MENDELIAN HEREDITY IN MAN
By Major C. C. Hurst, F.L.S.

Director of the Burbage Experiment Station

DURING the last ten years the Mendelian principles of heredity
have been applied with remarkable success to various characters
in many kinds of plants and animals. These Mendelian experi-
ments have given rise to the new science of Genetics. In the
absence of controlled matings, the analytical methods of
Genetics are obviously difficult to apply to Man, and for his
analyses, the student has torely on observations of the results of
random matings. In spite of this heavy handicap, and notwith-
standing the almost equally serious drawback of having to deal
with small families, considerable progress has been made, and
there is no doubt that at the present time we have sufficient
positive evidence to demonstrate the working of Mendelian
heredity in Man, and to encourage students to pursue their
investigations over a wider field. A large amount of work has
been done recently in the way of collecting pedigrees from
different sources, illustrating the inheritance of various characters
in Man, and in certain cases where the pedigrees have been
critically analysed in the light of the Mendelian principles, some
valuable results have been obtained. The results obtained by
Farabee (10), Nettleship (21), Davenport (3-7), Gossage (12),

301560
2

Mudge (19), Drinkwater (9), Goddard (11), Salaman (23), and
Jordan (16), provide good illustrations of the value of this method
of work. :

During the past ten years the writer has adopted the some-
what different method of making personal observations on the
individual characters of the parents and children living in and
about the village of Burbage, Leicestershire. (This district
consists of an industrial village with outlying hamlets, altogether
comprising a population of about 3,000 persons.) In all Men-
delian researches, whether in plants, animals or Man, the advan-
tages of personal observation seem far to outweigh the
disadvantages of small numbers in a limited area.

In view of the various characters touched upon in this
paper, it is, of course, ympossible to attempt to deal with any
single character exhaustively, consequently the paper can only
be regarded as an introductory sketch. Further details may be
found in the literature cited and in the forthcoming Reports of
the Burbage Experiment Station.

Eye-Colour.

Human eye-colour depends mainly on the colour of the iris,

which is largely determined by the presence or absence of two

distinct layers of pigment. In the true blue eye only one of

these pigmentary layers is visibly present, the posterior purple

pigment of the choroid, which, being reflected through the

fibrous structure of the iris produces the blue colour. In the

absence or partial absence of this pigment the eye appears to be

“ pink ” as in albinos. In the ordinary brown eye two layers: of

pigment are present, for in addition to the posterior purple layer
there is also an anterior brown layer, in front of the iris.

In order to distinguish eyes with two layers from eyes with
one layer of visible pigment, the writer (14) in 190% called the one
Duplex and the other Simplex, and further found that these two
types of eyes in their genetics follow the ordinary Mendelian
rules, Duplex being dominant and Simplex recessive. Or apply-
ing the presence and absence method—presence of the brown

_ front layer is dominant to its absence.
| Simplex eyes may be any shade of blue or grey according to
3

the fineness or coarseness of the structure of the iris, conse-
quently baby-blue eyes may develop into adult grey eyes.
Duplex eyes may be any colour from ‘‘ black ’’ (very dark brown)
to blue, according to the amount of brown pigment present on the
front of the iris. If the amount of brown pigment is small such
Duplex eyes may be easily mistaken for Simplex when viewed
at a short distance or in a bad light. A close lateral inspec-
tion in a good light will, however, generally determine the
presence or absence of visible pigment in front of the iris, with-
out the assistance of a lens.

An attempt to differentiate the various grades of blue and
grey Simplex eyes has so far proved unsuccessful owing partly to
their apparent continuity and partly to their changes with age.
An attempt to differentiate the various grades of Duplex eyes
has been more successful, though experience shows that great
caution is necessary to avoid the many pitfalls that exist even
for the wary.

In 1907 the writer suggested that Duplex eyes might be
classified into three definite patterns, Self, Ringed, and Spotted.

In the Self pattern, the brown pigment is distributed over
the entire surface of the iris, the rays being pigmented to the”
periphery. To this class belong eyes generally known as
“plack,” dark brown, brown, red-brown, yellow-brown, green-
brown and green, according to the” density of the melanic pig-
ment covering the front of the iris, and possibly also in some
cases to the existence of a distinct yellow or orange pigment
(lipochrome) diffused with the brown melanin.

In the Ringed pattern the brown pigment is confined to a
ringed area round the pupil, leaving a solid blue or grey rim round
the periphery. In somecases the ring of pigment is clearly defined
and narrow, while in others it is irregular in outline and broader
with fimbriated rays. To this class belong eyes generally
known as grey-brown, dark grey, grey, grey-green, and blue-
green.

In the Spotted pattern the inner ring is altogether absent or
broken up into discrete patches, blotches or spots. These spots
are usually irregular in size, number and distribution over the

“blue or grey ground colour. Similar spots are to be found
4

in certain eyes of the Self and Ringed patterns, but these are on
a lighter duplex ground, while those of the true spotted pattern
are on a simplex ground colour.’

3 ~ With regard to the genetics of these Duplex patterns, so far
as adults are concerned the Self seems to behave as a dominant
to the Ringed which is recessive, following the ordinary Mendelian
rules. In the case of juveniles, however, great care is necessary,
because since 19o5 I have found several cases in which the
Ringed pattern has developed into the Self pattern with age.
The Spotted pattern is rather rare in my material, but, so far as
the evidence goes, it appears to behave as a dominant both to the
Self and Ringed patterns, suggesting that it may be due to
the presence of an inhibitor. It is interesting to note the close
analogy between the genetics of the Self, Ringed and Spotted
patterns of eyes in Man, and the Self, Dutch and English
patterns of coat colour in rabbits.

Dr. C. B. Davenport and Mrs. Davenport (3), of the
Carnegie Institution of Washington (who discovered the Men-
delian nature of eye-colour almost simultaneously with the
writer (13)), make three grades of eye-colours, brown, grey and
blue in the order of their dominance. This is certainly a
more simple interpretation of the facts which, though broadly in
agreement with the writer's scheme, differs from it in one
important particular, inasmuch as it leaves untouched the
peculiar problem of the Spotted eye.

In following up their scheme, Davenport and Davenport find
that the darker grade of eye-colour is always dominant to the
lighter grade, so that no children have darker eyes than the
darker parent. In order to test this broad generalisation of the
“ Non-transgressibility of the Upper Limit,” I have attempted to
grade the eye-colour shades in some of the Burbage families from
o-10 according to the amount of brown pigment present on
the iris.

Thus o represents the blue or grey Simplex eye with
no brown pigment: 1, 2 and 3 are blue or grey Duplex eyes of
the Spotted or Ringed patterns: 4 is a dark grey, grey-green, or
grey-brown, a broadly ringed pattern: 5, 6, 7, 8 and g are Self
patterns varying in shade from light green to dark brown, and
5

10 represents “black ” eyes (really a black-brown) like those
found in some negroes, but not met with in the Burbage
families.

According to this method a numerical grading of the brown
pigment is obtained by the two-fold consideration of distribution
and density, the shade of the blue or grey ground-colour being
ignored as far as possible.

The following families have been worked out on this system,
the figures underneath each square and circle representing the
estimated grade of brown pigmentation in the eyes.

Fic. I.

 

 

 

© fomate, wil M08 grads bf fain colour].

Oo mile, wdhouZ visible net Loih.
ed = male, wR Zo1 Gee of oie UAT fg itor ie
Q T female, wk Hoo prads 4 eye colour J 4

o T mah, wilh Yollid dupls eyes

male, wil ard Lor

R
° D =r itt ir tr
3 Jenete. out sprlln depts yer

ka a = mals) wh No® grade 9) da colon,

The above pedigrees, on the whole, tend to confirm the
Davenports’ generalisation, the striking exceptions being the
cases of Spotted Duplex eyes which, notwithstanding their
lowness of grade, give offspring of a higher grade than either
parent (see families 40, 88, 95 and 33). The single case of
family 29 (4), where the higher parental grade 1 gives a solitary
instance of grade 3 is at present inexplicable, as both the 1 and 3
6

grades concerned are apparently of the ordinary ringed pattern.
Family 25 is interesting as a case of an apparently pure-breeding 5
grade, and the youngest daughter of this family (254) is a specific
case of the change with age from a Ringed to a Self Duplex: in
1905 her eyes were classed as Ringed Duplex representing
grade 3, whereas in 1911 they were Self Duplex representing
grade 5. The increase of pigment in this case was twofold,

representing an increase of I grade in distribution and 1 grade
in density.

«Hair Colour.

Davenport & Davenport (5) have shown that two distinct
pigments may be present in human hair, a granular brown melanin
and a diffused red lipochrome. Somatic interaction between
these two pigments gives complicated results, so that it will be
convenient to deal with them separately.

Brown Pigment.

According to the Davenports, human hair with brown
pigment varies in shade from black to light yellow according to
the amount of brown pigment present. Unlike similar hair
colours in rabbits, mice, and guinea-pigs, which experiments
have shown to be discontinuous, the shades of human hair
appear to be continuous. The Davenports, however, conclude
thalin all cases the higher and darker grade is dominant to the
lower and lighter grade, which is recessive. | Consequently,
children in no case have darker hair than their darker parent,
being another illustration of the Davenports’ generalisation of
the ‘ Non-transgressibility of the Upper Limit.” In order
to test this generalisation with my own material, I have attempted
to grade the brown hair colours of some of the Burbage families
from o-10, grade o representing white “albino” hair with
no pigment, and grade To representing jet black hair with no
trace of brown. Neither of these grades has yet been found
in my material. Grades 1 and 2 represent silvery cream and
pale yellow hair, so far only met with in juveniles. Grades
3 and 4 represent yellow and yellow-brown hair : grades 5 and 6
represent medium brown and deep brown hair: while grades
7

7, 8, and g represent dark-brown, black-brown and ordinary
‘ black ”’ hair respectively.

A serious difficulty however arises, a difficultywhich at one
time I feared would be insurmountable, and that is the frequent
and considerable changes of colour that human hair undergoes
in the progress of development and decay during the life of
the individual. These changes of colour from birth to death
appear to be very variable in different individuals. In many
individuals the true adult hair-colour of the head can only
be observed during a brief period of time, and often before
the children reach the adult stage the parents are already grey.

In view of these facts any first-hand investigation of the
inheritance of human hair-colour by comparison of parents with
children would appear to be extremely difficult, if not altogether
impracticable. Fortunately, however, the writer has found that
the colour of the eye-lashes at practically any age seems to be a
fairly reliable index to the adult head-hair. Generally speaking,
one finds that in very young children the pigment of the eye-
lashes develops much more quickly than that of the eye-brows and
head hair, and with few exceptions the colour of the eye-lashes in
juveniles may be taken as a fairly approximate index of what the
adult hair will be. Similarly in old people with grey hair,
the colour of the eye-lashes will generally give one a fair idea of
what the adult hair was like.

It is true that a few cases have been observed in normal
adults where the eye-lashes are darker or lighter than the hair of
the head and eye-brows, but generally the eye-lashes seem to
provide a fairly reliable index of the mature head-hair colour at
all ages.

The brown hair-colour gradings in Fig. 1 are all based
on the colour of the eye-lashes in both parents and children,
the grade numbers being inserted within the squares and circles.
As far as these observations go, they seem to confirm the
Davenports’ generalisation that no children are darker than
the darker parent. The slight exceptions are seen in family
‘41, where a single child apparently oversteps the higher parental
grade by 1 point, and in family 29 one child is apparently 2
grades darker than the darker parent. It will be noted that
8

it was in this family that an elder brother was similarly
exceptional in eye-colour.

Red Pigment.

In Man red hair exists in many continuous shades from a
light yellow-red to a dark chocolate-red. The Davenports
provisionally suggest that the red or reddish-yellow pigment may
exist in different dilutions and intensities, and also apparently,
with or without the sepia brown pigment. This may possibly be
so, but so far as my observations go a more simple scheme seems
to explain the facts equally well. All the various shades of red
hair known to me in humans, can, I think, be accounted for on
the basis of the somatic association of a uniform red or orange
pigment with each of the grades of brown pigment noted above.
Thus if. N1-Ng represent the various grades of the brown hair

from cream-coloured to “ black ”” and R a uniform red or orange
pigment, then :—

Ni} R=Silvery Cream with Red producing Pale red

Nz-+R=Pale Yellow ’ ” Light ved
N3-+R=Yellow os " rol Fiery red
N4+R=Yellow-brown ,, ’ " Sandy ved
N5+R=Medium brown ,, ’ Auburn ved
N6-+R=Deep brown 5 i » Chestnut ved
Ny7+R=Dark brown ” " “ Chocolate ved

N8-+R=Black-brown ,, 5 o Black-brown (Red masked)
Nog+R=* Black ”’ 2 " 5 “ Black” (Red masked)

If the above interpretation of the facts is correct there is
apparently no need to presume that the red pigment is found
alone in the complete absence of the brown pigment. Indeed I
imagine that such a presumption would be not only exceedingly
difficult to establish by observation, but would also invelve the
more frequent occurrence of albinos in red-haired families than
actually observed. Nor does it seem necessary to presume that
the red pigment itself exists in various intensities and dilutions
when its associations with the various grades of brown pigment
appear to provide all the sorts of red observed.

With regard to the genetics of red hair in Man, in 1908 the
9

writer (15) published a brief account of his investigations in
the Burbage families and at that time pointed out that Red hair
seemed to behave as a Mendelian recessive to Brown which
appeared to be dominant. These conclusions were based on the

following observations :—
First, when both parents had red hair, all the children

were red.

Second, when both parents had brown hair, either all the
children were brown, or most of them were brown and a few
were red. :

Third, when one parent was brown and the other red,
either all the children were brown, or about one half were brown
and one half were red.

Since 1908 further investigations have been made which not
only confirm and extend the previous observations but demon-
strate clearly that in all the families where red-haired children
appear, the brown-haired parents are without exception
descended from families containing red-haired children. While,
on the other hand, in those families in which red-haired children
do not appear at all, the brown-haired parents are in many
cases descended from families which have not thrown reds. All
these observations seem to point to the recessive nature of red
hair and the dominance of brown.

The adoption of the presence and absence” method in
genetics has, however, led us to regard such a character as red
hair as hypostatic (rather than recessive) to brown hair, which
is epistatic (rather than dominant). The Davenports’ demon-
stration of the fact that the red pigment in human hair is a
lipochrome distinct in its nature from the brown pigment, which
is a melanin, goes far to confirm this idea. It would appear,
therefore, that we must now regard the red pigment as due to
the presence of a distinct unit-factor dominant to its absence.
On this view, presence of the factor for red should give
visible red hair (except where it is masked by dark melanin as
shown above), while absence of the factor for red should give
hair with no traces of red. If (R) represents presence of red and
(r) its absence, then all individuals should be gametically (RR),
(Rr), or (rr). Red-haired individuals should be either (RR) or
10

(Rr), while brown-haired individuals should be all (rr), except as
aforesaid in those cases where the grades of brown are so dark
(8, 9 and 10) as to mask the red which may be present though
invisible.

In order, if possible, to avoid the masking complication
it seems necessary to consider those brown-haired individuals
where there is presumably no masking of the red pigment,
namely, in the lighter grades of brown 1-6. These low grade
browns should be gametically (rr), and, therefore, cannot be
expected to give red-haired children when mated together.
Similarly these low grade browns (rr) when mated with reds
(RR) or (Rr) may be expected to give either all reds
(RR) X (rr) = (Rr), or one-half reds and one-half browns
(Rr) x (rr) = 1 (Rr) + 1 (rr). While two red-haired parents
(RR) or (Rr) should give either all red-haired children (RR) X
(RR) = (RR), (RR) x (Rr) =1 (RR) + 1 (Rr), or about 3 reds
to 1 brown (Rr) Xx (Rr) = 1 (RR) + 2 (Rr) + 1 (rr).

In no case, however, do these expected results accord with
the observed facts, for :—

First, the low grade browns do, in many cases, give reds
when mated together.

Second, the low grade browns mated with reds in no case
give all reds, so far as observed.

Third, two red-haired parents in no case give children with
brown hair, so far as known.

The question arises, therefore, what is the explanation of the
whole matter? So far as one can see, the case seems to admit
of only one explanation, which though simple in itself, may
present certain difficulties to the orthodox Mendelian. The
suggested explanation is, that in order to have visible red
in the hair two doses of the factor (R) are necessary (RR), and
that in the presence of one dose (Rr) the red colour is either not
developed at all, or only developed so slightly as to be masked
by the melanin, and therefore invisible.

On this scheme two brown-haired parents (rr) would give all
brown-haired children (rr). A brown (Rr) mated with a brown
(rr) would give all browns (Rr) + (rr). Two brown (Rr) would
give about one quarter reds 1 (RR) and three-fourths brown
11

2 (Rr) + 1 (rr), giving the novel and heterodox Mendelian ratio
of 1 Dominant to 3 Recessive.

Similarly brown (rr) mated with red (RR) would give all
brown (Rr). The brown (Rr) mated with the red (RR) would
give about equal numbers of red (RR) and brown (Rr). While
two red (RR) parents wouldj give, of course, all red (RR)
children.

On this scheme all the expected results seem to be in
accordance with the observed facts. This suggested solution
of the puzzling problem of the inheritance of red hair intro-
duces a novel Mendelian ratio, which, if confirmed and extended
to other characters in ‘plants and animals, would be of consider-
able importance, inasmuch as it might go far to explain several

™% Fie. 11.

 

 

 

 

 

 

 

 

 

 

 
12

well-known cases of apparent reversal of dominance in plants and
animals which have so far remained inexplicable.

At the same time this suggested solution is advanced
with considerable reluctance, because the writer feels that it
would have been more acceptable, had it been first found in
some cultivated plant or domesticated animal, where the matter
could have been quickly and easily put to the test by conclusive
experiments. The only excuse for putting it forward in the case
of a human character is that no other explanation seems to be
available.

Skin-Colour in * White” Races.

Davenport & Davenport (6) have recently published an
important paper on the ‘ Heredity of Skin Pigmentation in
Man,” in which the Skin-colours of typical Caucasians are
divided into three categories, ‘ Brunet,” ‘ Intermediate” and
‘ Blond.”

Professor H. E. Jordan, of the University of Virginia (17),
in his microscopical studies on the melanin content of human
skins, has since demonstrated clearly that in the ‘ Brunet ” the
melanic pigment granules are more numerous than in the
‘ Blond” skin.

The Davenports have collected numerous data from various
sources, with the result that they find that their principle of the
‘“ Non-transgressibility of the Upper Limit” seems to apply to
these skin-colours much in the same way as in hair and eye-
colours. That is to say the higher and darker grade ‘‘ Brunet ”
is dominant (epistatic) to the lower and lighter grades ‘ Inter-
mediate” and “ Blond,” while ‘‘ Intermediate” skin-colour
is dominant (epistatie) to “ Blond,” which is recessive (hypostatic)
to both.

Consequently the children of ¢ Brunet,” ‘ Intermediate ”
and “ Blond ” parents are in no case darker than the darker
parent.

In order to test this generalisation in my material I have
attempted to divide the skin-colours in the Burbage families into
the three categories, Brunet,” “Intermediate” and ‘ Blond.”
So far this attempt has not proved at all satisfactory, as there
13

seem to be various grades both of darkness and fairness in
different families. A further attempt to divide skin-colours into
ten grades as in the hair and eye colours has not been at all
successful owing to the elusive nature of the different shades.
At the same time it is evident [that in many families there is a
definite segregation of dark and fair skins, and in this respect the
Davenports’ suggestion that the higher and darker grade is
dominant to the lower and lighter grade is certainly confirmed
so far as my observations go.

The above pedigrees (fig. 2) will serve to illustrate the
dominance of dark to fair skins, their segregation in the same
family, and the apparent purity of the extracted fair individuals.

It should, however, be carefully noted that while the dark
individuals in each family may be sensibly uniform in tint, yet
in the different families there are different grades of darkness.
A similar state of things appears also to exist amongst the fair
individuals. It is quite possible, therefore, that a certain amount
of overlapping might occur with these two categories ‘ Dark ”
and “ Fair” in certain pedigrees, though in the above pedigrees
the two categories are quite distinct.

For instance, the dark members of families 19 and 22 are
decidedly darker than the dark members of families 3 and 4,
while the fair members of families 13, 14 and 20 are certainly
fairer than the fair members of families 3, 4 and 8, yet the
segregation of the dark and fair in families 3,4 and 8 is quite
palpable and clear cut.

Complexion Colour.

While investigating the question of skin-colour in the
Burbage families, the writer has found his observations some-
what complicated by the different colourings of the complexion.
Thus both dark and fair skins may be either pale or coloured.
These differences in complexion colour appear to depend on the
thickness of the skin, pale skins being thick and coloured skins
thin. Generally speaking, a dark, pale skin makes a sallow or
muddy complexion : a fair, pale skin a clear complexion : a dark,
coloured skin a ruddy complexion: and a fair, coloured skin a
florid complexion.
14

So far as my observations go, the pale thick skin seems to
behave as a Mendelian dominant to the coloured thin skin which
is recessive. At the same time, owing to the scarcity of coloured
by coloured matings the evidence is not yet sufficient to establish
the statement fully. The few families with two coloured parents
have so far given all the children with coloured complexions.
Cases of two pale parents ‘giving either all pale, or a few
coloured, are quite numerous, as are the cases of pale by
coloured giving either all pale or about one-half with coloured
complexions.

It is possible, moreover, that coloured complexions in adults
really consist of two distinct classes, viz.:— red,” where the
colouring extends over the whole of the face and neck, and
“fresh ” where the colouring is confined to a definite area in
the cheeks. The precise relationship between these two classes
is not yet known.

The above notes illustrate once more that the apparently
simple question of the dark and fair colouring in “ white *’ races
is really a most intricate and complicated problem, which cannot
be satisfactorily dealt with by the easy methods of the anthro-
pologist. Sufficient evidence, however, has been obtained to
show that, notwithstanding the apparently continuous nature of
the pigmentation of the eyes, hair and skin of ¢ white” races,
the discrete Mendelian factors are at work in Man as well as in
domesticated animals and cultivated plants.

Further investigation will no doubt determine many other
Mendelian factors which are concerned in the heredity of
colouring in Man, and also the interesting question as to how far
these different factors behave as independent units, and what
amount of association exists between them in the production of
the visible soma of the two sexes.

With regard to the possibility of sex limitation or distur-
bance, it may be interesting to note that in the pigmentation
characters so far observed only two families (quite unrelated)
have shown any signs of such a phenomenon. Curiously enough
in both families the dark hair and skin of the father were
apparently transmitted to the daughters only, the sons (with a
single exception in I family) having fair hair and skin similar to
15

those of the mother. The numbers (9 males and 10 females),
however, are too small to attach much importance to the
observation, and both may be merely curious coincidences.

Left-handedness.

The occasional appearance of left-handed individuals among
normal right-handed brethren is a familiar fact, and there is a
widespread popular belief that the peculiarity is inherited.

An interesting paper by Jordan (16) gives a concise history
of the probable causes of left-handedness, together with a
number of pedigrees which certainly support Jordan’s conclusion
that left-handedness is hereditary.

Jordan very cautiously suggests as a guiding hypothesis,
that left-handedness may be a Mendelian character recessive to
right-handedness which is dominant, and further, that ambi-
dexterity may possibly represent a condition of imperfect
dominance of the right-handed over the left-handed character.

In view of Jordan’s paper, the writer has made some
investigations with regard to the appearance of left-handedness
in the Burbage families.

The following diagram (fig. 3) illustrates the results so far

obtained.
Pie. 111.

 

 

= ag ll handed, mate

BE - Onto Oar Oss male

@ 2 mld ows, female
cbf Landed mate a

* Rel faparyr

a
Oo = mpl Lamdsd., female
a
©

© lp Aamidsst. farmta
Cm tr Ta ee

16

On Jordan’shypothesis thatright-handedness is dominantand
left-handedness recessive, most of the matings illustrated in fig. 3
‘would be of the nature of heterozygous right-handed individuals,
which should give on the average one-fourth of the children left-
handed. Counting the ambidexterous among the right-handed
as Jordan suggests (imperfect dominants), and omitting the
family with only one child, we get 74 right-handed to 14 left-
handed children, a ratio nearer 5 : 1 than the expected 3: 1.
If, however, on the other hand, we regard the ambidexterous
as left-handed, we get 67 right-handed to 21 left-handed
children, a ratio very near to the expected 3 : 1. Further
inquiry into the ambidexterous cases shows that, with the

exception of one or two doubtful cases, the congenital bias

was undoubtedly to left-handedness, the ambidexterity being
apparently acquired as an adaptation to circumstances. In view
of this it seems more reasonable to classify these ambidexterous

«cases as left-handed rather than right-handed.

Jordan’s cases of similar parentage, omitting families with

only one child, apparently give 21 right-handed to 12 left-handed
«children, a ratio nearer 2 : 1 than the expected 3: 1, this, however,

may be due to the fact that some of his families were admittedly

incomplete, and consequently somewhat smaller than the

Burbage families, which are fairly complete. Two left-handed
parents have not yet been found in the Burbage families, but
Jordan gives two cases in which the two offspring were left-
handed in accordance with expectation. Jordan also gives
11 families of left-handed mated with heterozygous right-handed
parents giving 24 right-handed and 22 left-handed children,
which is in accordance with the expectation of equality. The
Burbage families only supply one case of this mating giving one
of each kind (family 28).

On the whole the evidence, so far as it goes, seems to favour
Jordan’s hypothesis that left-handedness in Man is a Mendelian
recessive character, but his idea that ambidexterity is the
condition of imperfect dominance of the right-handed over
left-handed is not so far confirmed.

Hand-Clasp.
All persons naturally clasp their hands in one of two ways,
17

either, with the right thumb over the left thumb, in which case
it is called a right-handed clasp, or, with the left thumb over the
right thumb, in which case it is called a left-handed clasp.
In practice one finds that certain nervous or self-conscious
people when asked to clasp their hands are apt to wander with
their thumbs, in such cases the position of the fingers provides
an equally accurate index to their hand-clasp, if the fingers of
each hand are alternated as they should be when the hand is
properly clasped.

The manner of the clasping of the hands—right or left—
seems to be congenital, being quite constant in very young
children, and one naturally expects to find such a character
inherited. The first to draw attention to this matter was
apparently Dr. Frank E. Lutz, of U.S.A. (18), who, with the
assistance of Prof. J. Arthur Thomson, of Aberdeen University,
collected data for about 600 individuals. The result was
certainly unexpected, inasmuch as neither position bred true,
and did not seem to follow the Mendelian or any other
system. Thus,

Two right-clasped parents gave 72.5 per cent. right-clasped
children.

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Two left-clasped parents gave 42.2 per cent. right-clasped
children.

While right-clasped mated with left-clasped parents gave
about 56 per cent. right-clasped children.

In view of these curious results steps have been taken
to investigate the matter in some of the Burbage families. It
was thought that a separate study of individual families might
throw some light on the question. The following pedigrees
(fig. 4) will suffice to illustrate the results obtained.

It will be seen that the observations made in the Burbage
families give similar results to those collected by Thomson
and Lutz. Neither right nor left position breeds true. Right
by Right give 15 rights and 11 lefts, no family giving all rights.
Left by Left give, curiously enough, the same result—15 rights
and 11 lefts, no family giving all lefts. Right by Left and
Left by Right give 20 rights and 27 lefts. All the families taken
together with all kinds of matings give 50 rights and 49 lefts. In
fact, there seems to be an equality of rights and lefts all round,
no matter what the parents are. Neither does there seem to be
any question of sex limitation or disturbance, for out of the
67 males and #3 females concerned there are 37 right males,
34 right females, 30 left males, and 39 left females. Naturally
the question arises whether in a case of this kind we are really
dealing with an hereditary character at all, and yet it is difficult to
imagine that a perfectly discontinuous, congenital, and apparently
instinctive character like hand-clasp is not inherited.

One thing, however, seems clear, and that is, that the
inheritance, if any, is not Mendelian, at any rate so far as
one can analyse it from the data. Somatic segregation is
evident, but there is no dominance, and above all, no gametic
purity, so far as one can see. It is often pointed out, with
truth, that dominance is not an essential part of Mendelism, only
segregation matters. The case of hand-clasp, however, suggests
that even segregation (somatic) is not always evidence of
Mendelian heredity, but that pure breeding alone is the true
test. The case of hand-clasp is particularly interesting inasmuch
as it is, so far as the writer knows, the only human character
investigated that defies Mendelian analysis, and as such it
19

appears to constitute a single real exception to the general
rule.

A comparison of figs. 3 and 4 shows at a glance that in
dealing with the inheritance of right and left-handedness (fig. 3),
and right and left hand-clasp (fig. 4), we are up against two
totally different characters, and that, however much they may
appear to resemble one another superficially, their fundamental
natures must be wide asunder.

Tuberculosis.

In venturing to touch upon such an intricate and com-
plicated question as tuberculosis, the writer is well aware of the
many difficulties involved, but the question is of such over-
whelming importance that a few observations noted while con-
ducting other researches in the same families may possibly be of
some interest to those engaged in a study of the question from
the point of view of Eugenics.

The following pedigrees of the adult Burbage families that
have suffered from the disease deal only with pulmonary tubercu-
losis and comprise three definite categories :—

First, those who have died from the disease.

Second, those who are seriously affected with it.

Third, those who have resisted it.

The first and second categories are alas self-evident. The
third category consists of adult individuals who, so far, have
proved themselves resistant to infection, in the sense that the
disease, if present at all, has never seriously affected them,
though it has proved fatal in their homes.

From the point of view of heredity, the significance, if any,
of the above facts seems to lie in the following observations :—

First, when both parents are resistant to the disease (i.e.,
have come under infection but have not been seriously affected
by it) and yet have one or more children who have succumbed to
its ravages, the death-rate is about one-fourth. Thus out of 48
adult children of such parents, 18 died of the disease, 3 have
been seriously affected by it, and 57 have so far proved
resistant,
   

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Second, the single case in which both parents died of the
disease produced 7 children, of whom 4 have died of the disease,
2 have been seriously affected by it, and 1 has so far proved
resistant.

Is it possible that the power of resistance to tuberculosis in
certain individuals is due to the presence of a definite factor
which is absent in those who succumb to the disease? Owing
to the extraordinary difficulties which beset this question the
above suggestion is only submitted with a note of interrogation,
in the hope that some expert student of tuberculosis will apply
it and test it in a more extended form, with due regard to the
many environmental complications involved.

Musical Temperament.

That certain individuals have a natural disposition for
music, while others have not, is evident to the most casual
observer. That this natural disposition or temperament is innate
and hereditary there can be little doubt. Musical associations
21

and careful training can, of course, do much in enriching the
musical qualities in an individual of a musical disposition, but in
the absence of the musical temperament these outer stimuli are
practically powerless. My observations in the Burbage families
go to show that individuals with a natural disposition for music
almost always display it early in life, usually between the 2nd
and 6th year. Musical children soon pick up airs and melodies
after hearing them a few times and are found constantly humming
them over to themselves, almost unconsciously. So far as my
experience goes, non-musical children do not do this, when they
are not silent they either drone monotonously or are merely noisy.
¥ Musical children as they grow up begin to harmonise naturally,
putting in their own parts without any tuition. If their singing
voice 1s poor they may take up the playing of some instrument,
otherwise they are content with mere humming or whistling.

In musical children and adults all this is done naturally and
spontaneously without conscious effort, and in many cases with-
out knowing a note of music throughout their lives. Of course, in
all musical individuals there are found varying degrees of ability
in regard to powers of expression, musical memory, sense of
time and rhythm and other musical minutie, all of which may be
developed to a considerable extent by careful training. There
appears also to be a distinct class of non-musical individuals
who are practically tone-deaf, who for instance are unable to
distinguish between the ‘National Anthem ” and the ¢ Old
Hundredth,” and to whom apparently there is little, if any, dis-
tinction between low and high notes, such cases, however, are
rare, probably as rare as the talented musician among ordinary
musical people. There seems also to be a striking difference be-
tween musical and non-musical persons in the nature of their
ordinary speaking voice, the musical individual speaks naturally
in melodious tones, the voice rising and falling with correct
intervals, while the non-musical individual speaks naturally in
monotones without a trace of melody. So far, only one case

has been observed by the writer where a non-musical individual -

has a melodious speaking voice, and not a single case where a
musical individual has a monotonous voice.
A study of the distribution of musical and non-musical

a
22

individuals in the Burbage families led the writer (15) to point
out in 1908 that the musical temperament appeared to behave
as a Mendelian recessive character, the ordinary non-musical
temperament being dominant. For it was found that when both
parents were musical all the children were musical. When
neither parent was musical either none of the children was
musical, or a few were musical and most of them were not.
When one parent was musical and the other not, esther none of
the children was musical, or about one-half were musical and
one-half not.

Professor F. O. Grover, of Oberlin College, U.S.A., has
kindly informed me that his investigations in some American
families confirm the above results. Further observations made
here since 1908 generally confirm and extend the previous
work. One interesting case, however, has recently turned up
which seems to be an exception to the rule that when both
parents are musical all the children are musical. This is the
case of a boy in family 35 (b) (fig. 6), 7 years old, who though
fond of singing, cannot at present sing an air at all correctly, it is
not so much that he sings out of tune, but he sings a kind of
monotonous vamp that is not altogether in harmony with the
air. Whether this is due to some defect of control of the vocal
chords, or whether it is due to a non-musical temperament is a
question that may be decided at a later stage. This boy’s elder
brother is a sweet and correct singer with an undoubted musical
temperament, as are both his parents.

The following pedigrees will serve to illustrate the genetics
_of the musical and non-musical temperaments.

All the individuals classed as ‘‘ musical” in the following
pedigrees have given some definite expression of their musical
temperament, in one way or another, either as singers, players,
whistlers or hummers. On the other hand, the individuals
classed as ‘ non-musical >’ have not done so, that is, so far as my
observation of them has gone. Itis, of course, possible that a
few of those classed as * non-musical ” may be of the nature of
mute musicians (especially those early affected with deafness),
but in any case, if such do exist they must be rare. The pedi-
grees given serve to illustrate the dominance of the non-
 

 

 

 

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musical and the recessive nature of the musical temperament,
the segregation of these two characters in the same family and
the apparent purity of the extracted recessives.

The above results raise two interesting points :—

First, the fact that the musical temperament is inherited on
Mendelian lines is interesting because as a temperamental
character it is probably of a different order from purely physical
or structural characters like pigmentation and left-handedness
and the question naturally arises if a psychical character like
musical temperament follows simple Mendelian rules, may not
other characters of a similar nature, but more important perhaps
to the race from the Eugenic standpoint, do soalso? This opens
up possibilities of a wider field of research for the worker in
human Genetics and Eugenics.

Second, the fact that the musical temperament behaves as a
recessive character and not a dominant, would seem to imply,
according to the presence and absence” method, that an
individual is non-musical owing to the presence of an inhibitory
factor preventing the expression of the musical temperament
which is hypostatically present in everyone.

In other words, it would appear that while probably every-
one possesses the fundamental musical temperament, yet owing
24

to the presence of an inhibitory factor non-musical people are
unable to give expression to it, whereas, on the other hand, in the
absence of such an inhibitor the musical temperament is
expressed and the individual is musical.

The interesting question as to whether an individual with a
double dose of the inhibitory factor differs in any way from an
individual with a single dose, cannot be answered on the present
evidence. It is tempting, however, to suppose that a tone-deaf

person differs in gametic constitution from an ordinary non-
musical individual.

Genetics and Eugenics.

In the above paper on Mendelian heredity in Man, an
attempt has been made to show how the new principles of
Genetics can be applied to Man with some degree of success. In
studying the inheritance of the more simple physical characters
in Man, it is evident that we are as yet only feeling our way
towards the solution of certain larger and more complicated
problems which are of vital importance to the human race. The
future of Eugenics depends very largely on the solution of these
problems. I do not wish for one moment to suggest that the art
of Eugenics has been born before its time, but I do feel that
before we can venture to apply the scientific principles of
Genetics to human life we must first make our foundations sure.
For this reason, I am convinced that a good deal of spade work
in human Genetics will have to be done before any considerable
amount of practical good can be accomplished in Eugenics.
Eugenics is simply applied Genetics, and sound Eugenics can only
be founded on sound Genetics.

REFERENCES.

1. Bateson, W. Mendel’s Principles of Heredity (Camb. Univ. Press), 1909,
VI., pp. 107-110; XII., pp. 205-234.

2. DARBISHIRE, A. D. Breeding and the Mendelian Discovery (Cassell & Co.),
1911, IV., pp. 40-49.

3. DAVENPORT, G.C.and C.B. * Heredity of Eye-Colour in Man,” Science,
1907 (Nov. 1st), Vol. 26, pp. 589-592.

4. i “ Heredity of Hair-form in Man,” Amer. Nat., 1908 (May),
XLII, pp. 341-349.
5. + “ Heredity of Hair-Colour in Man,” Amer. Nat., 1909 (April),

XLIII., pp. 193-211.
I0.

II.

12.

13.

14.
15.

16.

18.

19.

20.

21.

22.

23.

25

DavENPORT, G. C. and C. B. ¢ Heredity of Skin Pigment in Man,”
Amer. Nat., 1910 (Nov. and Dec.), XLIV., pp. 641-672, 705-731."

7) Eugenics (Holt & Co., N.Y.).

DONCASTER, L. Heredity in the Light of Recent Research (Camb. Univ.
Press), 1910, VIII., pp. 99-115.

DRINKWATER, H. “An Account of a Brachydactylous Family,” 1908,
Proc. Roy. Soc. Edin., XXVIII, p. 35.

FARABEE, W. C. “Inheritance of Digital Malformations in Man,”
Papers Peabody Museum Amer. Arch. & Ethn. Harv. Univ., 1905,
111.,'p. 60.
GoppARD, H. H. ‘Heredity of Feeble-mindedness,” 1910, Amer. Breed.
Mag., 1., pp. 165-178.
Amer. Breed. Assoc., 1910, pp. 103-116 (reprint).
Eugenics Review, 1911, 111., pp. 46-60 (reprint).
GOSSAGE, A. M. ¢ The Inheritance of Certain Human Abnormalities,”
Quart. Journ. Med., 1908, p. 331.
Hurst, C. C. A Reference to the Mendelian Nature of Eye-Colour
in Man, Nature, 1907 (Sept. 26th), Vol. 76, p. 558.
“ On the Inheritance of Eye-colour in Man,” 1908, Proc. Roy.
Soc., B., Vol. 80, pp. 85-96.
4, “ Mendel’s Law of Heredity and its Application to Man,” 1908,
Trans. Leic. Lit. & Phil. Soc., X11., Pt. 1, pp. 35-48.
JorpaN, H. E. “Inheritance of Left-Handedness,” 1911, Amer. Bresd.
Mag., 11., pp. 19-20.

5 “A Comparative Microscopic Study of the Melanin Content
of Pigmented Skins,” 1911, Amer. Nat. (Aug.), XLV., pp. 449-
470.

Lutz, F. E. “The Inheritance of the Manner of Clasping the Hands,”
Amer. Nat., 1908 (March), XLII., pp. 195-6.

MupGe, G. P. On Segregation in the Second Generation of Crosses
between Red and White Races of Man, Nature, 1907, Nov.
7th, p. 9.

2 “ Segregation of European Skin Colour,” Mendel Journal, 1909,

No. 1, p. 24.

NEeTTLEsHIP, E. ¢ Hereditary Diseases of the Eye (Bowman Lecture),
Trans. Opthalm. Soc., 1909, XXIX., pp. lvii.-cxcviii.

PunNeTT, R. C. Mendelism (Macmillan), 1911 (3rd ed.), Chap. XII.
pp. 120-122; XV., pp. 156-170.

SarLamaN, R. N. “Heredity and the Jew,” Journ. Genetics, 1911, I.,
PP. 273-292 ; Eugenics Rev., 1911, I11., pp. 187-200.
 
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