MENDELISM. To define what some biologists call
Mendelism briefly is not possible. Within recent years there has
come to biologists a new idea of the nature of living things,
a new conception of their potentialities and of their limitations;
and for this we are primarily indebted to the work of Gregor
Mendel. Peasant boy, monk, and abbot of Brünn, this remarkable
man at one time interested himself in the workings of
heredity, and the experiments devised by him and carried out
in his cloister garden are to-day the foundation of that exact
knowledge of the physiological process of heredity which biologists
are rapidly extending in various directions. This extension
Mendel never saw. Born in 1822 he published the account
of his experiments in 1865, but it was not until 1900, eighteen
years after his death, that biologists came to appreciate what
he had accomplished. That year marked the simultaneous
rediscovery of his work by three distinguished botanists: Hugo
de Vries, C. Correns and E. Tschermak. Thenceforward
Mendel’s ideas have steadily gained ground, and, as the already
strong body of evidence in their favour grows, they must come
to exert upon biological conceptions an influence not less than
those associated with the name of Darwin.
Dominant and Recessive.—Mendel chose the common pea
(Pisum sativum) as a subject for experiment, and investigated
the effects of crossing different varieties. In his method he
differed from previous investigators in concentrating his attention
on the mode of inheritance of a single pair of alternative
characters at a time. Thus on crossing a tall with a dwarf
and paying attention to this pair of characters alone, he found
that the hybrids (or F1 generation) were all tall and that no
intermediates appeared. Accordingly he termed the tall
character dominant and the dwarf character recessive. On
allowing these hybrids to fertilize themselves in the ordinary way he obtained a further generation which on the average
was composed of three talls to one dwarf. Subsequent experiment
showed that the
dwarfs always bred true,
as did also one out of
every three talls; the two
remaining talls behaved
as the original hybrids in
giving three talls to one
dwarf. Having regard to
the characters, tallness
and dwarfness, three and
only three kinds of peas
exist, viz. dwarfs which
breed true, talls which
breed true, and talls which give a fixed proportion of talls
and dwarfs. The relation between these three forms may
be briefly summarized in the subjoined scheme, in which
pure tall and dwarf are represented by T and D respectively,
while [T] denotes the talls which do not breed true. Experiments
were also made with several other pairs of characters,
and the same mode of inheritance was shown to hold good
throughout.
Fig. 1.
Fig. 2.
Unit-Characters.—As Mendel clearly perceived, these definite
results lead inevitably to a precise conception of the constitution
of the reproductive cells, or gametes; and to appreciate
fully the change wrought in our point of view necessitates
a brief digression into the essential features of the reproductive
process. A sexual process (see Sex) is almost universal among
animals and plants, and consists essentially of the union of
two gametes, of which one is produced by either parent. Every
gamete contains small definite bodies known as chromosomes,
and the number of these is, with few known exceptions, constant
for the gametes of a given species. On the fusion of two
gametes the resulting cell or zygote has therefore a double
structure, for it contains an equal number of chromosomes
brought in by the paternal and by the maternal gamete—in
the case of a plant by the pollen grain as well as by the
ovule. By a process of repeated
division the zygote
gives rise to a plant (or an
animal) whose cells apparently
retain the double
structure throughout. Certain
of the cells of such a
zygote become the germ
cells and are set apart, as
it were, for the formation
of gametes. Histology has
shown that when this occurs
the cells lose the double
structure which they had
hitherto possessed, and that
as the result of a process
known as the reduction
division gametes are formed
in which the number of
chromosomes is one half
of that which characterizes
the cells of the zygote. It
is generally acknowledged
that the chromosomes play
an important part in the
hereditary process, and it is
possible that the divisions which they undergo in gametogenesis
are connected with the observed inheritance of characters.
We shall refer later to the few observations which seem to connect
the two sets of phenomena.
Our conception of what occurs when a cross is made between
two individuals may be illustrated by the diagram which forms
fig. 2. Zygotes are here represented by squares and gametes
by circles. The dominant and recessive characters are indicated
by small plain and black rectangles. Each zygote must contain
two and each gamete but one of these unit-characters.
Zygotes such as the original parents which breed true to a given
character are said to be homozygous for that character, and
from their nature such homozygotes must produce identical
gametes. Consequently when a cross is made only one kind
of zygote can be formed, viz. that containing both the
dominant and recessive unit-characters. When the germ-cells
of such a heterozygote split to form gametes, these, as indicated
in fig. 2, will be of two sorts containing the dominant and recessive
characters respectively, and will be produced in equal
numbers. If we are dealing with a hermaphrodite plant such as
the pea the ovules will consist of one half bearing only the
dominant character and one half bearing only the recessive
character; and this will be true also of the pollen grains.
Consequently each dominant ovule has an equal chance of
being fertilized by a dominant or by a recessive pollen grain,
and the dominant ovules must therefore give rise to equal
numbers of dominant homozygous and of heterozygous plants.
Similarly the recessive ovules must give rise to equal numbers
of recessive homozygotes and of heterozygotes. Hence of
the total offspring of such a plant one quarter will be pure
dominants, one quarter recessives, and one half heterozygotes
as indicated in fig. 2. Where one character is completely
dominant over the other, heterozygotes will be indistinguishable
in appearance from the homozygous dominant, and the
F2 generation will be composed of three plants of the dominant
form to each recessive. These are the proportions actually
found by Mendel in the pea and by many other more recent
observers in a number of plants and animals. The experimental
facts are in accordance with the conception of unit characters
and their transmission from zygote to gamete in
the way outlined above; and the numerical results of breeding
experiments are to be regarded as proving that in the formation
of gametes from the heterozygote the unit-characters are
treated as unblending entities separating cleanly, or segregating,
from one another. From this it follows that any gamete can
carry but one of a pair of unit-characters and must therefore
be pure for that character. The principle of the segregation
of characters in gametogenesis with its natural corollary,
the purity of the gametes, is the essential part of Mendel’s
discoveries. The quite distinct phenomenon of dominance
observed by him in Pisum occurs in many other cases, but, as
will appear below, is by no means universal.
Illustrations.—Mendelian inheritance in its simplest form,
i.e. for a single pair of characters, has already been shown to
occur in many species of animals and plants, and for many
very diverse characters. In some cases complete dominance
of one of the pair of unit-characters occurs; in others the form
of heterozygote is more or less intermediate. Fresh cases
are continually being recorded and the following short list
can but serve to give some idea of the variety of characters in
which Mendelian inheritance has been demonstrated.
A. Dominance nearly or quite complete. (The dominant
character is given first).
Tall and dwarf habit (pea, sweet pea).
Round seed and wrinkled seed (pea).
Long pollen and round pollen (sweet pea).
Starch and sugar endosperm (maize).
Hoariness and absence of hairs (stocks, Lychnis).
Beardless and bearded condition (wheat).
Prickliness and smoothness of fruits (Datura).
Palm and fern leaf (Primula).
Purple and red flowers (sweet pea, stocks, &c.).
Fertility and sterility of anthers (sweet pea).
Susceptibility and immunity to rust (wheat).
Rose comb and single comb (fowls).
Black and white plumage (Rosecomb bantams).
Grey and black coat colour (rabbits, mice).
Bay and chestnut coat colour (horses).
Pigmentation and albinism (rabbits, rats, mice).
Polled and horned condition (cattle).
Short and long “Angora” coat (rabbits).
Normal and waltzing habit (mice).
Deformed hand with but two phalanges in digits and normal
hand (man).
B. Absence of dominance, the heterozygote being more or less
intermediate in form.
Black and white splashed plumage (Andalusian fowls).
Lax and dense ears (wheat).
Six rowed and two rowed ears (barley).
Dominance.—The meaning of this phenomenon is at present
obscure, and we can make no, suggestion as to why it should
be complete in one case, partial in another, and entirely absent
in a third. When found it is as a rule definite and orderly,
but there are cases known where irregularity exists. The
extra toe characteristic of certain breeds of fowls, such as Dorkings,
behaves generally as a dominant character, but in certain
cases it has been ascertained that a fowl without an extra toe
may yet carry the extra toe character. It is possible that in
some cases dominance may be conditioned by the presence of
other features, and certain crosses in sheep lend colour to the
supposition that sex may be such a feature. A cross between
the polled Suffolk and the horned Dorset breeds results in
horned rams and polled ewes only, though in the F2 generation
both sexes appear with and without horns. At present the
simplest hypothesis which fits the facts is that horns are dominant
in the male and recessive in the female. It is important
not to confuse cases of apparent reversal of dominance such
as the above with cases in which a given visible character may
be the result of two entirely different causes. One white hen
may give only colour chicks by a coloured cock, whilst the
same cock with another white hen, indistinguishable in appearance
from the former, will give only white chickens containing
a few dark ticks. There is here no reversal of dominance,
but, as has been abundantly proved by experiment, there are
two entirely distinct classes of white fowls, of which one is
dominant and the other recessive to colour.
The Presence and Absence Hypothesis.—Whether the phenomenon
of dominance occur or not, the unit-characters exist
in pairs, of which the members are seemingly interchangeable.
In virtue of this behaviour the unit-characters forming such
a pair have been termed allelomorphic to one another, and
the question arises as to what is the nature of the relation
between two allelomorphs. The fact that such cases of heredity
as have been fully worked out can all be formulated in terms
of allelomorphic pairs is suggestive, and has led to what may
be called the “presence and absence” hypothesis. An allelomorphic
pair represents the only two possible states of any
given unit-character in its relation to the gamete, viz. its presence
or its absence. When the unit-character is present the
quality for which it stands is manifested in the zygote: when
it is absent some other quality previously concealed is able
to appear. When the unit-character for yellowness is present
in a pea the seeds are yellow, when it is absent the seeds are
green. The green character is underlying in all yellow seeds,
but can only appear in the absence of the unit-character for
yellowness, and greenness is allelomorphic to yellowness because
it is the expression of absence of yellowness.
Dihybridism.—The instances hitherto considered are all
simple cases in which the individuals crossed differ only in
one pair of unit-characters. Mendel himself worked out cases
in which the parents differed in more than one allelomorphic
pair, and he pointed out that the principles involved were
capable of indefinite extension. The inheritance of the various
allelomorphic pairs is to be regarded as entirely independent.
For example, when two individuals AA and aa are crossed
the composition of the F2 generation must be AA + 2Aa + aa.
If we suppose that the two parents differ also in the allelomorphic
pair B–b, the composition of the F2 generation for
this pair will be BB + 2Bb + bb. Hence of the zygotes which
are homozygous for AA one quarter will carry also BB, one
quarter bb, and one half Bb. And similarly for the zygotes
which carry Aa or aa. The various combinations possible
together with the relative frequencies of their occurrence may
be gathered from fig. 3. Of the 16 zygotes there are:—
9
containing
A and B
3
containing
B but not A
3
”
A but not B
1
”
neither A nor B
In a case of dihybridism the F1 zygote must be heterozygous for
the two allelomorphic pairs, i.e. must be of the constitution
AaBb. It is obvious that such a result may be produced in two
ways, either by the union of two gametes,
Ab and aB, or of two gametes AB and
ab. In the former case each parent
must be homozygous for one dominant
and one recessive character; in the
latter case one parent must be homozygous
for both the dominant and the
other for both recessive characters.
The results of a cross involving
dihybridism may be complicated in
several ways by the reaction upon one
another of the unit-characters belonging to the separate
allelomorphic pairs, and it will be convenient to consider
the various possibilities apart.
AA
BB
AA
Bb
Aa
BB
Aa
Bb
AA
bB
AA
bb
Aa
bB
Aa
bb
aA
BB
aA
Bb
aa
BB
aa
Bb
aA
bB
aA
bb
aa
bB
aa
bb
Fig. 3.
1. The simplest case is that in which the two allelomorphic
pairs affect entirely distinct characters. In the pea tallness
is dominant to dwarfness and yellow seeds are dominant to
green. When a yellow tall is crossed with a green dwarf the
F1 generation consists entirely of tall yellows. Precisely the
same result is obtained by crossing a tall green with a dwarf
yellow. In either case all the four characters involved are
visible in one or other of the parents. Of every 16 plants
produced by the tall yellow F1, 9 are tall yellows, 3 are tall
greens, 3 are dwarf yellows, and 1 is a dwarf green. If we
denote the tall and dwarf characters by A and a, and the yellow
and green seed characters by B and b respectively, then the
constitution of the F2 generation can be readily gathered from
fig. 3.
Fig. 4.
The four types of comb referred to in the text are shown here.
All the drawings were made from male birds. In the hens the
combs are smaller. All four types of comb are liable to a certain
amount of minor variation, and the walnut especially so. The
presence of minute bristles on its posterior portion, however,
serves at once to distinguish it from any other comb.
2. When the two allelomorphic pairs affect the same structure
we may get the phenomenon of novelties appearing in F1 and
F2. Certain breeds of fowls have a “rose” and others a “pea”
comb (fig. 4). On crossing the two a “walnut” comb
results, and the offspring of such walnuts bred together consist
of 9 walnuts, 3 roses, 3 peas, and 1 single comb in every
16 birds. This case may be brought into line with the scheme
in fig. 3 if we consider the allelomorphic pairs concerned to be rose (A) and absence of rose (a), and pea (B) and absence
of pea (b). The zygotic constitution of a rose is therefore
AAbb, and of a pea aaBB. A zygote containing both rose
and pea is a walnut: a zygote containing neither rose nor pea
is a single. The peculiar feature of such a case lies in the fact
that absence of rose and absence of pea are the same thing,
i.e. single; and this is doubtless owing to the fact that the
characters rose and pea both affect the same structure, the
comb.
3. Cases exist in which the characters due to one allelomorphic
pair can only become manifest in the presence of a
particular member of the other pair. If in fig. 3 the characters
due to B–b can only manifest themselves in the presence of
A, it is obvious that this can happen in twelve cases out of
sixteen, but not in the remaining four, which are homozygous
for aa. An example of this is to be found in the inheritance
of coat colour in rabbits, rats and mice where the allelomorphic
pairs concerned are wild grey colour (B) dominant to black
(b) and pigmentation (A) dominant to albinism (a). Certain
albinos (aaBB) crossed with blacks (AAbb) give only greys
(AaBb), and when these are bred together they give 9 greys,
3 blacks and 4 albinos. Of the 4 albinos 3 carry the grey
character and 1 does not, but in the absence of the pigmentation
factor (A) this is not visible. The ratio 9 : 3 : 4 must be
regarded as a 9 : 3 : 3 : 1 ratio, in which the last two terms are
visibly indistinguishable owing to the impossibility of telling
by the eye whether an albino carries the character for grey
or not.
4. The appearance of a zygotic character may depend upon
the coexistence in the zygote of two unit-characters belonging
to different allelomorphic pairs. If in the scheme shown in
fig. 3 the manifestation of a given character depends upon
the simultaneous presence of A and B, it is obvious that 9 of
the 16 zygotes will present this character, whilst the remaining
7 will be without it. This is shown graphically in fig. 5, where
the 9 squares have been shaded
and the 7 left plain. The sweet pea
offers an example of this phenomenon.
White sweet peas breed true
to whiteness, but when certain strains
of whites are crossed the offspring
are all coloured.. In the next generation
(F2) these F1 plants give rise to 9
coloured and 7 whites in every 16
plants. Colour here is a compound
character whose manifestation depends
upon the co-existence of two factors
in the zygote, and each of the original parents was homozygous
for one of the two factors necessary to the production
of colour. The ratio 9 : 7 is in reality a 9 : 3 : 3 : 1 ratio
in which, owing to special conditions; the zygotes represented
by the last three terms are indistinguishable from one another
by the eye.
AA
BB
AA
Bb
Aa
BB
Aa
Bb
AA
bB
AA
bb
Aa
bB
Aa
bb
aA
BB
aA
Bb
aa
BB
aa
Bb
aA
bB
aA
bb
aa
bB
aa
bb
Fig. 5.
The phenomena of dihybridism, as illustrated by the four
examples given above, have been worked out in many other
cases for plants and animals. Emphasis must be laid upon
the fact that, although the unit-characters belonging to two
pairs may react upon one another in the zygote and affect
its character, their inheritance is yet entirely independent.
Neither grey nor black can appear in the rabbit unless the
pigmentation factor is also present; nevertheless, gametic
segregation of this pair of characters takes place in the normal
way among albino rabbits, though its effects are never visible
until a suitable cross is made. In cases of trihybridism the
Mendelian ratio for the forms appearing in F2 is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1, i.e. 27 showing dominance of three characters, three
groups of 9 each showing dominance of two characters, three
groups of 3 each showing dominance of one character, and
a single individual out of 64 which is homozygous for all three
recessive characters. It is obvious that the system can be
indefinitely extended to embrace any number of allelomorphic
pairs.
Reversion.—Facts such as those just dealt with in connexion
with certain cases of dihybridism throw an entirely new light
upon the phenomenon known as reversion on crossing. This
is now seen to consist in the meeting of factors which had in
some way or other become separated in phylogeny. The
albino rabbit when crossed with the, black “reverts” to the
wild grey colour, because each parent supplies one of the two
factors upon which the manifestation of the wild colour depends.
So also the wild purple sweet pea may come as a reversion
on crossing two whites. In such cases the reversion appears
in the F1 generation, because the two factors upon which it
depends are the dominants of their respective allelomorphic
pairs. Where the reversion depends upon the simultaneous
absence of two characters it cannot appear until the F2 generation.
When fowls with rose and pea combs are crossed the
reversionary single comb characteristic of the wild Gallus bankiva
first appears in the F2 generation.
CRB
CRB
CRB
CRb
CRb
CRB
CRb
CRb
CRB
cRB
CRB
cRb
CRb
cRB
CRb
cRb
CrB
CRB
CrB
CRb
Crb
CRB
Crb
CRb
CrB
cRB
CrB
cRb
Crb
cRB
Crb
cRb
CRB
CrB
CRB
Crb
CRb
CrB
CRb
Crb
CRB
crB
CRB
crb
CRb
crB
CRb
crb
CrB
CrB
CrB
Crb
Crb
CrB
Crb
Crb
CrB
crB
CrB
crb
Crb
crB
Crb
crb
cRB
CRB
cRB
CRb
cRb
CRB
cRb
CRb
cRB
cRB
cRB
cRb
cRb
cRB
cRb
cRb
crB
CRB
crB
CRb
crb
CRB
crb
CRb
crB
cRB
crB
cRb
crb
cRB
crb
cRb
cRB
CrB
cRB
Crb
cRb
CrB
cRb
Crb
cRB
crB
cRB
crb
cRb
crB
cRb
crb
crB
CrB
crB
Crb
crb
CrB
crb
Crb
crB
crB
crB
crb
crb
crB
crb
crb
Fig. 6.
Gametic Coupling.—In certain cases the distribution of characters
in heredity is complicated by the fact that particular
unit-characters tend to become associated or coupled together
during gametogenesis. In no case have we yet a complete
explanation of the phenomenon, but in view of the important
bearing which these facts must eventually have on our ideas
of the gametogenic process an illustration may be given.
The case in which two white sweet peas gave a coloured on
crossing has already been described, and it was seen that the
production of colour was dependent upon the meeting of two
factors, of which one was brought in by each parent. If the
allelomorphic pairs be denoted by C–c and R–r, then the zygotic
constitution of the two parents must have been CCrr and
ccRR respectively. The F1 plant may be either purple or red,
two characters which form an allelomorphic pair in which
the former is dominant, and which may be denoted by the
letters B–b. If B is brought in by one parent only the F1
plant will be heterozygous for all three allelomorphic pairs,
and therefore of the constitution Cc Rr Bb. In the F2 generation
the ratio of coloured to white must be 9 : 7, and of purple to
red 3 : 1; and experiment has shown that this generation is
composed on the average of 27 purples, 9 reds and 28 whites
out of every 64 plants. The exact composition of such a family
may be gathered from the accompanying table (fig. 6). So
far the case is perfectly smooth, and it is only on the introduction
of another character that the phenomenon of partial coupling
is witnessed. Two kinds of pollen grain occur in the sweet
pea. In some plants they are oblong in shape, whilst in others
they are round, the latter condition being recessive to the
former. If the original white parents were homozygous for long
and round respectively the F1 purple must be heterozygous, and
in the F2 generation, as experiment has shown, the ratio of
longs to rounds for the whole family is 3 : 1. But among the
purples there are about twelve longs to each round, the excess
of longs here being balanced by the reds, where the proportion is 1 long to about 3·5 rounds. There is partial coupling
of long pollen with the purple colour and a complementary
coupling of the red colour with round pollen. This result
would be brought about if it were supposed that seven out of
every eight purple gametes produced by the F1 plant carried
the long pollen character, and that seven out of every eight
red gametes carried the round pollen character. The facts
observed fit in with the supposition that the gametes are produced
in series of sixteen, but how such result could be brought
about is a question which for the present must remain open.
Spurious Allelomorphism.—Instances of association between
characters are known in which the connection is between the
dominant member of one pair and the recessive of another.
In many sweet peas the standard folds over towards the wings,
and the flower is said to be hooded. This “hooding” behaves
as a recessive towards the erect standard. Red sap colour
is also recessive to purple. In families where purples and
reds as well as erect and hooded standards occur it has been
found, as might be expected, that erect standards are to hooded
ones, and that purples are to reds as 3:1. Were the case one
of simple dihybridism the F2 generation should be composed
of 9 erect purples, 3 hooded purples, 3 erect reds and 1
hooded red in every 16. Actually it is composed of 8 erect
purples, 4 hooded purples and 4 erect reds. The hood will not
associate with the red, but occurs only on the purples. Cases
like this are best interpreted on the assumption that during
gametogenesis there is some form of repulsion between the
members of the different pairs—in the present instance between
the factor for purple and that for the erect standard—so that
all the gametes which contain the purple factor are free from
the factor for the erect standard. To the process involved
in this assumption the term spurious allelomorphism has been
applied.
Sex.—On the existing evidence it is probable that the inheritance
of sex runs upon the same determinate lines as that
of other characters. Indeed, there occurs in the sweet pea
what may be regarded as an instance of sex inheritance of
the simplest kind. Most sweet peas are hermaphrodite, but
some are found in which the anthers are sterile and the plants
function only as females. This latter condition is recessive to
the hermaphrodite one and segregates from it in the ordinary
way. Most cases of sex inheritance, however, are complicated,
and it is further possible that the phenomena may be of a
different order in plants and animals. Instructive in this
connexion are certain cases in which one of the characters
of an allelomorphic pair may be coupled with a particular sex.
The pale lacticolor variety of the currant moth (Abraxasgrossulariata) is recessive to the normal form, and in families
produced by heterozygous parents one quarter of the offspring
are of the variety. Though the sexes occur in approximately
equal numbers, all the lacticolor in such families are females;
and the association of sex with character exhibiting normal
segregation is strongly suggestive of a similar process obtaining
for sex also. Castle has worked out similar cases in other
Lepidoptera and has put forward an hypothesis of sex inheritance
on the basis of the Mendelian segregation of sex
determinants. An ovum or spermatozoon can carry either the
male or the female character, but it is essential to Castle's
hypothesis that a male spermatozoon should fertilize only a
female ovum and vice versa, and consequently on his view all
zygotes are heterozygous in respect of sex. Whether any such
gametic selection as that postulated by Castle occurs here or
elsewhere must for the present remain unanswered. Little
evidence exists for it at present, but the possibility of its
occurrence should not be ignored.
More recently evidence has been brought forward by Bateson
and others (3) which supports the view that the inheritance
of sex is on Mendelian lines. The analysis of cases where there
is a closer association between a Mendelian character and a
particular sex has suggested that femaleness is here dominant
to maleness, and that the latter sex is homozygous while the
former is heterozygous.
Chromosomes and Unit-Characters.—Breeding experiments
have established the conception of definite unit-characters
existing in the cells of an organism: in the cell histology has
demonstrated the existence of a small definite bodies—the
chromosomes. During gametogenesis there takes place what
many histologists regard as a differentiating division of the
chromosomes: at the same period occurs the segregation of the
unit-characters. Is there a relation between the postulated
unit-character and the visible chromosome, and if so what is
this relation? The researches of E. B. Wilson and others have
shown that in certain Hemiptera the character of sex is definitely
associated with a particular chromosome. The males of
Protenor possess thirteen chromosomes, and the qualitative division
on gametogenesis results in the production of equal numbers
of spermatozoa having six and seven chromosomes. The
somatic number of chromosomes in the female is fourteen, and
consequently all the mature ova have seven chromosomes.
When a spermatozoon with seven chromosomes meets an
ovum the resulting zygote has fourteen chromosomes and is a
female; when a spermatozoon with six chromosomes meets
an ovum the resulting zygote has thirteen chromosomes and
is a male. In no other instance has any such definite relation
been established, and in many cases at any rate it is certain
that it could not be a simple one. The gametic number of
chromosomes in wheat is eight, whereas the work of R. H. Biffen
and others has shown that the number of unit-characters in
this species is considerably greater. If therefore there exists
a definite relation between the two it must be supposed that a
chromosome can carry more than a single unit-character. It
is not impossible that future work on gametic coupling may
throw light upon the matter.
Heredity and Variation.—It has long been realized that the
problems of heredity and variation are closely interwoven,
and that whatever throws light upon the one may be expected
to illuminate the other. Recent as has been the rise of the
study of genetics, it has, nevertheless, profoundly influenced
our views as to the nature of these phenomena. Heredity
we now perceive to be a method of analysis, and the facts of
heredity constitute a series of reactions which enable us to
argue towards the constitution of living matter. And essential
to any method of analysis is the recognition of the individuality
of the individual. Constitutional differences of a radical
nature may be concealed beneath apparent identity of external
form. Purple sweet peas from the same pod, indistinguishable
in appearance and of identical ancestry, may yet be fundamentally
different in their constitution. From one may come
purples, reds and whites, from another only purples and reds,
from another purples and whites alone, whilst a fourth will
breed true to purple. Any method of investigation which
fails to take account of the radical differences in constitution
which may underlie external similarity must necessarily be
doomed to failure. Conversely, we realize to-day that individuals
identical in constitution may yet have an entirely different
ancestral history. From the cross between two fowls with
rose and pea combs, each of irreproachable pedigree for generations,
come single combs in the second generation, and these
singles are precisely similar in their behaviour to singles bred
from strains of unblemished ancestry. In the ancestry of the
one is to be found no single over a long series of years, in the
ancestry of the other nothing but singles occurred. The creature
of given constitution may often be built up in many ways,
but once formed it will behave like others of the same constitution.
The one sure test of the constitution of a living thing
lies in the nature of the gametes which it carries, and it is
the analysis of these gametes which forms the province of
heredity.
The clear cut and definite mode of transmission of characters
first revealed by Mendel leads inevitably to the conception
of a definite and clear-cut basis for those characters. Upon
this structural basis, the unit-character, are grounded certain
of the phenomena now termed variation. Varieties exist as
such in virtue of differing in one or more unit-characters from what is conventionally termed the type; and since these unit-characters
must from their behaviour in transmission be regarded
as discontinuous in their nature, it follows that the variation
must be discontinuous also. A present tendency of thought
is to regard the discontinuous variation or mutation as the
material upon which natural selection works, and to consider
that the process of evolution takes place by definite steps.
Darwin’s opposition to this view rested partly upon the idea
that the discontinuous variation or sport would, from the
rarity of its occurrence, be unable to maintain itself against
the swamping effects of intercrossing with the normal form.
Mendel’s work has shown that this objection is not valid, and
the precision of the mode of inheritance of the discontinuous
variation leads us to inquire if the small or fluctuating variation
can be shown to have an equally definite physiological basis
before it is admitted to play any part in the production of
species. Until this has been shown it is possible to consider
the discontinuous variation as the unit in all evolutionary
change, and to regard the fluctuating variation as the uninherited
effect of environmental accident.
The Human Aspect.—In conclusion we may briefly allude to
certain practical aspects of Mendel’s discovery. Increased
knowledge of heredity means increased power of control over
the living thing, and as we come to understand more and more
the architecture of the plant or animal we realize what can
and what cannot be done towards modification or improvement.
The experiments of Biffen on the cereals have demonstrated
what may be done with our present knowledge in establishing
new, stable and more profitable varieties of wheat and barley,
and it is impossible to doubt that as this knowledge becomes
more widely disseminated it will lead to considerable improvements
in the methods of breeding animals and plants.
It is not, however, in the economic field, important as this
may be, that Mendel’s discovery is likely to have most meaning
for us: rather it is in the new light in which man will come
to view himself and his fellow creatures. To-day we are almost
entirely ignorant of the unit-characters that go to make the
difference between one man and another. A few diseases,
such as alcaptonuria and congenital cataract, a digital malformation,
and probably eye colour, are as yet the only cases
in which inheritance has been shown to run upon Mendelian
lines. The complexity of the subject must render investigation
at once difficult and slow; but the little that we know to-day
offers the hope of a great extension in our knowledge at no
very distant time. If this hope is borne out, if it is shown
that the qualities of man, his body and his intellect, his immunities
and his diseases, even his very virtues and vices, are
dependent upon the ascertainable presence or absence of definite
unit-characters whose mode of transmission follows fixed laws,
and if also man decides that his life shall be ordered in the light
of this knowledge, it is obvious that the social system will have
to undergo considerable changes.
Bibliography.—In the following short list are given the titles of
papers dealing with experiments directly referred to in this article.
References to most of the literature will be found in (11), and a
complete list to the date of publication in (3).
(1) W. Bateson, Mendel’s Principles of Heredity (Cambridge,
1902), contains translation of Mendel’s paper. (2) W. Bateson, An
Address on Mendelian Heredity and its Application to Man,
“Brain,” pt. cxiv. (1906). (3) W. Bateson, Mendel’s Principlesof Heredity (1909). (4) R. H. Biffen, “Mendel’s Laws of Inheritance
and Wheat Breedings,” Journ. Agr. Soc., vol. i. (1905)
(5) W. E. Castle, “The Heredity of Sex,” Bull. Mus. Comp. Zool.
(Harvard, 1903). (6) L. Cuénot, “L’Hérédité de la pigmentation
chez les souris,” Arch. Zool. Exp. (1903–1904). (7) H. de Vries,
Die Mutationstheorie (Leipzig, 1901–1903). (8) L. Doncaster and
G. H. Raynor, “Breeding Experiments with Lepidoptera,” Proc.Zool. Soc. (London, 1906). (9) C. C. Hurst, “Experimental
Studies on Heredity in Rabbits,” Journ. Linn. Soc. (1905). (10)
G. J. Mendel, Versuche über Pflanzen-Hybriden, Verh. natur. f. ver.in Brünn, Bd. IV. (1865). (11) Reports to the Evolution Committeeof the Royal Society, vols. i.–iii. (London, 1902–1906, experiments by
W. Bateson, E. R. Saunders, R. C. Punnett, C. C. Hurst and others).
(12) E. B. Wilson, “Studies in Chromosomes,” vols. i.–iii. Journ.Exp. Zool. (1905–1906). (13) T. B. Wood, “Note on the Inheritance
of Horns and Face Colour in Sheep,” Journ. Agr. Soc. vol. i.
(1905). (R. C. P.)
