Mendelism Part 8

[Ill.u.s.tration: FIG. 32.

Normal and brachydactylous hands placed together for comparison. (From Drinkwater.)]

[Ill.u.s.tration: FIG. 33.

Radiograph of a brachydactylous hand.]

Though the interest attaching to heredity in man is more widespread than in other animals, it is far more difficult to obtain evidence that is both complete and accurate. The species is one in which the differentiating characters separating individual from individual are very numerous, while the number of the offspring is comparatively few, and the generations are far between. For these reasons, even if it were possible, direct experimental work with man would be likely to prove both tedious and expensive. There is, however, another method besides the direct one from which something can be learned. This consists in collecting all the evidence possible, arranging it in the form of pedigrees, and comparing it with standard cases already worked out in animals and plants. In this way it has been possible to demonstrate in man the existence of several characters showing simple Mendelian inheritance. As few besides medical men have hitherto been concerned practically with heredity, such records as exist are, for the most part, records of deformity or of disease. So it happens that most of the {171} pedigrees at present available deal with characters which are usually cla.s.sed as abnormal. In some of these the inheritance is clearly Mendelian. One of the cases which has been most fully worked out is that of a deformity known as brachydactyly. In brachydactylous people the {172} whole of the body is much stunted, and the fingers and toes appear to have two joints only instead of three (cf. Figs.

32 and 33). The inheritance of this peculiarity has been carefully investigated by Dr. Drinkwater, who collected all the data he was able to find among the members of a large family in which it occurred. The result is the pedigree shown on p. 173. It is a.s.sumed that all who are recorded as having offspring were married to normals. Examination of the pedigree brings out the facts (1) that all affected individuals have an affected parent; (2) that none of the unaffected individuals, though sprung from the affected, ever have descendants who are affected, and (3) that in families where both affected and unaffected {173} occur, the numbers of the two cla.s.ses are, on the average, equal. (The sum of such families in the complete pedigree is thirty-nine affected and thirty-six normals.) It is obvious that these are the conditions which are fulfilled in a simple Mendelian case, and there is nothing in this pedigree to contradict the a.s.sertion that brachydactyly, whatever it may be due to, behaves as a simple dominant to the normal form, _i.e._ that it depends upon a factor which the normal does not contain. The recessive normals cannot transmit the affected condition whatever their ancestry. Once free they are always free, and can marry other normals with full confidence that none of their children will show the deformity.

[Ill.u.s.tration: FIG. 34.

Pedigree of Drinkwater's brachydactylous family. The affected members are indicated by black and the normals by light circles.]

{174}

The evidence available from pedigrees has revealed the simplest form of Mendelian inheritance in several human defects and diseases, among which may be mentioned presenile cataract of the eyes, an abnormal form of skin thickening in the palms of the hands and soles of the feet, known as tylosis, and epidermolysis bullosa, a disease in which the skin rises up into numerous bursting blisters.

Among the most interesting of all human pedigrees is one recently built up by Mr. Nettles.h.i.+p from the records of a night-blind family living near Monpelier in the south of France. In night-blind people the retina is insensitive to light which falls below a certain intensity, and such people are consequently blind in failing daylight or in moonlight. As the Monpelier case had excited interest for some time, the records are unusually complete. They commence with a certain Jean Nougaret, who was born in 1637, and suffered from night-blindness, and they end for the present with children who are to-day but a few years of age. Particulars are known of over 2000 of the descendants of Jean Nougaret. Through ten generations and nearly three centuries the affection has behaved as a Mendelian dominant, and there is no sign that long-continued marriage with folk of normal vision has produced any amelioration of the night-blind state. {175}

[Ill.u.s.tration: FIG. 35.

Pedigree of a haemophilic family. Affected (all males) represented by black, and normals of both s.e.xes by light circles. (From Stahel.)]

Besides cases such as these where a simple form of Mendelian inheritance is obviously indicated, there are others which are more difficult to read. Of some it may be said that on the whole the peculiarity behaves as though it were an ordinary dominant; but that exceptions occur in which affected children are born to unaffected parents. It is not impossible that the condition may, like colour in the sweet pea, depend upon the presence or absence of more than one factor. In none of these cases, however, are the data sufficient for determining with certainty whether this is so or not.

A group of cases of exceptional interest is that in which the incidence of disease is largely, if not absolutely, restricted to one s.e.x, and so far as is. .h.i.therto known the burden is invariably borne by the male. In the inheritance of colour-blindness (p. 117) we have already discussed an instance in which the defect is rare, though not {176} unknown, in the female. s.e.x-limited inheritance of a similar nature is known for one or two ocular defects, and for several diseases of the nervous system. In the peculiarly male disease known as haemophilia the blood refuses to clot when shed, and there is nothing to prevent great loss from even a superficial scratch. In its general trend the inheritance of haemophilia is not unlike that of horns among sheep, and it is possible that we are here again dealing with a character which is dominant in one s.e.x and recessive in the other. But the evidence so far collected points to a difference somewhere, for in haemophilic families the affected males, instead of being equal in number to the unaffected, show a considerable preponderance. The unfortunate nature of the defect, however, forces us to rely for our interpretation almost entirely upon the families produced by the unaffected females who can transmit it. Our knowledge of the offspring of "bleeding"

males is as yet far too scanty, and until it is improved, or until we can find some parallel case in animals or plants, the precise scheme of inheritance for haemophilia must remain undecided.

Though by far the greater part of the human evidence relates to abnormal or diseased conditions, a start has been made in obtaining pedigrees of normal characters. From the ease with which it can be observed, it was natural that eye-colour should be early selected as a subject of investigation, and the work of Hurst and others {177} has clearly demonstrated the existence of one Mendelian factor in operation here. Eyes are of many colours, and the colour depends upon the pigment in the iris. Some eyes have pigment on both sides of the iris--on the side that faces the retina as well as on the side that looks out upon the world. Other eyes have pigment on the retinal side only. To this cla.s.s belong the blues and clear greys; while the eyes with pigment in front of the iris also are brown, hazel, or green in various shades according to the amount of pigment present. In albino animals the pigment is entirely absent, and as the little blood-vessels are not obscured the iris takes on its characteristic pinkish-red appearance.

The condition in which pigment is present in front of the iris is dominant to that in which it is absent. Greens, browns, or hazels mated together may, if heterozygous, give the recessive blue, but no individuals of the brown cla.s.s are to be looked for among the offspring of blues mated together. The blues, however, may carry factors which are capable of modifying the brown. Just as the pale pink-tinged sweet pea (Pl. IV., 9) when mated with a suitable white gives only deep purples, so an eye with very little brown pigment mated with certain blues produces progeny of a deep brown, far darker than either parent. The blue may carry a factor which brings about intensification of the brown pigment. There are doubtless other factors which modify the brown when present, but we do not yet know enough of the {178} inheritance of the various shades to justify any statement other than that the heredity of the pigment in front of the iris behaves as though it were due to a Mendelian factor.

Even this fact is of considerable importance, for it at once suggests that the present systems of cla.s.sification of eye-colours, to which some anthropologists attach considerable weight, are founded on a purely empirical and unsatisfactory basis. Intensity of colour is the criterion at present in vogue, and it is customary to arrange the eye-colours in a scale of increasing depth of shade, starting with pale greys and ending with the deepest browns. On this system the lighter greens are placed among the blues. But we now know that blues may differ from the deep browns in the absence of only a single factor, while, on the other hand, the difference between a blue and a green may be a difference dependent upon more than one factor. To what extent eye-colour may be valuable as a criterion of race it is at present impossible to say, but if it is ever to become so, it will only be after a searching Mendelian a.n.a.lysis has disclosed the factors upon which the numerous varieties depend.

A discussion of eye-colour suggests reflections of another kind. It is difficult to believe that the markedly different states of pigmentation which occur in the same species are not a.s.sociated with deep-seated chemical differences influencing the character and bent of the individual.

{179} May not these differences in pigmentation be coupled with and so become in some measure a guide to mental and temperamental characteristics?

In the National Portrait Gallery in London the pictures of celebrated men and women are largely grouped according to the vocations in which they have succeeded. The observant will probably have noticed that there is a tendency for a given type of eye-colour to predominate in some of the larger groups. It is rare to find anything but a blue among the soldiers and sailors, while among the actors, preachers, and orators the dark eye is predominant, although for the population as a whole it is far scarcer than the light. The facts are suggestive, and it is not impossible that future research may reveal an intimate connection between peculiarities of pigmentation and peculiarities of mind.

The inheritance of mental characters is often elusive, for it is frequently difficult to appraise the effects of early environment in determining a man's bent. That ability can be transmitted there is no doubt, for this is borne out by general experience, as well as by the numerous cases of able families brought together by Galton and others. But when we come to inquire more precisely what it is that is transmitted we are baffled. A distinguished son follows in the footsteps of a distinguished father. Is this due to the inheritance of a particular mental apt.i.tude, or is it an instance of general mental ability displayed in a field rendered attractive by early a.s.sociation? We have {180} at present very little definite evidence for supposing that what appear to be special forms of ability may be due to specific factors. Hurst, indeed, has brought forward some facts which suggest that musical sense sometimes behaves as a recessive character, and it is likely that the study of some clean-cut faculty such as the mathematical one would yield interesting results.

The a.n.a.lysis of mental characters will no doubt be very difficult, and possibly the best line of attack is to search for cases where they are a.s.sociated with some physical feature such as pigmentation. If an a.s.sociation of this kind be found, and the pigmentation factors be determined, it is evident that we should thereby obtain an insight into the nature of the units upon which mental conditions depend. Nor must it be forgotten that mental qualities, such as quickness, generosity, instability, etc.,--qualities which we are accustomed to regard as convenient units in cla.s.sifying the different minds with which we are daily brought into contact,--are not necessarily qualities that correspond to heritable units. Effective mental ability is largely a matter of temperament, and this in turn is quite possibly dependent upon the various secretions produced by the different tissues of the body. Similar nervous systems a.s.sociated with different livers might conceivably result in individuals upon whose mental ability the world would pa.s.s a very different judgment. Indeed, it is not at all impossible {181} that a particular form of mental ability may depend for its manifestation, not so much upon an essential difference in the structure of the nervous system, as upon the production by another tissue of some specific poison which causes the nervous system to react in a definite way. We have mentioned these possibilities merely to indicate how complex the problem may turn out to be. Though there is no doubt that mental ability is inherited, what it is that is transmitted, whether factors involving the quality and structure of the nervous system itself, or factors involving the production of specific poisons by other tissues, or both together, is at present uncertain.

Little as is known to-day of heredity in man, that little is of extraordinary significance. The qualities of men and women, physical and mental, depend primarily upon the inherent properties of the gametes which went to their making. Within limits these qualities are elastic, and can be modified to a greater or lesser extent by influences brought to bear upon the growing zygote, provided always that the necessary basis is present upon which these influences can work. If the mathematical faculty has been carried in by the gamete, the education of the zygote will enable him to make the most of it. But if the basis is not there, no amount of education can transform that zygote into a mathematician. This is a matter of common experience. Neither is there any reason for supposing that the superior education of a {182} mathematical zygote will thereby increase the mathematical propensities of the gametes which live within him. For the gamete recks little of quaternions. It is true that there is progress of a kind in the world, and that this progress is largely due to improvements in education and hygiene. The people of to-day are better fitted to cope with their material surroundings than were the people of even a few thousand years ago. And as time goes on they are able more and more to control the workings of the world around them. But there is no reason for supposing that this is because the effects of education are inherited. Man stores knowledge as a bee stores honey or a squirrel stores nuts. With man, however, the h.o.a.rd is of a more lasting nature. Each generation in using it sifts, adds, and rejects, and pa.s.ses it on to the next a little better and a little fuller. When we speak of progress we generally mean that the h.o.a.rd has been improved, and is of more service to man in his attempts to control his surroundings. Sometimes this h.o.a.rded knowledge is spoken of as the inheritance which a generation receives from those who have gone before.

This is misleading. The handing on of such knowledge has nothing more to do with heredity in the biological sense than has the handing on from parent to offspring of a picture, or a t.i.tle, or a pair of boots. All these things are but the transfer from zygote to zygote of something extrinsic to the species. Heredity, on the other hand, deals with the {183} transmission of something intrinsic from gamete to zygote and from zygote to gamete. It is the partic.i.p.ation of the gamete in the process that is our criterion of what is and what is not heredity.

Better hygiene and better education, then, are good for the zygote, because they help him to make the fullest use of his inherent qualities. But the qualities themselves remain unchanged in so far as the gamete is concerned, since the gamete pays no heed to the intellectual development of the zygote in whom he happens to dwell. Nevertheless, upon the gamete depend those inherent faculties which enable the zygote to profit by his opportunities, and, unless the zygote has received them from the gamete, the advantages of education are of little worth. If we are bent upon producing a permanent betterment that shall be independent of external circ.u.mstances, if we wish the national stock to become inherently more vigorous in mind and body, more free from congenital physical defect and feeble mentality, better able to a.s.similate and act upon the stores of knowledge which have been acc.u.mulated through the centuries, then it is the gamete that we must consult. The saving grace is with the gamete, and with the gamete alone.

People generally look upon the human species as having two kinds of individuals, males and females, and it is for them that the sociologists and legislators frame their schemes. This, however, is but an imperfect view to {184} take of ourselves. In reality we are of four kinds, male zygotes and female zygotes, large gametes and small gametes, and heredity is the link that binds us together. If our lives were like those of the starfish or the sea-urchin, we should probably have realised this sooner.

For the gametes of these animals live freely, and contract their marriages in the waters of the sea. With us it is different, because half of us must live within the other half or perish. Parasites upon the rest, levying a daily toll of nutriment upon their hosts, they are yet in some measure the arbiters of the destiny of those within whom they dwell. At the moment of union of two gametes is decided the character of another zygote, as well as the nature of the population of gametes which must make its home within him. The union once affected the inevitable sequence takes its course, and whether it be good, or whether it be evil, we, the zygotes, have no longer power to alter it. We are in the hands of the gamete; yet not entirely. For though we cannot influence their behaviour we can nevertheless control their unions if we choose to do so. By regulating their marriages, by encouraging the desirable to come together, and by keeping the undesirable apart we could go far towards ridding the world of the squalor and the misery that come through disease and weakness and vice. But before we can be prepared to act, except, perhaps, in the simplest cases, we must learn far more about them. At present we are woefully ignorant {185} of much, though we do know that full knowledge is largely a matter of time and means. One day we shall have it, and the day may be nearer than most suspect. Whether we make use of it will depend in great measure upon whether we are prepared to recognise facts, and to modify or even destroy some of the conventions which we have become accustomed to regard as the foundations of our social life. Whatever be the outcome, there can be little doubt that the future of our civilisation, perhaps even the possibility of a future at all, is wrapped up with the recognition we accord to those who live unseen and inarticulate within us--the fateful race of gametes so irrevocably bound to us by that closest of all ties, heredity.

* * * * * {187}

APPENDIX

As some readers may possibly care to repeat Mendel's experiments for themselves, a few words on the methods used in crossing may not be superfluous. The flower of the pea with its standard, wings, and median keel is too familiar to need description. Like most flowers it is hermaphrodite. Both male and female organs occur on the same flower, and are covered by the keel. The anthers, ten in number, are arranged in a circle round the pistil. As soon as they are ripe they burst and shed their pollen on the style. The pollen tubes then penetrate the stigma, pa.s.s down the style, and eventually reach the ovules in the lower part of the pistil.

Fertilisation occurs here. Each ovule, which is reached by a pollen tube, swells up and becomes a seed. At the same time the fused carpels enclosing the ovules enlarge to form the pod. When this, the normal mode of fertilisation, takes place, the flower is said to be SELFED.

In crossing, it is necessary to emasculate a flower on the plant chosen to be the female parent. For this purpose a young flower must be taken in which the anthers have not yet burst. The keel is depressed, and the stamens bearing the anthers are removed at their base by a {188} pair of fine forceps. It will probably be found necessary to tear the keel slightly in order to do this. The pistil is then covered up again with the keel, and the flower is enclosed in a bag of waxed paper until the following day. The stigma is then again exposed and dusted with ripe pollen from a flower of the plant selected as the male parent. This done, the keel is replaced, and the flower again enclosed in its bag to protect it from the possible attentions of insects until it has set seed. The bag may be removed in about a week after fertilisation. It is perhaps hardly necessary to add that strict biological cleanliness must be exercised during the fertilising operations. This is readily attained by sterilising fingers and forceps with a little strong spirit before each operation, thereby ensuring the death of any foreign pollen grains which may be present.

The above method applies also to sweet peas, with these slight modifications. As the anthers ripen relatively sooner in this species, emasculation must be performed at a rather earlier stage. It is generally safe to choose a bud about three parts grown. The interval between emasculation and fertilisation must be rather longer. Two to three days is generally sufficient. Further, the sweet pea is visited by the leaf-cutter bee, _Megachile_, which, unlike the honey bee, is able to depress the keel and gather pollen. If the presence of this insect is suspected, it is desirable to guard against the risk of admixture of {189} foreign pollen by selecting for pollinating purposes a flower which has not quite opened. If the standard is not erected, it is unlikely to have been visited by _Megachile_. Lastly, it not infrequently happens that the little beetle _Meligethes_ is found inside the keel. Such flowers should be rejected for crossing purposes.