent had itself been extracted from Himalayans. The degree of albinism put in by the parents comes out in F2 and in the same degree. Hence it is not possible from similar parents to breed all three kinds, but, on the other hand, each family can contain at most two of them.

This phenomenon can be interpreted in either of two ways. The Himalayan pattern may be regarded simply as a quantitative diminution or fraction of the sum total of colour needed to make the selfcoloured type. The real albino is thus produced by the absence of the whole unit needed for colour, and the Himalayan by the absence of part of this total. It is then obvious that the heterozygote, coloured X albino, could never produce a Himalayan unless the colour-complex broke up again de novo. But on the analogy of the behaviour of other colour patterns the self and the Himalayan might: be conceived as each consisting of two units: one for colour and one a factor determining its pattern, intensity or distribution. If there were a very close linkage between each " pattern " factor and colour the observed facts could then be represented; but by continued breeding the supporters of this view would expect the missing crossover eventually to appear as either a Himalayan associated with recessive albinos or an albino associated with recessive Himalayan. On the ground of simplicity the former view seems preferable. The significance of these two alternatives will presently appear.

More complex illustrations of these possibilities have been described by Nabours in certain grasshoppers (Paratettix). The species studied presents a long series of colour forms, and experimental breeding showed that with certain exceptions all the pure forms behaved as if allelomorphic to each other. In other wcrds, whichever two pure forms A and B were crossed together, the Fl generation was AB giving in F2 a family approximating to IAA :2AB :IBB. The whole series of colours is thus often described as a vast set of multiple allelomorphs. Nevertheless there are curious features in that case which raise a doubt whether this account is really correct. Many of the distinctions are plainly quantitative degrees in development of some one type of coloration which are, as might be expected, allelomorphic to each other (c f. the Himalayan rabbit): but among the elements comprising the total coloration of these grasshoppers there are several in which both the pigments and the positions they occupy are so distinct that the characters cannot easily be represented as determined by factors allelomorphic to each other. Only by a very loose application of the term colour can the distinctions be said to apply to the same character. Hence, in this hitherto generally accepted illustration it seems probable that, in so far as the distinctions are actually quantitative differences in one respect, true allelomorphism may be recognized, but that the appearance of an allelomorphism between factors of differing scope is more probably spurious, and referable to close linkage (cf. Haldane). No decision on this question can yet be made with any confidence.

Allelomorphic Complexes

Among recent extensions of genetical theory none is more remarkable than the discovery that large and apparently miscellaneous groups of characters are sometimes governed by elements capable of segregating collectively as a single complex. Nevertheless, in the case of sex,we have long been familiar with one example. Since the distinction between the two sexes in many animals is known to behave in segregation as if it depended on a single Mendelian factor, we have to recognize that a number of distinctions of all kinds, structural and functional, may be treated in segregation as factorially single. In the special case of sex we know further that particular genetic elements may be detached from the complex (e.g. the elements governing spur and broodiness in fowls, the beard in man, etc.), though the possible limits of such disintegration are unknown.

Renner's experiments have shown that the inheritance of the protean variations of several Oenotheras is largely effected by the transmission of similar complexes. Each of these large composite factors or groups of factors (in so far as they prove to be divisible) may govern many characters of form, colour, habit, etc., and the whole group is transmitted as a single heritable entity. Similar discoveries will probably be made in regard to other forms. The details are beyond the scope of this article, but it may be remarked that these complexes in Oenothera supply one of the most striking illustrations of the phenomenon which may be called unilaterality (see " Somatic Segregation," infra) or the relegation of a factor or factors exclusively to one sex-side of a plant. For instance, whereas Oenothera Lamarckiana, the species which provided de Vries with his most celebrated but unsound evidence of mutation, can be proved to be a permanently heterozygous form having two complexes equally distributed in segregation to both the male and the female gametes, the species biennis and many more, though similarly heterozygotes of two complexes, in segregation pass the whole of the one complex into the male gametes and the whole of the other into the female gametes. The question whether the apparently simple factors which commonly behave as Mendelian units are capable of further resolution is of much theoretical importance in its bearing on the problem of the nature of variation. Such a complex factor as that which determines sex may evidently break up into simpler components, but for various reasons some geneticists incline to the belief that factors in general are permanent and irresoluble. Whenever a series in F2 i derived from two clearly distinct and true-breeding types, consists of a number of intergrading forms it is possible to interpret this result as due to the operation of a multitude of originally distinct factors, or to the fractionation of some one or more of them. Not very rarely in such series an extreme parental type fails to reappear at all (e.g. the many-feathered tail of the fantail pigeon [Staples-Brown], or the long glumes of Polish wheat) from crosses with ordinary types. It is difficult to interpret the absence of the extremes simply as an indication of their statistical infrequency. The recent production of an innumerable series of colour-forms, as in the sweet pea, is almost certainly due to the fractionation of the colour-complex. Until systematic crossing was undertaken, the extremes existed but the intergrades did not. So also in Drosophila, of which the normal eye is red, a profusion of intergrades ranging to the white eye, which was discovered first, has now appeared. Though "mutation " is involved, the essential change is probably the disintegration or fractionation of the originally integral complex.

(2) Cytological Interpretations of Genetic Phenomena. - Soon after the rediscovery of Mendelian analysis the plausible suggestion was made that the behaviour of the chromosome in the course of the maturation divisions was consistent with what might be expected if they were actually the bearers of segregable factors. Since, however, the number of segregating factors in many forms far exceeds the number of chromosomes possessed by those forms, it is clear that if the chromosomes are the carriers of factors they must be capable of carrying many. The discovery of linkage, and especially of the fact that linkage was determined by the parental associations of the factors, pointed in the same direction, for, as hinted (by Punnett) in the earlier article on Mendelism (see 18.118), linkage or "gametic coupling," as it was then called, might not unreasonably be supposed to be based on chromosomal association. The first development of this conception was made by T. H. Morgan, whose investigations, relating mainly to the fruit-fly Drosophila, have inaugurated a new phase in the development of genetical theory. This insect is a subject unusually favourable for experiment inasmuch as it offers a profusion of variations or " mutations," and reproduces itself with great rapidity under laboratory conditions.

The work began with the observation that the eyes, normally red, may be white, and that this variation is sex-linked, behaving genetically precisely as colour-blindness does in man. The whiteeyed male mated with normal females produces offspring all normal. Of these the sons cannot transmit the abnormality at all, whereas the daughters mated with normal males transmit the white eye to half their sons. White-eyed females can only be produced as daughters of white-eyed fathers and all the sons of such females are whiteeyed. Supposing the male to possess an X-chromosome, this system of descent would be represented if it were assumed that in the normal the X-chromosome carried the dominant factor for-red eye (see SEX). The linkage with sex is thus found to bean expression of the association of the two determining factors for sex and red eye in the same chromosome.

Numerous other sex-linked characters were soon after discovered, to which the same considerations apply, all collectively composing one linkage-group. The other factors identified in Drosophila, amounting to more than a hundred, can all be represented as grouped in three separate linkage-systems which, with the sex-linked group, make four; and since from cytological observations the haploid number of chromosomes in this animal is also 4, the inference is drawn that the factors composing each linkage-group are borne in one chromosome. Developing this conception, Morgan suggests that the factors are arranged in the chromosomes as beads on a string, each having a position normally fixed in relation to the rest. Crossing-over is thus represented as the consequence of an exchange of material between homologous pairs of chromosomes in synapsis (see Cytology). The pairs of chromosomes which then conjugate are with much probability regarded as respectively of maternal and paternal origin. The conjugating pairs seem to twist round each other, and occasionally there is (especially in Amphibia) an appearance of anastomosis between them which is regarded as providing for an exchange of material between the homologous pairs, and thus for the formation of cross-overs. According as the linkage between two factors is more or less complete it is supposed that the distance between the position of the two factors in the chromosome is smaller or greater, and in proportion as factors are placed close together the probability of their being separated in the process of twisting and anastomosis is regarded as diminished. The proportion of crossovers is thus taken as a measure of the position of two factors in the chromosome. If A, B, and C are three factors in one linkage group, and the closeness of the linkages between A and B and between B and C respectively be determined experimentally, then from these two the linkage between A and C can be calculated, and the result of the calculation is commonly found to agree with the value found experimentally for that linkage. In this way the relative " loci "of numerous factors have been determined with fair consistency, and the fact that this can be done forms a strong argument for the belief that in some way at least the factors must be disposed in linear systems. That these systems are actually arranged along the lines of the chromosomes is as yet a matter of inference. Attention must be called to the curious fact that in Drosophila crossing-over never occurs in the males in any of the 4 linkage-systems. As in every example of sex-linkage studied, the linkage with the sex-factor is always complete; but all the other factors are liable to crossingover in the female, though among the male gametes the original parental combinations reappear unchanged. Conversely Tanaka, examining linkages in the silkworm, observed that a pair of linked factors show crossing-over in the male but not in the female, and the two facts together suggest some limitation of crossing-over to the sex which is homozygous in sex, the female in Diptera, the male in Lepidoptera. The development of the idea here outlined has become the subject of very active research and is described in a copious but somewhat esoteric literature which can be followed only with difficulty by those not personally engaged in the work. That the outcome of these researches has led to a valuable codification of genetic principles is not in dispute; but until the main thesis, that the number of independent factors or of linkage-systems is never greater than the haploid number of chromosomes, has been shown to hold generally for animals and plants, this account of the nature of linkage, though probable, cannot be regarded as proved. The defect of the theory at the present time is that it rests on many subordinate hypotheses which are not all capable of independent verification. The position of the factors, for example, is believed to be liable to changes due to the action of other factors, the effects of age and miscellaneous influences difficult to distinguish. Errors of cell-division, long regarded as the most probable source of variation, may also cause disturbance. In two very remarkable instances it has been found possible to connect a disturbance in the normal course of heredity with a visible cytological irregularity - called by Bridges " non-disjunction." In a certain family he observed that a sex-linked character failed to follow its normal distribution to the sexes, and he was able to find that in this family the sex-chromosomes showed corresponding irregularities. More recently (1921) he obtained similar evidence in regard to the fourth chromosome and the group of genes attributed to it. Thus a definite association between particular chromosomes and the transferable factors must certainly exist.

Giant-forms

The interrelation of genetical and cytological phenomena is further illustrated by the behaviour of " Giantforms." This name is applied to certain varieties (chiefly of plants) in which the haploid and diploid numbers of chromosomes are double those of normal forms. R. P. Gregory bred such varieties of Primula sinensis, and found that in respect of various allelomorphs they might be quadripartite and not merely bipartite as the normals are. A plant, for example, might be Drrr in colour or leaf-shape, and, in consequence of the extra recessive elements, not distinguishable from the ordinary recessive, though in fact capable of throwing a small proportion of dominants. Since recent cytological studies have shown that series of allied forms may contain various multiples of the lowest haploid number (Chrysanthemum, for instance, having 9, 18, 27, 36 or45), various extensions on these lines may be expected.

Somatic Segregation

In the genetics of plants a number of phenomena have been encountered which are difficult to reconcile with the view, otherwise not unacceptable, that the distribution of the factors occurs exclusively in the maturation processes of the germ-cells. Apart from certain special conditions, best known in variegated plants (which are sometimes irregular mosaics and sometimes consist of an outer " skin " and an inner " core," dissimilar in their genetical potentialities), there are many plants in which the distribution of factors must have been laid down before the formation of germ-cells. E. R. Saunders's results proved that in certain stocks (Matthiola) the pollen all carried doubleness though the ovules were mixed in character, single and double. C. Pellew showed that in the hermaphrodite Campanula carpatica " pelviformis " the pollen bore exclusively femaleness and preponderantly white flower-colour (the plant being heterozygous for blue). The pollen of Begonia Davisii (a wild species with single flowers) carries doubleness exclusively, and several similar examples are known, in all of which the segregation of characters must precede the maturation of the germ-cells. Thus, while it is not in question that segregation depends on some cell-division, and very possibly on a differentiation of the chromosomes, there is evidence that the cell-division in which this differentiation occurs must at least sometimes precede germ-formation. As mentioned, in Oenothera this " unilateral " distribution is exceptionally frequent.

Bearing on Evolutionary Theory

This aspect of genetics can only be briefly treated here (see also under Mendelism). Genetic analysis has shown that the appearance of variability as a contemporary and widespread phenomenon is largely illusory. On studying a variable species critically it is found that the various forms cannot all produce each other as was formerly assumed, but that they stand in a regular descending order, being terms in a series of combinations of definite factors. Such series are no evidence of contemporary variability. Many of the terms can be separated in the homozygous condition, and thereafter may breed perfectly true. Even such an appearance of variability as that seen in polymorphic species is frequently not above suspicion of being the consequence of a cross, more or less remote. Contemporary variation certainly may occur; but of the contemporary origination of new species, or of the occurrence of genetic changes which can be colourably interpreted as likely to lead to the production of incipient species in a strict sense, no indication has been found. That the forms of life have been evolved from dissimilar precedent forms we know from the geological record, but as to the process by which this evolution has come to pass we are still in ignorance. All that can be said with any confidence is that variation most commonly arises as an error of cell-division, and that conceivably new species have so arisen.

Bibliography

Text-books: W. Bateson, Mendel's Principles of Heredity (3rd ed. 1913); E. Baur, Einfiihrung in die experimentelle Vererbungslehre (4th ed. 1919); T. H. Morgan, Heredity and Sex (1913); The Mechanism of Mendelian Heredity (1915); The Physical Basis of Heredity (1919); R. C. Punnett, Mendelism (5th ed. 1919). Special references: W. Bateson and I. Sutton, " Double Flowers in Begonia," Jour. Gen., viii., 1919; C. B. Bridges, " Nondisjunction as Proof of the Chromosome Theory of Heredity," Genetics, i., 1916; L. Cuenot, " L'Heredite chez les Souris," 4me Note, Arch. Zool. exp. et gen. iii., 1905; E. M. East, " A. Mendelian Interpretation of a Variation that is Apparently Continuous," Amer. Nat., 1910; R. P. Gregory, " Experiments with Primula sinensis," Jour. Gen., i., 1911; " Genetics of Tetraploid Plants," Proc. Roy. Soc., B, 1914; J. B. S. Haldane, " Note on a Case of Linkage in Paratettix," Jour. Gen., x., 1920; H. L. Ibsen and E. Steigleder, " Evidence for the Death in utero of the Homozygous Yellow Mouse," Amer. Nat., 1917; R. K. Nabours, " Studies of Inheritance and Evolution in Orthoptera," i., ii., iii., Jour. Gen., iii., 19134, and vii., 1917-8; H. Nilsson-Ehle, " Kreuzungsunters. an Hafer u. Weizen," Lunds Unirersitets A rsskrift, 1909 and 1911; A. Pascher, eber d. Kreuzun. g einzelliger haploide Organismen," Ber. dent. bot. Ges., xxxiv., 1916; C. Pellew, "Types of Segregation," Jour. Gen., vi., 1917; 0. Renner, Versuche iib. d. gametische Konstitution d. Onotheren," Zeits. f. ind. Abst. u. Vererbungslehre, xviii., 1917; E. R. Saunders, " Further Experiments on the Inheritance of Doubleness and Other Characters in Stocks," Jour. Gen., i., 1911.

The following periodicals are devoted to the subject: The Journal of Genetics (Cambridge); Genetics (Princeton); Genetica (The Hague); Hereditas (Stockholm); Zeits. f. ind. Abst. u. Vererbungslehre (Berlin). (W. BN.)

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