MAN
[Illustration: FIG. 32.
Normal and brachydactylous hands placed together for comparison. (From
Drinkwater.)]
[Illustration: 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 classed 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 assumed 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
classes 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
assertion 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.
[Illustration: 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. Nettleship 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}
[Illustration: FIG. 35.
Pedigree of a haemophilic family. Affected (all males) represented by
black, and normals of both sexes 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 sex, and so far as
is hitherto 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. Sex-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 sex 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 class 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 class 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 classification 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 analysis 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 associated 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 aptitude, or is it an
instance of general mental ability displayed in a field rendered attractive
by early association? 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 analysis of mental characters will no doubt be very difficult, and
possibly the best line of attack is to search for cases where they are
associated with some physical feature such as pigmentation. If an
association 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 classifying 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 associated with different livers might conceivably result in
individuals upon whose mental ability the world would pass 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 hoard is of a more lasting nature. Each generation in using it
sifts, adds, and rejects, and passes it on to the next a little better and
a little fuller. When we speak of progress we generally mean that the hoard
has been improved, and is of more service to man in his attempts to control
his surroundings. Sometimes this hoarded 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 title, 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 participation 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 circumstances, 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 assimilate and act upon the stores of knowledge which have been
accumulated 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, pass 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.
* * * * *
{191}
INDEX
_Abraxas grossulariata_, 99
"Acquired" characters, 14
Adaptation, 143
Agouti mice, 50
Albino mice, 50
Albinos, nature of, 53
_Amauris_, 144
Analysis of types, 156
Ancestral Heredity, Law of, 13
Andalusian fowls, 70
Axil colour in sweet peas, 93
Bateson, W., 14, 29, 55, 116, 132, 141
Biffen, R. H., 157
Blue Andalusian fowls, 71
Brachydactyly, 171
Bryony, 120
Bush sweet peas, 63
Castle, 132
Cattle, horns in, 86, 166
Colour, nature of, in flowers, 48
Colour-blindness, 117
Combs of fowls, 33, 43
Correns, C., 29, 120
Coupling of characters in gametes, 93
Cuenot, 50, 119
"Cupid" sweet peas, 62
Currant moth, 99
Darwin, C., 10, 65, 147, 163
De Vries, H., 15, 29, 141
Discontinuity in variation, 14
Dominant characters, 18
Doncaster, L., 99
Drinkwater, H., 172
Dutch rabbits, 60
Eggs, 2
Environment, influence of, 137
_Euralia_, 144
Evolution, 10, 85, 139
Eye, in primulas, 55
Eye-colour, in man, 176
Factor, definition of, 31
Factors, interaction of, 42
Fertilisation, 3
Fertilisation, self- and cross-, 163
Fixation of varieties, 153
Fluctuations, 138
Fowls, coloured from whites, 49, 73
Galton, 13, 179
Gametes, nature of, 6
Gregory, R. P., 55, 93
Haemophilia, 176
Hardy, G. H., 147
Heterozygote, definition of, 28
Heterozygote, of intermediate form, 68
_Hieracium_, 27, 132
Himalayan rabbits, 60
Homostyle primulas, 56
Homozygote, definition of, 28
Hooded sweet peas, 89
Horses, bay and chestnut in, 167
Hurst, C. C., 62, 176, 180
Immunity in wheat, 158
Individuality, 135
Inhibition, factors for, 74, 108
Intermediates, 125
{192}
Johannsen, W., 160
Lop-eared rabbits, 132
Mendel, 8, 17, 26, 132
Mental characters, 180
Mice, inheritance of coat colour in, 50
Mimicry, 143
_Mirabilis_, 151
Morgan, T. H., 116
Mulattos, 129
Mutation, 83, 138
Naegeli, C., 26
Natural selection, 11, 140, 142, 149
Nettleship, E., 175
Night-blindness, 175
_Pararge egeria_, 132
Parkinson, J., 122
Pea comb, 33
Peas, coloured flowers in, 24
Peas, tall and dwarf, 18
Pigeons, 86
Pin-eye in primulas, 55
_Pisum_, 17
Primulas, 31, 55, 68, 93
Pollen, 3
Pollen of sweet peas, 92
Pomace fly, 115
Population, inheritance of characters in a, 147
Presence and Absence theory, 35
Pure lines, 162
Purity of gametes, 24
Purity of type, 155
Rabbits, 53, 60
Ratios, Mendelian--
3 : 1, 20
9 : 3 : 3 : 1, 25, 34
9 : 3 : 4, 51
9 : 7, 49
Ray, John, 143
Recessive characters, 19
Repulsion between factors, 90
Reversion, 59, 165
in rabbits, 59
in sweet peas, 62
in fowls, 65
in pigeons, 65
Rose comb, 33
Saunders, E. R., 54, 122
Seeds, nature of, 4
Segregation, 22
Selection, 162
Sheep, horns in, 76
Silky fowls, 30, 105
Single comb, 32
Species, nature of, 150
Species, origin of, 11
Speckled wood butterfly, 132
Spermatozoa, 3
Sports, 147
Staples-Browne, R., 66
Sterility, 151
Sterility in sweet peas, 93
Stocks, double, 122
Stocks, hoariness in, 54
Sweet pea, colour in, 44, 79
history of, 82
inheritance of hood in, 89
inheritance of size in, 62
Telegony, 167
Thrum-eye in primulas, 55
Toe, extra toe in poultry, 76
Tschermak, E., 29
Unit-character, definition of, 31
Variation, 14, 137, 139
Walnut comb, 33
Weismann, A., 13
Wheat, beard in, 74
experiments with, 157
White, dominant in poultry, 72
Wilson, J., 168
Yellow mice, 119
Zygotes, nature of, 5
* * * * *
Notes
* * * * *
[1] Cf. note on p. 171.
[2] It has been found convenient to denote the various generations
resulting from a cross by the signs F_1, F_2, F_3, etc. F_1 on this system
denotes the first filial generation, F_2 the second filial generation
produced by two parents belonging to the F_1 generation, and so on.
[3] Hurst's original cross was between a Belgian hare and an albina Angora,
which turned out to be a masked Dutch.
[4] The Spot is an almost white bird, the colour being confined to the tail
and the characteristic spot on the head.
[5] The reader who searches florists' catalogues for these varieties will
probably experience disappointment. The sweet pea has been much "improved"
in the past few years, and it is unlikely that the modern seedsman would
list such unfashionable forms.
[6] It is to be understood that wherever a given factor is present the
plant may be homozygous or heterozygous for it without alteration in its
colour.
[7] It should be mentioned that as the shape of the pollen coat, like that
of the seed coat, is a maternal character, all the grains of any given
plant are either long or else round. The two kinds do not occur together on
the same plant.
[8] For the most recent discussion of this peculiar case the reader is
referred to Professor Castle's paper in _Science_, December 16, 1910.
[9] _Paradisus Terrestris_, London, 1629, p. 261.
* * * * *
Transcriber's note:
Corrections made to printed original.
Page 36, "two sorts, RP and Rp": 'PR and Rp' in original.
Page 51, "9 contain both C and G": 'c and G' in original.
Page 184, "in the simplest cases": 'simples' in original.
Footnote 3, "turned out to be a masked Dutch": 'turned
to out be' in original.