'"""A LECTURE ON

MENDELISM

BY H. DRINKWATER

THE LIBRARY

OF

THE UNIVERSITY

OF CALIFORNIA

LOS ANGELES

GIFT OF

Santa Monica Public Library

A LECTURE ON MENDELISM

Gregor Mendel

[Frontispiece.

A LECTURE ON
' MENDELISM

BY

H. LDRINKWATER
M.D., F.R.S. (Edin.), F.L.S.

WITH NUMEROUS ILLUSTRATIONS

LONDON
J. M. DENT & SONS, Ltd
29 & 30 BEDFORD STREET, W.C.
1910

All rights reserved

M-31

P632L

INTRODUCTION

The following Lecture is published at the
earnest request of several friends who are
interested in the subject and who heard it
delivered as one of a series of " Science
Lectures for the People " early in the present
year.

I have to thank Mr. Bateson and Mr. Punnett
for kindly allowing me to include their portraits
and also must acknowledge my indebtedness
to their works on Mendelism for many of the
statements contained in this lecture.

H. Drinkwater

Wrexham,

Sept. igio.

A LECTURE ON MENDELISM

It is never easy, and sometimes impossible, to explain
the facts of any science without using its technical language.
For technical terms used by scientists are not, as is
commonly supposed, arbitrary conventions designed for
the exclusion of the layman, but are really abbreviations
and simplifications which enable the student to handle in
single terms complicated sets ot ideas and associations.
I will, however, do my best to avoid these technicalities
and will only use them when their content has been made
clear by the course of the narrative.

As Darwinism is the theory of the origin of species
which takes its name from Charles Darwin, so by Men-
delism we mean the biological theory of heredity first
propounded by Mendel.

Gregor Mendel was born on the 27th July, 1822, at
Heinzendorf in Austria. His parents were peasant farmers.
They were enabled to send him to Vienna to pursue his
studies, and there he took a University degree. When
twenty-one years old he joined the Augustinian Order, and
three years later was ordained priest. Soon afterwards he
removed to Brunn, a few miles north of Vienna, and here
he taught natural science in the Realschule from 1853 to
1868, when he was appointed abbot of the Monastery.

During the years he was teaching science in the
Monastery School at Brunn, he spent a great deal of time
in his little garden, working quietly, patiently, and alone,
engaged in his favourite hobby, the cultivation of the edible
and the sweet pea. He made careful observations, and

2 MENDELISM

kept exact records of about 10,000 plants which he had
cultivated. He was well versed in other branches of science,
especially meteorology.

Eventually he became president of the Brunn Society
of Naturalists.

In 1866 he published an account of his observations on
Peas, but the scientific world of Germany did not appreciate
their importance, and no one in England seems to have
heard anything about them. His work was indeed lost
sight of, until it was re-discovered by Prof. De Vries
of Amsterdam about the beginning of the present century.

In 1902 Prof. Wm. Bateson of Cambridge trans-
lated Mendel's monograph into English, and published
it under the title Mendel s Principles of Heredity.

Mendel died in 1884, at the age of sixty-two years.

That branch of science which Mendel founded is con-
cerned with living things — both animals and plants — that
is, with Biology. Mendel's work has supplied to Biology
a key or guide which may be compared to the mariner's
compass ; for, just as by the aid of the compass the mariner
is able to take the most direct course from one place to
another, so, by following Mendel's method, the biologist is
now able to achieve results much more quickly, and with
greater certainty than hitherto.

Mendel's work constitutes "one of the most fascinating
chapters in the history of science." It is interesting not
only to the theoretical biologist, but will, I believe, before
long be recognized as of great economic, commercial and
social importance.

I hope to be able to show that each one of you can, if
you like, make observations without cost, and with com-
paratively little trouble, by which you may not only test
the truth of the laws which I shall explain to you, but you
may be able to contribute some useful facts. " Mendel's

Professor Bateson

[ To face p. 2.

MENDEL1SM 3

clue has shown the way into a realm of nature which, for
surprising novelty and adventure, is hardly to be excelled.
It has led us into a new world the very existence of
which was unsuspected before." I shall first show you
how Mendel gradually developed his theories, and then
proceed to describe some of the later results achieved by
his followers.

Let us first consider Mendel's Method.

His practice was to select two plants, such as two peas,
which differ in some well-marked character — such as height
of stem, colour of flowers, or shape and colour of the seeds,
and he produced offspring from these contrasted plants,
using one as the male and the other as the female parent.

In order that this may be clearly understood it is neces-
sary to know how plants are normally reproduced ; and in
case there may be some one present who does not under-
stand this, it will be well briefly to describe the process.
Almost any plant will answer the purpose, but we will take
a primrose shown in section in Fig. i. Within the tube
of the flower (the corolla), and attached to it, are some
small bodies called anthers, which
contain a fine powder called pollen.
When the anther is ripe it bursts, and
the pollen becomes dispersed. In the
very centre of the flower is the pistil,
surmounted at the top by a little
knob called the stigma, which is
generally covered with a sticky juice. Fig. i.

The lowest part of the pistil is called the ovary — a hollow
chamber containing small bodies called ovules. These
ovules eventually develop into ripe seeds, as a result of
uniting with the pollen contained in the anthers.

How does the pollen from the anthers reach the ovules
in the closed ovary ? When the anther bursts, and the

MENDELISM

liberated pollen is scattered about, some of it alights on the
stigma, and is retained there by the sticky juice (Fig. 2).
Very soon the pollen grain begins to shoot out little finger-
like processes called "pollen-tubes," which grow down the

Fig. 2.

A and B are pollen grains with commencing " pollen tubes."
C — Section (vertical) of stigma ; showing two pollen grains resting
on the upper surface, with their downward growing tubes.

whole length of the pistil, until they eventually reach the
ovules. The pollen-tube then enters an ovule, and the two
bodies (pollen grain and ovule) become blended into one
body, and a ripe seed is the result of the union.

The union of pollen grain with ovule constitutes what is
termed " fertilization " : the pollen is said to fertilize the
ovule ; and without the addition of the pollen the ovules
will not ripen into "seeds." The pollen is the male ele-
ment, the ovule is the female element. We may have " self-
fertilization," and " cross-fertilization." When pollen fer-
tilizes the ovules of its own flowers, we get " self-fertilization,"
but the pollen of one flower may be carried by such insects
as bees, butterflies and moths to another flower, and effect
"cross-fertilization." If the pollen from A (Fig. 3) fer-

MENDEL ISM

tilizes the ovules of B, or if the pollen from B fertilizes
the ovules of A we get cross-fertilization.

Gardeners sometimes effect cross-fertilization, or
" crosses," by placing the pollen from one flower on to
the stiema of another flower.

The flower from which the pollen is derived is the male
parent ; that which contains the ovules (and seeds) is the
female parent.

A

Fig.

The Two Types of Primrose.

Male and female may be represented by the letters

M (male) and F (female), but they are best represented

by certain astronomical signs employed by scientists of

all countries —

£ stands for male ;

£ stands for female.

The male and female elements, the pollen grains and
ovules, are now called "gametes," which means marry ing-
cells, but it will be easier to follow me if I use the better-
known term "germ-cells." The germ-cells of plants, then,
are the pollen grains and ovules. The same term (germ-
cells) is also applied to animals ; the process of reproduction
is essentially the same in them as it is in plants.

Reproduction or fertilization may be represented in
diagrammatic form thus —

? x t

6 MENDELISM

The cross ( x ) is to be read as " fertilized by." This therefore
shows that a female germ-cell is fertilized by a male germ-
cell. A vertical line placed under the cross points to
the offspring which results from the fertilization, a simple
circle indicating the offspring without showing the sex.
Thus- x

When the two germ-cells which unite in fertilization are
derived from two plants which are essentially similar, the
resulting offspring is said to be "pure-bred." When the
two parents show some well-marked difference, as in stature
or colour of flowers, the offspring is said to be " cross-bred,"
so far as this particular character is concerned, and such a
cross-bred individual is called a " hybrid."

The similarity or difference of the parents may be
represented by the sign, thus —

£ and $ represent two similar parents ;

$ and ?, or $ and ? represent two parents which differ in

some important feature.
? x $

represents the production of pure-bred
o offspring ;

$ x^ $ x

or
o o

represents the production of cross-bred
or hybrid offspring.

Now, when the parents show a marked difference, what
will the hybrid offspring be like ? Will it be like one of
the parents, and if so, which one ? Or will it be inter-
mediate between the two ? Clearly it cannot be like both.

This is what Mendel studied experimentally. He
sought to discover the Law of Inheritance in these hybrid
offspring ; in other words, the law or laws governing the
resemblance between parent and offspring.

MENDELISM 7

Mendel's method of experimentation was as follows —
First of all he removed the anthers from a flower just
before they were ripe, in order to prevent self-fertilization :
this would be the female parent. Then he placed upon its
stigma the ripe pollen from another flower, the male parent.
Fig. 4 shows the pollen from a r<?«/^-seeded plant
being placed upon the stigma of a plant which produces
angular seeds. The round seeds and the angular seeds
constitute a pair of contrasted characters. These angular
seeds when ripe were sown, and produced plants which

Anther
*1*»«'.X% discharging
»...v\A Pollen.

Ripe Angular ggA \2 b/ ( 'Wi Ripe Round

Seed. v-*^ ^J^/ V^ Seed

Ovary containing
Nine Ovules.

Fig. 4.

Cross fertilization.

were, of course, hybrids, so far as the shape of the seeds
was concerned, for the shape differed in the two parent
plants.

What Mendel wished to observe in this experiment was
the shape of the seeds in this hybrid offspring. Other
characters were crossed in a similar manner, such as —

A tall with a dwarf plant ;
Yellow-seeded with green-seeded plants ;
Coloured flowers with white flowers, and so on.

In each case the male plant showed one character, it
mattered not which, and the female showed the contrasted
character.

8 MENDELISM

You would probably expect the offspring to be inter-
mediate in character between the two parents, but such was
never the case. In each instance the offspring exactly
resembled one parent, i. e. it showed only one of any pair
of contrasted characters possessed by the parents, to the
entire exclusion of the other character.

Thus (Fig. 5) a tall plant crossed with a dwarf produced
only tall offspring : e.g. the ordinary tall pea was crossed
with the cupid variety, which is only 3 or 4 inches high ;
the ripe seeds were sown, and produced only tall
plants —

Coloured flowers crossed with white produced coloured

flowers only ;
A yellow-seeded plant crossed with a green-seeded one

produced only yellow seeds ;
Smooth-seeded crossed with wrinkled-seeded plants gave

smooth seeds only.

Thus there was always one of the two contrasted
characters in each cross that appeared exclusively in the
hybrid offspring, and to this character Mendel applied the
term Dominant, while that character which did not appear
in the offspring he termed Recessive.

These two terms ("dominant" and "recessive") you
should particularly remember. They are terms for which
I am not able to give you any simpler equivalents. You
will meet with them repeatedly in the literature of
Mendelism.

As a cross between a tall and a dwarf plant produces
only tall offspring, tallness is said to be dominant to
dwarfness —

Coloured flowers are dominant to white flowers ;
Yellow seeds are dominant to green seeds ;
Smooth seeds are dominant to wrinkled.

Tall

Plant

Cl ILOURED

Flowers

Yellow-
Seeds

Smooth
Seeds

Dwarf
Plant

White

Flowers

Green

Seeds

Wrinkled

Seeds

Dominant Characters

Fig.

Recessive Characters
[ To face p.

MENDELISM 9

This is the first of Mendel's results, and I hope it has
been made clear to you. It forms the foundation of what
follows.

We require some short formula which will express the
fact of dominance. If we are considering stature, we can
employ the following formula —

Tall x Dwarf (parents)

Tall (offspring)

which means that when a tall plant is crossed with a dwarf
all the offspring are tall.

But it is desirable to use some formula which will
express the fact of dominance of any character, and this
can be done by using D to represent a dominant and R to
represent a recessive ; thus —

D x R

D

You see this applies not only to stature, but to any of

the characters above mentioned.

Another formula very commonly employed is the

following —

• x o

where the black circle stands for the dominant, and the
white circle represents the recessive.

Both the diagrams show that the offspring of certain
crosses resemble one of the parents, and not the other.
The parent which is like the offspring is the dominant
parent.

The next step was to breed from these hybrid plants
by allowing them to become self-fertilized (the usual method
of reproduction in peas). Mendel found that the two

10 MENDELISM

contrasted characters of the parents reappeared in one or

other of the offspring of the hybrids.

Pure-bred tall peas bred amongst themselves breed

true, i. e. they produce only tall plants ; similarly, the

dwarfs breed true amongst themselves ; but these hybrids,

though apparently like the tall parent, do not breed true,

but produce both tails and dwarfs. Now this is a most

remarkable fact. In the first generation of hybrids the

dwarf character has disappeared ; apparently it has been

destroyed, and yet it reappears in the offspring of these

hybrids. The two kinds, the dwarfs and tails produced by

the hybrids, appeared, however, in unexpected proportions,

for there were always three tails to one dwarf, i. e. three

dominants to one recessive ; and this proportion of three

dominants to one recessive holds good, not only for

stature, but for whatever pair of contrasted characters be

experimented with. This is represented by the following

diagram —

D x R

D

ill i

D D D R

It is useful to employ some concise terms to indicate
each of these three generations respectively. The first is
called the "parental generation," and is indicated shortly
by the letter P. The second is the " first filial generation,"
and indicated by Fj (the word "filial " being derived from
the Latin word for son or daughter). The third generation
is called the " second filial generation," and is indicated by
Fij. If D represents a coloured flower, and R a white
one, then in the Fj generation we get only coloured flowers,
and in the Fij generation we get three plants bearing
coloured flowers to one bearing white flowers, and so on
with the other characters named.

MENDELISM 11

Observe ! the recessive character (in Fij) has been
inherited from a hybrid parent which looked exactly like
the original dominant parent ; it follows, therefore, that
this recessive character must have been present, though
hidden, in the hybrid, and that the hybrid is really different
from the original parent, though like it in appearance.
This difference may be expressed in the diagram by letting
D[R] represent the hybrid, putting the R in brackets, to
show that the recessive character is present, though latent,
and by mere inspection quite unrecognizable. In other
words, the hybrid has a double nature : the dominant and
the recessive factors are both present in the hybrid, though
only the dominant shows. The hybrid is an "impure
dominant," and does not breed true to the dominant
character. The fact that pure dominants and pure
recessives breed true to their respective characters is
indicated thus —

D x D R x R

and

D R

You will see that it is necessary to amend the diagram
on page 10 in order to indicate that the Fj generation
consists of impure dominants, thus —

D x R

D[R]

ill i

D D D R

Thus we cannot always tell what the offspring will be
like from simple inspection of the parents, for tall plants
may produce dwarfs. The results are not what would be
looked for in accordance with the ordinary view of heredity
that "like produces like."

12 MENDELISM

How did Mendel explain these results ? Here he
showed his genius. He assumed that these hybrids —
D[R] — produce two kinds of germ-cells in each sex,
two kinds of pollen grain, and two kinds of ovules. One
kind possesses the dominant character derived from the
dominant parent, the other kind possesses the recessive
character of the recessive parent. In other words, the
dominant and the recessive properties of the parents are
conveyed to separate germ-cells of the hybrid. No single
germ-cell can carry both dominant and recessive properties.
The germ-cells are not hybrids, though the individuals
that produced them may be. This is' what Mendel called
Segregation and it is the most characteristic of his theories.

The Germ-cells produced by a dominant parent are
all purely dominant ; those of the recessive parent are all
purely recessive ; and of those produced by the hybrid,
some are dominant and some are recessive. If the dominant
is represented by a black circle and the recessive by a white
one, then the dominant parent and its germ-cells can be
diagrammatically illustrated thus —

D

i i I i

• • • •

The recessive thus —

R

i

I I ! I

o o o o
The hybrid thus —

D[R]

!

I I I I

• • o o

There is only one kind of germ-cells in the dominant,
one kind in the recessive, but there are two kinds in the
hybrid.

MENDELISM 13

The tall and dwarf pea crossed could be illustrated
thus, together with the germ-cells —

• • o o

produce

• o

According to Mendel, a germ-cell is either purely
dominant, or purely recessive, and never partly the one and
partly the other — always ooro, and never ©.

I must apologize for repeating myself, but this is the
essence of Mendel's theory, and no progress can be made in
Mendelism without a clear appreciation of the constitution
of the germ-cells.

As the character of a plant depends upon the nature of
the germ-cells from which it has sprung, it follows that the
Fj generation must resemble the dominant parent, and that
a tall pea crossed with a dwarf must produce tall offspring ;
but as the hybrids possess two kinds of germ-cells, their off-
spring are not of one kind only : some are tall, and others
are short (in the Fij generation).

So far, we have seen that Mendel established the fact of
dominance ; and, secondly, he assumed that hybrids contain
germ-cells similar in character to those of both parents.

How did he explain the 3 to 1 ratio in the Fij generation ?
Now we come to another important doctrine of Mendel —
another stroke of genius. Mendel concluded that not only
are both dominant and recessive germ-cells produced in the

14 MENDELISM

hybrid, but that they are produced in equal numbers. Of
four ovules, two will be dominant and two recessive ; and
the same will hold true for the pollen grains.

But how will equal numbers of dominant and recessive
germ-cells produce offspring in which the dominants exceed
the recessives by three to one ? For four individuals, we
shall require four pollen grains and four ovules. Let us
assume that a hybrid produces four ovules which are fertilized
by four of its own pollen grains. What is most likely to
happen, on the assumption that the dominant and recessive
germ-cells exist in equal numbers, is shown by this diagram —
Of the two dominant pollen grains-one will unite with a
dominant ovule (a) ; the other with a recessive ovule (6) ;
and of the two recessive pollen grains one will meet the
remaining dominant ovule (c), and the other the recessive
ovule (d). That is, all the possible combinations will occur.
Other combinations than the above four might occur, and
sometimes do so, — for instance, the pollen grain might
unite with the second dominant ovule ; but in the great
majority of cases the result is as above stated, and so we
get three individuals [a, 6, and c) produced by at least one
dominant germ-cell, and the result will be that they appear
like the dominant parent ; the fourth will be recessive.

Ovules. Pollen grains.

• < • •

a •

O

o °

"2 o

o <-
Germ-cells. Resulting individuals.

The result, you observe, is the same whether it be the
pollen or the ovule that is dominant. It is, however, clear
from the diagram that the three dominant offspring, though

MENDELISM 15

to all appearance quite similar, are yet differently constituted,
for a is formed of two dominant germ-cells, whereas in b the
pollen grain alone is dominant, and in c the ovule only is
dominant.

a is a "pure dominant";

b and c are " impure dominants " ;

d is a "pure recessive."

Thus in the Fij generation we get, not three dominants
to one recessive, but one pure dominant, two impure
dominants, and one pure recessive. We must therefore
amend the diagram on p. 1 1 thus —

D x R
D[R] = Fj

DD D[R] D[R] RR = Fij

The former diagram shows their appearance — this shows
their Constihition.

This is the usual result of the self-fertilization of
hybrids of any contrasted pairs, and thus

in the Fij generation we get 3 tails to 1 dwarf,

3 coloured to 1 white,

3 smooth seeds to 1 wrinkled,

3 yellow seeds to 1 green.

When breeding is further continued we find that the
pure dominant (DD) of Fij always breeds true, and pro-
duces— e. g. in case of stature, none but tall plants ; the
recessive (RR) produces none but dwarf plants ; and the
impure dominants (D[R]) produce the same kinds of off-
spring, and in the same proportions that the Fj generation
produced, viz. —

1 DD, 2 D[R], and 1 RR.

16 MENDELISM

The DD and D[R] plants look alike, and can only be
distinguished by breeding. This being so, it follows that
it is necessary for the horticulturist to keep the seeds of
each of these plants separate if he wishes to find out
which particular plant is the pure dominant. Hitherto it
has been customary to mix the seeds of all individuals
which appear alike, with the result that plants grown from
them have shown various characters.

How is dominance accounted for? Messrs. Bateson
and Punnett attribute it to the presence of some factor or
ingredient, and the recessive character to its absence.
Now, if recessives are marked by -the absence of some
factor, it follows that they can never reproduce the
dominant character which depends for its manifestation
upon the presence of this factor. A dwarf plant can never
reproduce a tall, for tallness depends upon something which
is absent from the dwarf. This has a most important
bearing upon certain inherited diseases, for if the disease
be dominant, owing to the presence of something, then
the recessive individuals will not inherit it, and will not
transmit it to their offspring.

Let us now summarize the Laws that appear to me
to have been established as regards a single pair of
contrasted characters.

(i) D x D produces DD.

(2) R x R .. RR.

(3) D x R

(4) D[R] x R

(5) D[R] x D

(6) D[R] x D[R]

D[RJ

D[R] + R in equal numbers.

D[R] + D

3D+1R,

or more correctly 1 DD + 2 D[R] + RR.
Law 4 may be shown to be true, thus —

Professor Punnf.i t

[ To face p. 17.

MENDELISM 17

D[R] R

• x o §

•
o

o
o

O X o §

• X o =
O X o

Law 6 is illustrated in the self-fertilization of hybrids.
These laws have been confirmed in several species of
plants and animals.

I must now draw your attention to the fact that
dominance is not invariably a feature of either member of
a pair of contrasted characters. Sometimes the hybrid
differs considerably from both parents. An exceedingly
interesting example of this fact has been worked out by
Prof. Punnett. He says, " Breeders have long recognized
the difficulty of obtaining a pure breed of the Blue
Andalusian Fowl. No matter how carefully the blues are
selected, they always throw ' wasters ' of two sorts, some
pure black, and others of a peculiar white with black
splashes. Careful breeding shows that, on an average, one
half of the offspring from a pair of blue andalusians come
blue, one quarter black, and one quarter white. These
numbers at once suggest that the blues are hybrids " (see
Law 6). " If this is so, it follows that the blacks and
the splashed whites are pure, and ought to breed true.
Experiment has shown that such is actually the case.
Further, we should expect hybrid offspring from a union of
the two pure breeds, the black and the white " (see Law 3).
" Here, again, experimental results accord with theory.
When a black and a white are bred together, all their
offspring, without exception, are blue. The black and the
white are pure-breeds, the so-called ' pure blue ' is, and
from its nature ever must be, a mongrel. The black and
the white are two contrasted characters, and the essential
feature of Mendel's theory, ' that the germ-cells remain

c

18 MENDELISM

pure in respect of each character,' could not be better
shown."1

In the following diagram the black circles represent
the black andalusians, the white circles the white birds,
and the dotted ones the blue birds.

0X0

I I i I
• X • 0 © oxo

X o

So far we have been considering Mendel's study of a
single pair of contrasted characters, but he also observed
the results of crossing plants which exhibited two pairs.

He called this " di-hybridism," or double-hybridism.
For instance, he studied height and colour in the same
hybrids by crossing a tall pea bearing coloured flowers
with a dwarf white. Here you have two plants contrasted
as to size and also as to the colour of the flowers. Of
course the results are more complicated than when a single
character only is dealt with. He found that each character
acts independently, for tallness and colour are always
dominant to dwarfness and absence of colour. You can
therefore tell me what will be the result of —

Tall coloured x Dwarf white.

The Fj generation will all be tall coloured.

The Fij generation will follow the result already
established (Law 6) and consist of coloured plants and white
in the proportion of 3 to I, and tails to dwarfs in the
proportion of 3 to 1. The smallest number of plants in

1 The wording of the above quotation has been slightly altered so as
to avoid technical terms.

MENDELISM 19

Fij which will show all these results is 16, as illustrated in
this diagram —

I

x D
i

i

iisi nil iiib nan

o

Here you observe 12 dark stripes representing the
plants with coloured flowers, and 4 light stripes to represent
the white ones. 12 to 4 is the 3 to 1 ratio. There are
also 3 tails to every dwarf. So that we get

9 tall coloured,
3 dwarf coloured,
3 tall white,
1 dwarf white.

This ratio of 9-3 -yi (9 with both dominant characters,
3 with only one such, 3 with the other, and 1 with both
recessive characters) is true of any two pairs of contrasted
characters.

I submit that a theory which will explain such a
complicated result is most satisfactory. Mendelism — and
so far Mendelism alone — has furnished the explanation.

I will now give you a problem to work out for yourselves.
What will be the result of crossing wrinkled yellow seeds
with smooth green if yellow is the dominant colour and
smoothness the dominant surface ? —

WY x SG

The Fj generation will be — what ? —

SY ( = smooth yellow)

Suppose a gardener ignorant of Mendelian laws wished
c 2

20 MENDELISM

to produce a wrinkled green pea as a new variety. Would
he not at once say, after seeing the above result, " I have
not only lost the wrinkled shape but I have lost the green
colour as well ; it is therefore of no use to continue the
experiment"? But with Mendelism as a guide he now
knows that he must get what he wants in the Fij generation,
which will consist of — what ? —

SY WY SG WG

(9) (3) (3) (i)

And the gardener would know that the wrinkled green,
being a pure recessive, must continue to breed true. He
would thus have produced a new variety — and, please note
he has obtained this result in the third year.

Some beautiful results have been obtained by Mr.
Lock with Maize. Maize seeds are mostly yellow or white,
and may be smooth or wrinkled. By crossing these we get

W x W = W ; YxY = Y; Y x W = Y.

This shows that Y is dominant to white. The Fj
yellow is a hybrid = D[R]. If it be crossed with W
(Recessive) we ought, according to Law 4, to get yellow
seeds and white seeds in about equal numbers. The actual
numbers obtained by Lock were —

26751 white,
26792 yellow —

a very close approximation to theoretical requirements.
The yellow hybrid self-fertilized should produce

3 Y, 1 W.

Lock obtained

16705 yellow,

5568 white— (5568 x 3 = 16704).

MENDELISM 21

Other results occurred as follows —

Smooth x Wrinkled

i
S

I

i i

s w

53Jo 1765 (1765 x 3 = 5295).

Smooth yellow x Wrinkled white.

SY

SY WrY SWh WrWh

The Fj SY x WrWh (see Law 4)

S^ WrY SW WrW

2869 2933 2798 2803

What is the inference from all these results? It seems
abundantly evident that plants and animals are built up
of a number of indivisible unit factors upon which their
characters depend, and that these units are capable of
acting independently of one another ; e.g. in the maize
seeds just referred to, the yellow, the white, the smooth
and the wrinkled characters depend each upon a separate
and quite independent factor. This is one of the most
important conclusions to which we are led by Mendel's
experiments.

"The fact," says Prof. Bateson, "that two cells are
concerned in the production of all ordinary forms of life was
discovered long ago, and has been part of the common
stock of elementary knowledge of all educated persons for
about half a century. The full consequences of this double
nature seem to have struck nobody before Mendel. Simple
though the fact is, to many it is difficult to assimilate as a
working idea. We are accustomed to think of a man, a

22 MENDELISM

butterfly, or an apple-tree as each one thing. In order to
understand the significance of Mendelism we must get
thoroughly familiar with the fact that they are each two
things, double throughout every part of their constitution.
It is a good exercise tp examine the people one meets with
in daily life and to try, in a rough way, to analyze them
into the two assemblages of characters which are united in
them. That we are assemblages or medleys of our parental
characteristics is obvious. We all know that a man may
possess some of his father's features, whilst in other respects
he resembles his mother. Now it is not generally seen
that in each feature he is double. Yet it is obvious that
the contribution of the male and female germ-cells may,
in respect of any of the ingredients, be either the same
or different. In any case in which the contributions made
by the two cells is the same, the resulting individual is pure-
bred for that ingredient ; and in all respects in which the
contribution from the two sides of the parentage is
dissimilar, the resulting individual is cross-bred."

Thus every character of an individual (animal or plant)
has a double origin.

What is the constitution of the germ-cells which any
individual produces? " If both parent germ-cells brought
a certain character in then all the daughter germ-cells will
have it" (Law i) ; "if neither brought it in, none of the
daughter cells have it" (Law 2) ; "if it came in from one
side and not from the other, then, on an average, in half
the immediate descendants it will be present, and in the
other half it will be absent" (Law 4; p. 17).

It follows that when a germ-cell transmits a certain
quality, it transmits it as an undivided unit to another
germ-cell — it transmits it undiluted, so that the resulting
germ-cells either contain a full share of a certain quality, or
none of it. That is, the quality is present or it is absent.

MENDELISM 23

It also follows — and this is very important — that purity
of type or breed has nothing to do with a prolonged course
of selection or inheritance, as hitherto supposed. The black
and the white andalusian fowls born of bine mongrel parents
are just as " pure " as those which never had any " blue "
ancestors.

There are some Apparent Exceptions to the rules I
have enunciated ; for instance, all white peas breed true,
and only produce white flowers when self-fertilized. Most
white crosses also breed true, but not invariably so.
Prof. Bateson found that amongst the white peas called
" Emily Henderson " the pollen grains are of two shapes,
though only one kind is found in any particular flower..
When he crossed these two kinds, there resulted, not white
flowers, but purple ones only, in the Fj generation, and in
the Fij generation 9 purples to 7 white in every 16.

White x White [" Emily Henderson "]

Purple

9 Purples 7 Whites

Here we meet with what, at first sight, seems an
exception to the rule. One might have expected 3 Purples
to 1 White in the Fij generation (Law 6). But we are
really dealing with a case of di-hybridism, in which the
9-3 -y 1 numbers are changed to 97 by including the 3-3-1 in
one form. The explanation is very ingenious, and throws
a flood of light on what is known as " reversion to an
ancestral type." This is perhaps the greatest difficulty
which one encounters at the beginning of the study of Men-
delism, but I will endeavour to clear up the difficulty. It is
assumed that, in order to produce colour, two factors must
be present. If one factor is present in one parent, and the
other factor in the other parent, their mating will result in

24

MENDELISM

coloured offspring; the Fj generation will all.be coloured
because the two factors meet and combine. You might
compare it to the action of one chemical upon another. I
have in these two bottles1 two different chemicals dissolved
in a large proportion of water. They are both colourless,
or nearly so.2 You observe that on mixing them together
a bright purple colour is produced. Something comparable
to this probably occurs in the production of purple flowers
from two white-flowered parents — i. e. two colourless
substances combine, and their union produces colour.

Supposing this theory to be correct, the following
diagram (borrowed from Mr. Punne'tt's Mendelisni) will

(i) 4 plants
where the
A factor
comes from
both pa-
rents.

(3) 4 plants
where A
comes from
the other
(female)
parent only.

(2) 4 plants
where A
comes from
one' (male)
parent only.

(4) 4 plants
where A is
entirely
absent.

help you to understand how it happens that in the Fij
generation we get 9 coloured- to 7- white-flowered plants.

Let A represent one factor and B the other factor
which must combine in order to produce colour, and small
a and b their respective absence ; then the diagram shows
all the possible combinations of the A and B factors derived
from the two parents in the 16 resulting individuals.

1 Shown at lecture.

2 One contained a few drops of perchloride of iron, and the other a
few grains of salicylate of sodium.

MENDELISM 25

There are 4 different A groups, each containing- 4
plants.

All the possible arrangements of B [= BB Bb bB bb]
are combined with each of the A groups.

Some of the squares are seen to contain both A and B ;
some contain A only ; some B only, and one neither A nor B.
Those which contain both A and B represent the coloured
flowers, and there are 9 such ; the other squares, of which
there are 7, contain A only, or B only, or neither, and these
represent the white flowers. You can, however, see that
these 7 plants will differ in constitution, for 3 contain A
only, 3 contain B only, and 1 contains neither A nor B ; so
that it is clearly an instance of the 9'3'3'i ratio, though
the last three kinds are identical in appearance. That these
7 are really different, and that some contain one factor and
some the other factor, can be shown by mating them
together ; for those which contain A only, if mated
with those which contain B only, will produce coloured
offspring.

Two breeds of white rabbits have produced in the
Fj generation all greys, and in the Fij generation 9 greys,
3 blacks, and 4 whites.

This is another modification of the 9*3'3'i ratio,
which I must not stop now to explain.

A very striking result has been obtained by Prof.
Bateson by mating a white Silky fowl with a white Breda.
The offspring from this cross were all brightly coloured
birds resembling the wild jungle fowl of India. These in
the Fij generation produced 9 coloured and 7 white.

Other experiments have been performed with rats, mice,
pigeons, silk mothsr snails, etc., and all without exception go
to support Mendel's theory.

When scientists talk of such a subject as Mendelism,
they are sometimes asked, " What is the use of it ? " This

26 MENDELISM

question has been put to me. It is said that Benjamin
Franklin was once asked a similar question, and he replied,
"What is the use of a new-born baby? " The aim of the
man of science is to gain knowledge of the world around
him, to study Nature, and learn as much as he possibly can
about its forces and phenomena, and he is not directly or
immediately concerned with the application of these facts.
Fortunately, I can give an answer which will be much
more satisfactory to utilitarians than was Benjamin
Franklin's.

For a great number of years agriculturists have been
endeavouring to improve British Wheat. They have
effected an immense number of crosses, without making
any very substantial progress, because they have not had
any guiding principle or theory to work upon. Now let
us see what has recently been done by Prof. Biffen of
Cambridge working on Mendelian lines. You are aware
that certain kinds of wheat, including British wheat, are
particularly liable to a disease which destroys the grain —
a disease called "Rust." About ^100,000,000 worth of
wheat is destroyed annually.

On the other hand, there are some kinds — mostly small-
eared wheats growing in America and elsewhere — which are
not liable to this disease. They are immune.

It occurred to Mr. Biffen to cross these two kinds. The
resulting hybrids were all affected by the disease, and so
badly affected that he had difficulty in preserving any of the
trains with which to continue the experiment. Now if you
have quite understood what has gone before, you will at
once see that he had achieved a great result, for this first
experiment showed that the susceptibility to disease was a
dominant character, and the immunity, or non-liability to
disease, a recessive character. You can easily follow the
further steps of the experiment, thus —

MENDELISM 27

D x R

(susceptible) (immune)

D[R] (all affected)

I

DD D[R] D[R] RR

i. e. one quarter would be insusceptible. But this does not
complete Biffen's results, for this large-eared grain was
found dominant, as regards size, to the small-eared grain.
So we have two dominant characters, viz. Susceptibility to
Rust and Large size, contrasted with the two characters
Immunity and Small she — a case of di-hybridism com-
parable to the tall coloured peas and the dwarf whites, and
we can construct the Fij generation thus —

LR x SI

LR

9 LR 3 SR 3 LI i SI 1

letting L = Large, R = Susceptibility to Rust, S = Small, and
I = Immunity.

According to this there should be, in this Fij generation,
about 3 large immune plants out of every 16 ; and these,
being immune, will breed true. So that Biffen has
transferred the immunity of the foreign wheat to what in
other respects closely resembles the British variety. Last
year there were several acres of this immune wheat growing
on the experimental farm at Cambridge. Surely this is
likely, from an economic and commercial point of view, to
prove one of the most important experiments ever performed.
He has also informed me that last year this corn yielded
about io bushels per acre more than the ordinary British
wheat.

1 This is what would almost certainly occur if these were only the factors
named, but, as a matter of fact, the result was much more complicated.

28 MENDEL1SM

I now come to the consideration of Mendelism in Man.
Here the evidence of the action of Mendelian laws is scanty,
but there has not been time for much investigation. The
best example, so far, is that supplied by a family certain
members of which are affected by a peculiar deformity of
the hands and feet — technically termed "Brachydactyly,"
and consisting in the apparent absence of the middle bone
of each finger and toe. The members of this family are
divided into two classes : one class is quite normal, having
ordinary hands and feet ; the other class shows the apparent
absence of the middle bone in every finger and toe (not
reckoning the thumb and big toe). These abnormal
individuals are also much below the average stature of their
normal relatives.

This figure shows that the middle bone is present in the
middle finger as a short cubical piece. It ought to be
intermediate in leno-th between the other two bones of the
finger. The cubical piece at the base of the terminal bone
of the other fingers is in reality the middle bone, which is
ill-developed, and united into one with the terminal bone.

The middle bone of the foot is even more degenerate
than the middle bone of the finger, and in each case it is
found to form one piece with the terminal bone.

This abnormality has been hereditary through seven
generations. It acts as a pure dominant. As each individual
has had one abnormal parent, whilst the other parent was
normal, they are impure dominants, or hybrids, so far as
this anatomical feature is concerned, and can be represented
by D[R]. Now when one of these abnormals marries a
normal, i. e. a recessive, we get D[R] x R.and you know from
Law 4 that one half of their children should show the
abnormality, and the other half should be normal. Well,
these abnormals have had 75 children, of whom 39 were
abnormal, that is, 52 per cent. — a sufficiently close approxi-

[ To face p. 28.

MENDELISM 29

mation to the 50 per cent, required by theory. Moreover,
you know the recessives breed true, and so we find it in
this family, for the normals have not in a single instance
transmitted this peculiar defect. Mr. Punnett refers to
this family as exhibiting " the most conspicuous example
of Mendelian heredity in man."

More recently Mr. Nettleship, a distinguished oculist,
has published an account of a family affected by " congenital
night-blindness" (an inability to see in a dim light). It has
been hereditary through ten generations, and has been
transmitted solely by the affected individuals. If we stop
at the abnormal family in each line of descent, the propor-
tion of affected to normals agrees almost exactly with
Mendelian ratios.

Here is the chart of a family affected by spasmodic
asthma, in which the numbers agree exactly with Mendel's
law : 10 individuals suffered, whilst 10 were free from the
disease (the descendants of the recessive must of course not
be counted) —

1

1
$

/

?

?

1
¥

1
t

1

1 1

X ?

t

1 1
t 2

1 1

1
?

1

1
t

1 1 1

r 2 ?

1 1

1
?

\

1

• = 10

Spasm'

3dic i

tsthn

1a.

0=10

There is a human disease called Haemophilia, charac-
terized by a tendency to bleed profusely from even slight
wounds ; as a rule the males alone suffer, and the females
escape, but it is quite clear that, as an unaffected mother
can transmit the disease to her male children, she herself
must possess the disease in a latent form (as an impure
dominant D[R] can transmit the recessive features.)

I have already referred to the opinion that living beings

30 MENDELISM

are essentially double in every character. This may be repre-
sented diagrammatically (see opposite page). Let A and B
represent two germ-cells, which we will assume (for the sake
of simplicity) to contain only 4 factors each (a, b, c, d, and
e,f,g,h). Then C will represent the resulting individual.
We see that it contains the two similar factors a and e, and
is pure-bred for this quality. It is also pure-bred for the two
bottom factors, but in respect of each of the other factors
it is cross-bred, for it has only one dose of each. Suppose
this individual to be a man, then he is fortunate if the top
factors (black) are good qualities, and also if the bottom
(white) factors represent defects which are absent. But
what if a and e are bad qualities — a tendency to tuber-
culosis, mental defect, idleness, intemperance, or crime ?
How can an individual battle against such an inheritance ?
I leave you to think this out for yourselves.

Mendelism has thrown a great deal of light upon some
hitherto obscure subjects, and, to say the least, it is a good
working hypothesis.

I submit that the various facts that have been mentioned
point out, with unmistakable clearness, the only way by
which hereditary disease can be eliminated, and the kind of
stock which should be produced. We hear now-a-days a
great deal about physical deterioration, and much benefit is
expected from education and from improved sanitary laws,
but neither education nor sanitary laws can eradicate
hereditary disease, except by preventing the marriage of the
affected individuals. Just as a purely short-fingered race
could be produced, so it would be equally easy to eradicate
this abnormality by preventing the mating of the abnormal
individuals, and there is little doubt that other hereditary
diseases and defects can be made to practically disappear
by the same means. Unfortunately modern legislation
tends to produce the opposite effect, for everything is done

[ To face p. 30.

MENDELISM

31

to encourage, not only the marriage, but the early marriage
of inferior members of the community ; at the same time
that the burdens of taxation retard more and more the

A

B

C

/

g

d

marriage of the middle and desirable classes. Free educa-
tion, free meals, free clothing may be very well for a certain
class, but they are not remedies, and the net result to this
nation will, in the lon^ run, be detrimental.

Had we but eyes to see, and ears to hear,
How perfect would this world appear !

Kichakd Clay & Sons, Limited,
bread street hill, e.c., and

bungay, suffolk.

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