parental and fraternal coefficients group about an average value of
about .50 or slightly less. Similar coefficients have been worked out
for other degrees of relationship; thus grandparental coefficients are
about .25.
Stated briefly, in less exact terms, these coefficients mean that,
with respect to such traits as deviate from the group average, the
resemblance of brothers and sisters to each other or of children to
their parents is, on the whole, approximately mid-way between being
complete in its deviation from the average and in not deviating at all
from the average in the direction of the fraternal or parental
characteristic. Grandchildren tend to deviate from the group average
only about one fourth as far as their grandparents. It should be
remembered that these are statistical and not individual statements,
and that as many "exceptions" will be found in the direction of
greater resemblance as in that of lesser resemblance.
One of the present objects of the student of heredity, perhaps his
chief object, is to be able to state the facts of human heredity in
Mendelian terms, reducing many of the complex human traits to their
simpler elements. Some of the chief objections to the use of the
statistical formula of heredity are that apparently it is applicable
only to the fluctuating variabilities of organisms; that it rarely
takes into account the presence of (and therefore the heredity of)
true variations or mutations--and we have seen that it is just these
characters that are of the greatest value in evolution; and that
heredity is after all fundamentally an individual relation which loses
much of its definiteness and significance when we merge the individual
in with a crowd. To some these seem fatal objections to any use of the
statistical formula and it is certainly true that they greatly limit
its value. But for the present at least the statistical statement of
certain facts of heredity is still useful in this bio-social field. We
may therefore use the statistical formulas of heredity as a kind of
temporary expedient, enabling us to make statements regarding
inheritance of certain characters in the group or class, pending the
time when we shall be able to give the facts a more precise and more
"final" expression in Mendelian formulas. Many human traits are indeed
already known to Mendelize. Most of these are, however, "abnormal"
traits or pathological conditions; we are still in the dark regarding
the actually Mendelian or non-Mendelian inheritance of most of man's
normal characteristics. We might enumerate the following Mendelizing
human characters--eye color, color blindness, hair color and
curliness, albinism (absence of pigment), brachydactylism (two joints
instead of three in fingers and toes), syndactylism (union of certain
fingers and toes), polydactylism (one or more additional fingers or
toes in each hand or foot), keratosis (unusually thick and horny
skin), haemophilia (lack of clotting property in the blood),
nightblindness (ability to see only in strong light--a retinal defect
usually), certain forms of deaf mutism and cataract, imbecility,
Huntington's chorea (a form of dementia).
In observing Mendelian heredity we should bear in mind that a given
character may be due either to the presence or to the absence of a
"determiner" in the germ. Long hair such as is characteristic of many
"Angora" varieties of the guinea pig and cat, for example, is believed
to be due to the absence of a determiner which stops its growth. Blue
eyes are due to the absence of a brown pigment determiner, _et
cetera_. The presence or absence in the offspring of such characters
as we know do Mendelize can be predicted when we know the parental
history for two generations.
Turning now to the inheritance of mental traits and including, of
course, moral traits here as well, we find that we are almost entirely
limited to the statistical statement of results. Pearson found upon
examining data from a large number of school children, brothers and
sisters, that the coefficients of heredity between them were the same
as for their physical traits. His results are summarized in Figure 12.
The physical traits measured were, in the order plotted in the
figure--health, eye color, hair color, hair curliness, cephalic index
(ratio between breadth and length of cranium), head length, head
breadth, head height. These gave an average of .54 in brothers, .53 in
sisters, and .51 in brothers and sisters. The psychical traits in
order were--vivacity, assertiveness, introspection, popularity,
conscientiousness, temper, ability, handwriting. The corresponding
averages were .52, .51, .52.
[Illustration: FIG. 12.--Coefficients of heredity of physical
and psychical characters in school children. Characters
enumerated in text. (From Pearson.)]
Galton's pioneer works on "Hereditary Genius," "English Men of
Science," and "Natural Inheritance" showed with great clearness the
fact of mental and moral heredity. Wood's recent extensive study of
"Mental and Moral Heredity in Royalty" shows the same thing, although
not all the results of these investigations are given in mathematical
form. Little can be said regarding Mendelian heredity of mental traits
because the psychologist has not yet told us how to analyze even the
common and simpler psychic characters into their fundamental units;
since we do not know what the mental hereditary units are, obviously
we cannot work with them. Much of our knowledge in this field does not
permit of very accurate summary, though pointing indisputably to the
fact of mental inheritance in spite of the very great influences of
training and education, environment and tradition, in moulding the
mental and moral characteristics--influences with much greater effect
here than in connection with physical characters.
Galton studied the parentage of 207 Fellows of the Royal Society, a
Fellowship which is a real mark of distinction. He assumed that one
per cent of the individuals represented by the class from which his
observations were drawn, that is the higher intellectual classes,
might be expected to be "noteworthy": among the general population the
average is really about one in 4,000 or one fortieth of one per cent.
On the one per cent basis Galton found that Fellows of the Royal
Society had noteworthy fathers with 24 times the frequency to be
expected in the absence of heredity; noteworthy brothers with 31 times
the expected frequency; noteworthy grandfathers 12 times; and so on
through various grades of relationship.
Schuster examined the class lists of Oxford covering a period of 92
years and found that first honor men had 36 per cent first or second
honor fathers; second honor men had 32 per cent first or second honor
fathers; ordinary degree men 14 per cent first or second honor
fathers. These percentages are far in excess of that to be
expected--perhaps 0.5 per cent--on the assumption that ability is not
inherited. Schuster also determined the coefficients of heredity
between fathers and sons as regards intellectual ability, the evidence
being class marks in Oxford and Harrow; these he found to be about .3
for the parental relation and .4 for the fraternal. The intensity of
heredity in many forms of insanity has been determined and this runs
up much higher--.57 parental and .50 fraternal.
It is clear I take it, that the fact of human heredity does not
concern only physical traits but extends to psychical traits as well,
and with about the same intensity. This fact has been found true also
for still less analyzable characters such as length of life, fertility
or infertility and the like, and again about the same intensity of
resemblance is found.
Human heredity is a fact then just as human variability is a fact. We
have truly the raw materials and the means for racial improvement. The
ability to direct the evolution of the human race makes this our
supremest duty.
The facts of human heredity can more easily be brought home to us by
the examination of some actual pedigrees and family histories. We may
look at a few representative cases which will serve to bring out some
additional aspects of the significance to society of the demonstrated
fact of heredity. In the examination of single family histories we
should remember that a single pedigree may not accurately illustrate a
general law of heredity--again, an individual case may belong to a
group of cases without representing them fairly. Even in observing
illustrations of Mendel's laws allowance has to be made for the
variability due to "chance" meetings of germ cells. It is only when
large numbers of individuals are observed that the typical Mendelian
fractions and ratios can be strictly observed. It must be borne in
mind then that the histories given below illustrate the nature of the
facts of heredity rather than the laws of heredity. Some special
cautions in the interpretation of certain pedigrees will be suggested
in particular cases. Many of the figures are taken from the extremely
valuable "Treasury of Human Inheritance," now being published by the
Eugenics Laboratory of the University of London. In these figures and
some others a uniform series of symbols is used. Successive horizontal
lines designated by Roman numerals indicate generations; within a
single generation the individuals are numbered consecutively simply
for purposes of reference. The meaning of the more common symbols is
as shown in Table IV. We may first consider a few pedigrees showing
the heredity of physical abnormalities or defects.
TABLE IV.
_Symbols used in Pedigrees. As adopted by the Galton Eugenics
Laboratory._
[Symbol] Male and female respectively, not possessing the trait
under consideration.
[Symbol] Male and female possessing the trait under consideration.
[Symbol] Unknown sex--normal or affected.
[Symbol] Trait incompletely developed.
[Symbol] Neither presence nor absence of trait can be affirmed.
[Symbol] With a deformity or disease of special character which
may possibly be associated with that under consideration.
[Symbol] Twins.
[Symbol] Indicates number of children.
[Symbol] Marriage.
[Symbol] Number of children unknown.
[Symbol] Number and character of children unknown.
_S. P._ _Sine prole._ (No offspring.)
Fig. 13 illustrates a family history where brachydactylism (an
abnormality of the digits commonly called shortfingeredness, due to
the lack of one joint in each digit) is present and frequently
associated with dwarfism. We may describe this case rather fully
because it illustrates nicely the heredity of a trait according to the
Mendelian formula. The parentage of the affected female (II, 1) who
started this line is uncertain. The marriage was with a normal male
whose parentage is unknown but evidently normal. This pair produced 11
children, the character of 8 of whom is known; 4 were affected, 4
unaffected, a Mendelian ratio resulting from the mating of a normal
with a hybrid individual, the observed character dominating (i. e.,
the abnormality appearing in the hybrid individuals). According to
Mendelian laws, the normal offspring of affected hybrids when mated
with normals should produce all normal offspring; this result is shown
clearly through generations IV-VI, where no affected individuals are
produced by two normal parents, although one or two of the
grandparents were affected. Marriage of a normal person with one
affected parent is fit because this individual is wholly without
germinal determiners for this character. Marriage between a normal and
an affected person is unfit (or it would be if the observed character
were a serious defect) because approximately one half their offspring
will be affected like the one parent. Thus in IV, 7-21, we see 12
children from one such marriage, 7 of whom are affected, 5 unaffected.
All of the 11 children of the 5 unaffected are normal, while of the 16
children of the affected persons, all of whom that married at all
married normal individuals, 9 were affected, 7 unaffected. Similar
relations are found in generation VI, where the 9 affected persons in
V married normals, producing 33 children, 15 of whom were affected, 18
unaffected. Taking all the offspring of marriages between unaffected
and affected (hybrid) persons through the four generations III-VI, we
find 35 affected and 33 unaffected, with the condition of 3 unknown.
There is no instance in this pedigree of the marriage of two affected
persons, but such a marriage would be highly unfit (again in the case
of a serious defect) because we know that all their offspring would be
affected. Mating of two unaffected persons, even though each had one
affected parent, would be fit because the offspring would all be
unaffected, barring the possibility of a new variation or mutation to
this character, which would be extremely unlikely. Such a pedigree as
this illustrates very well how a knowledge of Mendelian heredity may
be of the greatest value practically, in determining the fitness or
unfitness of marriages in families where an abnormality or defect is
known to occur. The course of the inheritance here illustrates the
simplest form of Mendelism. We have already indicated that there are
many other forms which we have not described and which we cannot
undertake to describe here on account of their complexity; in such
cases, however, it is still possible to predict with fair accuracy
the characters of the offspring of parents whose history is known for
one or two generations.
[Illustration: FIG. 13.--Family history showing
brachydactylism. Farabee's data. (From "Treasury of Human
Inheritance.")]
The defect we have just been considering is dominant. Many defects are
recessive, i. e., transmitted though not exhibited by a hybrid
individual. Viewed from the standpoint of the character of the
offspring, mating with such a person would be unfit only when both
persons were similarly recessives. Such a chance similarity would be
likely only in cases of blood relationship. Here lies the scientific
basis for many of the legal restrictions against cousin marriage or
the marriage of closer relatives, for here, although both persons may
appear normal, the chances for latent ills appearing in the progeny in
a pure and permanently fixed condition are greatly increased. Of
course the same relation holds for characteristics which are not
defects but really valuable traits. Marriage of cousins possessing
valuable characters, whether apparent or not, might be allowed or
encouraged as a means of rendering permanent a rare and valuable
family trait which might otherwise be much less likely to become an
established characteristic. Some discrimination should be exercised
in the control, legal or otherwise, of such marriages.
[Illustration: FIG. 14.--Family history showing
polydactylism. (From "Treasury of Human Inheritance.")]
Fig. 14 gives a brief pedigree of a family in which polydactylism
occurs. This is a condition in which one or more additional or
supernumerary fingers or toes are present in the extremities. The
Mendelian character of the heredity of this defect is less clear than
in the preceding, yet there are many indications that this is really
an illustration of a complex Mendelian formula. Probably if the
parentage of the individuals marrying into this family were known we
should be able to give a complete formula. At any rate the pedigree
illustrates the unfit character of the matings with affected persons,
for in no instance has such a marriage resulted in the production of
fewer than one half affected offspring.
Fig. 15 illustrates a form of what is known as "split hand" or
"lobster claw," where certain digits may be absent in the hands and
feet. In this case all the digits are absent except the fifth. This is
frequently associated with syndactylism or the fusion of the remaining
digits into one or two groups. When present this usually affects all
four extremities. Two pedigrees of this defect are illustrated in Fig.
16. Here again we have a defect whose inheritance follows quite
closely the Mendelian formula, although the character of the matings
is not fully known; it is unnecessary to describe the details--the
histories speak for themselves.
[Illustration: FIG. 15.--Mother and two daughters showing
"split hand." (From Pearson.)]
Fig. 17 illustrates a pedigree of congenital cataract. This history is
less satisfactory because the matings are given in only three
instances. It is known from other data that this defect follows simple
Mendelian laws. Normal individuals produce only normals, while
affected persons produce one half or all affected offspring according
to the character of the mating.
Fig. 18 illustrates the heredity of another defect of the eye called
night blindness. This is a retinal defect, the affected being able to
see only in strong illumination. The particular form of the disease in
this family resulted in total blindness later in life. Little is known
definitely concerning the character of the matings; no mating is known
to have been with an affected person and some are known to have been
with unaffected. Of the 42 descendants of the first affected person
only 6 are known to have been unaffected. Can there be any doubt
regarding the unfitness of these matings? In generation III a single
mating led to a family of 10 children _all_ affected by this serious
defect, rendering them dependents.
One of the most complete pedigrees of a defect on record is given in
condensed form in Fig. 19. This summarizes the extraordinarily
complete data of Nettleship covering nine, and in one branch ten,
consecutive generations. The defect is another form of night blindness
as it existed in a French family. The inheritance is obviously
Mendelian: no affected persons are produced by unaffected parents,
although their own brothers or sisters or one parent may have been
affected. The pedigree gives the history of 2,040 persons, all
descended from one affected individual. Of these 135 were known to
have been affected, and all were children of affected parentage. Of
the total number of progeny of affected persons mated with normals,
130 were reported as affected and 242 as unaffected.
[Illustration: FIG. 16.--Two family histories showing split
foot. (From "Treasury of Human Inheritance.")]
We may consider next the hereditary history of some forms of nervous
defect, the exact nature of the causes of which can be less definitely
stated than in all of the preceding instances of defect. Fig. 20 gives
a brief history of the heredity of Huntington's chorea--a form of
insanity which here resulted in the death of all but one of the
affected persons in the first four generations; the fifth generation
is the present and is incomplete. Although the matings were with
normals in every case, yet in four of the eight marriages all of the
offspring were affected. From one affected male 23 affected persons
descended in four generations and their multiplication is still going
on. There can be no doubt as to the unfitness of marriage into such a
family.
[Illustration: FIG. 18.--Family history showing a form of
night blindness. Character of matings incompletely known.
(Data from Bordley.)]
A very complete family history showing deaf-mutism is given in Fig.
21. It cannot be said that in every case here the defect is innate,
i. e., hereditary, and it is not known that the cause of the defect
was the same in every family concerned, for deaf-mutism may result
from several different causes. In most cases in this history, however,
the defect behaves like a Mendelian dominant. In certain other cases
it is clearly known to follow the Mendelian formula. Such pedigrees
as this show how dangerous it is to marry into a family in which this
defect exists.
[Illustration: FIG. 19.--Family history showing a form of
night blindness. (Condensed form of Nettleship's data.)]
Goddard has recently published several family histories showing
feeble-mindedness. One of the most significant of these--significant
both socially and eugenically--is summarized here in Fig. 22. Of this
Goddard writes: "Here we have a feeble-minded woman [IV, 3] who has
had three husbands (including one 'who was not her husband'), and the
result has been nothing but feeble-minded children. The story may be
told as follows:
"This woman was a handsome girl, apparently having inherited some
refinement from her mother, although her father was a feeble-minded,
alcoholic brute. Somewhere about the age of seventeen or eighteen she
went out to do housework in a family in one of the towns of this State
[New Jersey]. She soon became the mother of an illegitimate child. It
was born in an almshouse to which she fled after she had been
discharged from the home where she had been at work. After this,
charitably disposed people tried to do what they could for her, giving
her a home for herself and her child in return for the work which she
could do. However, she soon appeared in the same condition. An effort
was then made to discover the father of this second child, and when he
was found to be a drunken, feeble-minded epileptic living in the
neighborhood, in order to save the legitimacy of the child, her
friends [_sic_] saw to it that a marriage ceremony took place. Later
another feeble-minded child was born to them. Then the whole family
secured a home with an unmarried farmer in the neighborhood. They
lived there together until another child was forthcoming which the
husband refused to own. When, finally, the farmer acknowledged this
child to be his, the same good friends [_sic_] interfered, went into
the courts and procured a divorce from the husband, and had the woman
married to the father of the expected fourth child. This proved to be
feeble-minded, and they have had four other feeble-minded children,
making eight in all, born of this woman. There have also been one
child stillborn and one miscarriage.
"As will be seen from the chart, this woman had four feeble-minded
brothers and sisters [IV, 6, 10, 15, 16]. These are all married and
have children. The older of the two sisters had a child by her own
father, when she was thirteen years old. The child died at about six
years of age. This woman has since married. The two brothers have each
at least one child of whose mental condition nothing is known. The
other sister married a feeble-minded man and had three children. Two
of these are feeble-minded and the other died in infancy. There were
six other brothers and sisters that died in infancy."
[Illustration: FIG. 20.--Family history showing Huntington's
chorea. Last generation incomplete. (Data from Hamilton.)]
The paternal ancestry of this unfortunate woman is hardly less
interesting, as may be seen from the diagram. All told, this family
history, as far as it is known, includes 59 persons; the mental
character of 12 of these is unknown; 10 died in infancy or before
their characteristics were known; of the remaining 37, 30 were
feeble-minded.
[Illustration: FIG. 21.--Family history showing deaf-mutism.
(From "Treasury of Human Inheritance.")]
Turning now to defects of other kinds, an interesting history is
illustrated in Fig. 23. Here a single individual fatally affected with
angio-neurotic oedema gave rise, in four completed generations, to
113 persons, 43 of whom were affected. In 11 this disease was the
direct cause of death. The Mendelian character of the heredity here
can be neither asserted nor denied. In generations II-V matings
between normal and affected gave 42 affected and 35 unaffected
offspring.