EXPERIMENTS IN PLANT HYBRIDIZATION (1865)
GREGOR MENDEL
Read at the February 8th, and March 8th, 1865, meetingsof the Brünn Natural History Society

Mendel, Gregor. 1866. Versuche über Plflanzenhybriden. Verhandlungen des naturforschenden Vereines in Brünn, Bd. IV für das Jahr
1865, Abhandlungen, 3–47.

Mendel’s paper was first translated into English by William
Bateson in 1901. This present version derives from the Batesontranslation, with some minor corrections and changes provided byRoger Blumberg as part of the MendelWeb project. A few
additional corrections have been made in this version. For anannotated copy, which includes comments on the Batesontranslation and Blumberg's corrections, see the MendelWeb site:(http://www.netspace.org./MendelWeb/).

© 1996, Electronic Scholarly Publishing Project
http://www.esp.org
This electronic edition is made freely available for scholarly or
educational purposes, provided that this copyright notice isincluded. The manuscript may not be reprinted or redistributed forcommercial purposes without permission.

Mendel, Gregor. 1866. Versuche über Plflanzen-hybriden.
Verhandlungen des naturforschenden Ver-eines in Brünn,
Bd. IV für das Jahr 1865, Abhand-lungen, 3–47.

EXPERIMENTS IN PLANT HYBRIDIZATION (1865)
GREGOR MENDEL
Read at the February 8th, and March 8th, 1865, meetingsof the Brünn Natural History Society

INTRODUCTORY REMARKSEXPERIENCE OF ARTIFICIAL FERTILIZATION, such as is effected withornamental plants in order to obtain new variations in color, has led tothe experiments which will here be discussed. The striking regularity
with which the same hybrid forms always reappeared wheneverfertilization took place between the same species induced further
experiments to be undertaken, the object of which was to follow up
the developments of the hybrids in their progeny.
To this object numerous careful observers, such as Kölreuter,
Gärtner, Herbert, Lecoq, Wichura and others, have devoted a part oftheir lives with inexhaustible perseverance. Gärtner especially in his
work Die Bastarderzeugung im Pflanzenreiche [The Production ofHybrids in the Vegetable Kingdom], has recorded very valuable
observations; and quite recently Wichura published the results ofsome profound investigations into the hybrids of the Willow. That, sofar, no generally applicable law governing the formation anddevelopment of hybrids has been successfully formulated can hardly
be wondered at by anyone who is acquainted with the extent of thetask, and can appreciate the difficulties with which experiments ofthis class have to contend. A final decision can only be arrived atwhen we shall have before us the results of detailed experiments madeon plants belonging to the most diverse orders.

© 1996, Electronic Scholarly Publishing ProjectThis electronic edition is made freely available for scholarly or educational purposes,
provided that this copyright notice is included. The manuscript may not be reprinted or
redistributed for commercial purposes without permission.

2

GREGOR MENDEL (1865)

Those who survey the work done in this department will arrive at
the conviction that among all the numerous experiments made, notone has been carried out to such an extent and in such a way as tomake it possible to determine the number of different forms underwhich the offspring of the hybrids appear, or to arrange these forms
with certainty according to their separate generations, or definitely toascertain their statistical relations.
It requires indeed some courage to undertake a labor of such far–reaching extent; this appears, however, to be the only right way by
which we can finally reach the solution of a question the importanceof which cannot be overestimated in connection with the history of theevolution of organic forms.
The paper now presented records the results of such a detailed
experiment. This experiment was practically confined to a small plantgroup, and is now, after eight years’ pursuit, concluded in all
essentials. Whether the plan upon which the separate experiments
were conducted and carried out was the best suited to attain thedesired end is left to the friendly decision of the reader.

SELECTION OF THE EXPERIMENTAL PLANTS
The value and utility of any experiment are determined by thefitness of the material to the purpose for which it is used, and thus in
the case before us it cannot be immaterial what plants are subjected toexperiment and in what manner such experiment is conducted.
The selection of the plant group which shall serve for experimentsof this kind must be made with all possible care if it be desired to
avoid from the outset every risk of questionable results.
The experimental plants must necessarily:
1.
2.

Possess constant differentiating characteristics
The hybrids of such plants must, during the flowering
period, be protected from the influence of all foreignpollen, or be easily capable of such protection.
The hybrids and their offspring should suffer no marked
disturbance in their fertility in the successive generations.
Accidental impregnation by foreign pollen, if it occurred during
the experiments and were not recognized, would lead to entirely
erroneous conclusions. Reduced fertility or entire sterility of certain
forms, such as occurs in the offspring of many hybrids, would render

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the experiments very difficult or entirely frustrate them. In order todiscover the relations in which the hybrid forms stand towards eachother and also towards their progenitors it appears to be necessary that
all member of the series developed in each successive generations
should be, without exception, subjected to observation.
At the very outset special attention was devoted to theLeguminosae on account of their peculiar floral structure.
Experiments which were made with several members of this family
led to the result that the genus Pisum was found to possess thenecessary qualifications.
Some thoroughly distinct forms of this genus possess characterswhich are constant, and easily and certainly recognizable, and when
their hybrids are mutually crossed they yield perfectly fertile progeny.
Furthermore, a disturbance through foreign pollen cannot easily occur,
since the fertilizing organs are closely packed inside the keel and theanthers burst within the bud, so that the stigma becomes covered with
pollen even before the flower opens. This circumstance is especially
important. As additional advantages worth mentioning, there may becited the easy culture of these plants in the open ground and in pots,
and also their relatively short period of growth. Artificial fertilizationis certainly a somewhat elaborate process, but nearly always succeeds.
For this purpose the bud is opened before it is perfectly developed, thekeel is removed, and each stamen carefully extracted by means offorceps, after which the stigma can at once be dusted over with theforeign pollen.
In all, thirty–four more or less distinct varieties of Peas wereobtained from several seedsmen and subjected to a two year’s trial. In
the case of one variety there were noticed, among a larger number ofplants all alike, a few forms which were markedly different. These,
however, did not vary in the following year, and agreed entirely withanother variety obtained from the same seedsman; the seeds were
therefore doubtless merely accidentally mixed. All the other varieties
yielded perfectly constant and similar offspring; at any rate, noessential difference was observed during two trial years. For
fertilization twenty–two of these were selected and cultivated during
the whole period of the experiments. They remained constant without
any exception.
Their systematic classification is difficult and uncertain. If we
adopt the strictest definition of a species, according to which only
those individuals belong to a species which under precisely the same
circumstances display precisely similar characters, no two of thesevarieties could be referred to one species. According to the opinion of
CLASSICAL GENETICS4

GREGOR MENDEL (1865)

experts, however, the majority belong to the species Pisum sativum;
while the rest are regarded and classed, some as sub–species of P.
sativum, and some as independent species, such as P. quadratum, P.
saccharatum, and P. umbellatum. The positions, however, which may
be assigned to them in a classificatory system are quite immaterial forthe purposes of the experiments in question. It has so far been foundto be just as impossible to draw a sharp line between the hybrids ofspecies and varieties as between species and varieties themselves.

DIVISION AND ARRANGEMENT OF THE EXPERIMENTS
If two plants which differ constantly in one or several charactersbe crossed, numerous experiments have demonstrated that thecommon characters are transmitted unchanged to the hybrids and theirprogeny; but each pair of differentiating characters, on the other hand,
unite in the hybrid to form a new character, which in the progeny of
the hybrid is usually variable. The object of the experiment was toobserve these variations in the case of each pair of differentiating
characters, and to deduce the law according to which they appear insuccessive generations. The experiment resolves itself therefore into
just as many separate experiments are there are constantly
differentiating characters presented in the experimental plants.
The various forms of Peas selected for crossing showed
differences in length and color of the stem; in the size and form of theleaves; in the position, color, size of the flowers; in the length of theflower stalk; in the color, form, and size of the pods; in the form andsize of the seeds; and in the color of the seed–coats and of thealbumen (endosperm). Some of the characters noted do not permit ofa sharp and certain separation, since the difference is of a “more orless” nature, which is often difficult to define. Such characters could
not be utilized for the separate experiments; these could only beapplied to characters which stand out clearly and definitely in theplants. Lastly, the result must show whether they, in their entirety,
observe a regular behavior in their hybrid unions, and whether fromthese facts any conclusion can be reached regarding those characterswhich possess a subordinate significance in the type.
The characters which were selected for experiment relate:1.

To the difference in the form of the ripe seeds. These are
either round or roundish, the depressions, if any, occur

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2.

3.

4.

5.

6.

7.

*

5

on the surface, being always only shallow; or they are
irregularly angular and deeply wrinkled (P. quadratum).
To the difference in the color of the seed albumen(endosperm). The albumen of the ripe seeds is either paleyellow, bright yellow and orange colored, or it possesses
a more or less intense green tint. This difference of color
is easily seen in the seeds as their coats are transparent.
To the difference in the color of the seed–coat. This iseither white, with which character white flowers areconstantly correlated; or it is gray, gray–brown, leather–brown, with or without violet spotting, in which case thecolor of the standards is violet, that of the wings purple,
and the stem in the axils of the leaves is of a reddish tint.
The gray seed–coats become dark brown in boilingwater.
To the difference in the form of the ripe pods. These are
either simply inflated, not contracted in places; or they
are deeply constricted between the seeds and more orless wrinkled (P. saccharatum).
To the difference in the color of the unripe pods. They
are either light to dark green, or vividly yellow, in whichcoloring the stalks, leaf–veins, and calyx participate.*
To the difference in the position of the flowers. They are
either axial, that is, distributed along the main stem; orthey are terminal, that is, bunched at the top of the stem
and arranged almost in a false umbel; in this case theupper part of the stem is more or less widened in section(P. umbellatum).
To the difference in the length of the stem. The length of
the stem is very various in some forms; it is, however, aconstant character for each, in so far that healthy plants,
grown in the same soil, are only subject to unimportantvariations in this character. In experiments with thischaracter, in order to be able to discriminate withcertainty, the long axis of 6 to 7 ft. was always crossedwith the short one of ¾ ft. to 1½ ft.

One species possesses a beautifully brownish–red colored pod, which when
ripening turns to violet and blue. Trials with this character were only begun lastyear.

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GREGOR MENDEL (1865)

Each two of the differentiating characters enumerated above wereunited by cross–fertilization. There were made for the
1st experiment
2nd experiment
3rd experiment
4th experiment
5th experiment
6th experiment
7th experiment

60 fertilizations on 15 plants.
58 fertilizations on 10 plants.
35 fertilizations on 10 plants.
40 fertilizations on 10 plants.
23 fertilizations on 5 plants.
34 fertilizations on 10 plants.
37 fertilizations on 10 plants.

From a larger number of plants of the same variety only the mostvigorous were chosen for fertilization. Weakly plants always afford
uncertain results, because even in the first generation of hybrids, andstill more so in the subsequent ones, many of the offspring either
entirely fail to flower or only form a few and inferior seeds.
Furthermore, in all the experiments reciprocal crossings wereeffected in such a way that each of the two varieties which in one setof fertilizations served as seed–bearer in the other set was used as thepollen plant.
The plants were grown in garden beds, a few also in pots, andwere maintained in their natural upright position by means of sticks,
branches of trees, and strings stretched between. For each experimenta number of pot plants were placed during the blooming period in agreenhouse, to serve as control plants for the main experiment in the
open as regards possible disturbance by insects. Among the insects
which visit Peas the beetle Buchus pisi might be detrimental to theexperiments should it appear in numbers. The female of this species isknown to lay the eggs in the flower, and in so doing opens the keel;
upon the tarsi of one specimen, which was caught in a flower, somepollen grains could clearly be seen under a lens. Mention must also bemade of a circumstance which possibly might lead to the introductionof foreign pollen. It occurs, for instance, in some rare cases thatcertain parts of an otherwise normally developed flower wither,resulting in a partial exposure of the fertilizing organs. A defective
development of the keel has also been observed, owing to which thestigma and anthers remained partially covered. It also sometimes
happens that the pollen does not reach full perfection. In this event
there occurs a gradual lengthening of the pistil during the blooming
period, until the stigmatic tip protrudes at the point of the keel. This
remarkable appearance has also been observed in hybrids ofPhaseolus and Lathyrus.

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The risk of false impregnation by foreign pollen is, however, avery slight one with Pisum, and is quite incapable of disturbing thegeneral result. Among more than 10,000 plants which were carefully
examined there were only a very few cases where an indubitable false
impregnation had occurred. Since in the greenhouse such a case wasnever remarked, it may well be supposed that Brucus pisi, andpossibly also the described abnormalities in the floral structure, wereto blame.

THE FORMS OF THE HYBRIDS
Experiments which in previous years were made with ornamental
plants have already afforded evidence that the hybrids, as a rule, arenot exactly intermediate between the parental species. With some of
the more striking characters, those, for instance, which relate to theform and size of the leaves, the pubescence of the several parts, etc.,
the intermediate, indeed, is nearly always to be seen; in other cases,
however, one of the two parental characters is so preponderant that itis difficult, or quite impossible, to detect the other in the hybrid.
This is precisely the case with the Pea hybrids. In the case of eachof the seven crosses the hybrid–character resembles that of one of theparental forms so closely that the other either escapes observation
completely or cannot be detected with certainty. This circumstance isof great importance in the determination and classification of theforms under which the offspring of the hybrids appear. Henceforth in
this paper those characters which are transmitted entire, or almost
unchanged in the hybridization, and therefore in themselves constitutethe characters of the hybrid, are termed the dominant, and those whichbecome latent in the process recessive. The expression recessive has
been chosen because the characters thereby designated withdraw or
entirely disappear in the hybrids, but nevertheless reappear unchangedin their progeny, as will be demonstrated later on.
It was furthermore shown by the whole of the experiments that it
is perfectly immaterial whether the dominant character belongs to theseed plant or to the pollen plant; the form of the hybrid remains
identical in both cases. This interesting fact was also emphasized by
Gärtner, with the remark that even the most practiced expert is not ina position to determine in a hybrid which of the two parental species
was the seed or the pollen plant.

CLASSICAL GENETICS

8

GREGOR MENDEL (1865)

Of the differentiating characters which were used in theexperiments the following are dominant:1.

The round or roundish form of the seed with or without
shallow depressions.

2.
3.

The yellow coloring of the seed albumen.
The gray, gray–brown, or leather brown color of theseed–coat, in association with violet–red blossoms andreddish spots in the leaf axils.
The simply inflated form of the pod.
The green coloring of the unripe pod in association withthe same color of the stems, the leaf–veins and the calyx.
The distribution of the flowers along the stem.
The greater length of stem.

4.
5.
6.
7.

With regard to this last character it must be stated that the longerof the two parental stems is usually exceeded by the hybrid, a factwhich is possibly only attributable to the greater luxuriance whichappears in all parts of plants when stems of very different lengths arecrossed. Thus, for instance, in repeated experiments, stems of 1 ft. and6 ft. in length yielded without exception hybrids which varied inlength between 6 ft. and 7½ ft.
The hybrid seeds in the experiments with seed–coat are often
more spotted, and the spots sometimes coalesce into small bluish–
violet patches. The spotting also frequently appears even when it isabsent as a parental character.
The hybrid forms of the seed–shape and of the [color of the]albumen are developed immediately after the artificial fertilization by
the mere influence of the foreign pollen. They can, therefore, beobserved even in the first year of experiment, whilst all the othercharacters naturally only appear in the following year in such plants ashave been raised from the crossed seed.

THE FIRST GENERATION FROM THE HYBRIDS
In this generation there reappear, together with the dominantcharacters, also the recessive ones with their peculiarities fully
developed, and this occurs in the definitely expressed average
proportion of three to one, so that among each four plants of thisgeneration three display the dominant character and one the recessive.
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9This relates without exception to all the characters which were
investigated in the experiments. The angular wrinkled form of theseed, the green color of the albumen, the white color of the seed–coats
and the flowers, the constrictions of the pods, the yellow color of theunripe pod, of the stalk, of the calyx, and of the leaf venation, theumbel–like form of the inflorescence, and the dwarfed stem, allreappear in the numerical proportion given, without any essential
alteration. Transitional forms were not observed in any experiment.
Since the hybrids resulting from reciprocal crosses are formed
alike and present no appreciable difference in their subsequentdevelopment, consequently these results can be reckoned together ineach experiment. The relative numbers which were obtained for eachpair of differentiating characters are as follows:
Expt 1:

Expt 2:Form of seed. –– From 253 hybrids 7,324 seeds
were obtained in the second trial year. Amongthem were 5,474 round or roundish ones and1,850 angular wrinkled ones. Therefrom the ratio
2.96:1 is deduced.
Color of albumen. –– 258 plants yielded 8,023
seeds, 6,022 yellow, and 2,001 green; their ratio,
therefore, is as 3.01:1.

In these two experiments each pod yielded usually both kinds ofseed. In well–developed pods which contained on the average six tonine seeds, it often happened that all the seeds were round (Expt. 1) or
all yellow (Expt. 2); on the other hand there were never observed
more than five wrinkled or five green ones on one pod. It appears tomake no difference whether the pods are developed early or later inthe hybrid or whether they spring from the main axis or from a lateralone. In some few plants only a few seeds developed in the first formed
pods, and these possessed exclusively one of the two characters, but inthe subsequently developed pods the normal proportions weremaintained nevertheless.
As in separate pods, so did the distribution of the characters varyin separate plants. By way of illustration the first ten individuals fromboth series of experiments may serve.

CLASSICAL GENETICS10

GREGOR MENDEL (1865)

Plants
1
2
3
4
5
6
7
8
9
10

Experiment 1
Form of the Seedround
wrinkled
45
12
27
8
24
7
19
10
32
11
26
6
88
24
22
10
28
6
25
7

Experiment 2
Color of the Albumen
yellow
green
25
11
32
7
14
5
70
27
24
13
20
6
32
13
44
9
50
14
44
18

As extremes in the distribution of the two seed characters in one
plant, there were observed in Expt. 1 an instance of 43 round and only
two angular, and another of 14 round and 15 angular seeds. In Expt. 2
there was a case of 32 yellow and only one green seed, but also one of
20 yellow and 19 green.
These two experiments are important for the determination of theaverage ratios, because with a smaller number of experimental plants
they show that very considerable fluctuations may occur. In counting
the seeds, also, especially in Expt. 2, some care is requisite, since insome of the seeds of many plants the green color of the albumen isless developed, and at first may be easily overlooked. The cause ofthis partial disappearance of the green coloring has no connection
with the hybrid–character of the plants, as it likewise occurs in theparental variety. This peculiarity is also confined to the individual andis not inherited by the offspring. In luxuriant plants this appearance
was frequently noted. Seeds which are damaged by insects during
their development often vary in color and form, but with a little
practice in sorting, errors are easily avoided. It is almost superfluous
to mention that the pods must remain on the plants until they arethoroughly ripened and have become dried, since it is only then thatthe shape and color of the seed are fully developed.
Expt. 3:

Expt. 4:

Color of the seed–coats. –– Among 929 plants,
705 bore violet–red flowers and gray–brown seed–coats; 224 had white flowers and white seed–coats, giving the proportion 3.15:1.
Form of pods. –– Of 1,181 plants, 882 had themsimply inflated, and in 299 they were constricted.
Resulting ratio, 2.95:1.

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Expt. 5:

Expt. 6:

Expt. 7:

11

Color of the unripe pods. –– The number of trial
plants was 580, of which 428 had green pods and
152 yellow ones. Consequently these stand in theratio of 2.82:1.
Position of flowers. –– Among 858 cases 651 had
inflorescences axial and 207 terminal. Ratio,
3.14:1.
Length of stem. –– Out of 1,064 plants, in 787
cases the stem was long, and in 277 short. Hence amutual ratio of 2.84:1. In this experiment the
dwarfed plants were carefully lifted andtransferred to a special bed. This precaution wasnecessary, as otherwise they would have perishedthrough being overgrown by their tall relatives.
Even in their quite young state they can be easily
picked out by their compact growth and thickdark–green foliage.

If now the results of the whole of the experiments be brought
together, there is found, as between the number of forms with thedominant and recessive characters, an average ratio of 2.98:1, or 3:1.
The dominant character can have here a double signification ––viz. that of a parental character or a hybrid–character. In which of thetwo significations it appears in each separate case can only be
determined by the following generation. As a parental character it
must pass over unchanged to the whole of the offspring; as a hybrid–character, on the other hand, it must maintain the same behavior as in
the first generation.

THE SECOND GENERATION FROM THE HYBRIDS
Those forms which in the first generation exhibit the recessive
character do not further vary in the second generation as regards thischaracter; they remain constant in their offspring.
It is otherwise with those which possess the dominant character inthe first generation [bred from the hybrids –– i.e., the F2 in modern
terminology]. Of these two–thirds yield offspring which display thedominant and recessive characters in the proportion of three to one,and thereby show exactly the same ratio as the hybrid forms, while
only one–third remains with the dominant character constant.

CLASSICAL GENETICS12

GREGOR MENDEL (1865)
The separate experiments yielded the following results:
Expt. 1:

Expt. 2:

Among 565 plants which were raised from roundseeds of the first generation, 193 yielded roundseeds only, and remained therefore constant in thischaracter; 372, however, gave both round andwrinkled seeds, in the proportion of 3:1. The
number of the hybrids, therefore, as comparedwith the constants is 1.93:1.
Of 519 plants which were raised from seeds whosealbumen was of yellow color in the first
generation, 166 yielded exclusively yellow, while
353 yielded yellow and green seeds in the
proportion of 3:1. There resulted, therefore, adivision into hybrid and constant forms in the
proportion of 2.13:1.

For each separate trial in the following experiments 100 plants
were selected which displayed the dominant character in the first
generation, and in order to ascertain the significance of this, ten seedsof each were cultivated.
Expt. 3:

Expt. 4:

Expt. 5:

Expt. 6.:

Expt. 7:The offspring of 36 plants yielded exclusively
gray–brown seed–coats, while of the offspring of
64 plants some had gray–brown and some had
white.
The offspring of 29 plants had only simply
inflated pods; of the offspring of 71, on the other
hand, some had inflated and some constricted.
The offspring of 40 plants had only green pods; ofthe offspring of 60 plants some had green, someyellow ones.
The offspring of 33 plants had only axial flowers;of the offspring of 67, on the other hand, some
had axial and some terminal flowers.
The offspring of 28 plants inherited the long axis,of those of 72 plants some the long and some theshort axis.

In each of these experiments a certain number of the plants cameconstant with the dominant character. For the determination of the
proportion in which the separation of the forms with the constantly

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persistent character results, the two first experiments are especially
important, since in these a larger number of plants can be compared.
The ratios 1.93:1 and 2.13:1 gave together almost exactly the average
ratio of 2:1. Experiment 6 gave a quite concordant result; in the others
the ratio varies more or less, as was only to be expected in view of thesmaller number of 100 trial plants. Experiment 5, which shows thegreatest departure, was repeated, and then in lieu of the ratio of 60:40,
that of 65:35 resulted. The average ratio of 2 to 1 appears, therefore,
as fixed with certainty. It is therefore demonstrated that, of those
forms which possess the dominant character in the first generation,
two–thirds have the hybrid–character, while one–third remains
constant with the dominant character.
The ratio 3:1, in accordance with which the distribution of thedominant and recessive characters results in the first generation,
resolves itself therefore in all experiments into the ratio of 2:1:1, if thedominant character be differentiated according to its significance as a
hybrid–character or as a parental one. Since the members of the first
generation spring directly from the seed of the hybrids, it is now clearthat the hybrids form seeds having one or other of the twodifferentiating characters, and of these one–half develop again the
hybrid form, while the other half yield plants which remain constantand receive the dominant or the recessive characters in equal
numbers.

THE SUBSEQUENT GENERATIONS FROM THE HYBRIDS
The proportions in which the descendants of the hybrids develop
and split up in the first and second generations presumably hold goodfor all subsequent progeny. Experiments 1 and 2 have already been
carried through six generations; 3 and 7 through five; and 4, 5, and 6
through four; these experiments being continued from the third
generation with a small number of plants, and no departure from therule has been perceptible. The offspring of the hybrids separated ineach generation in the ratio of 2:1:1 into hybrids and constant forms.
If A be taken as denoting one of the two constant characters, forinstance the dominant, a, the recessive, and Aa the hybrid form inwhich both are conjoined, the expression
A + 2Aa + a

CLASSICAL GENETICS14

GREGOR MENDEL (1865)

shows the terms in the series for the progeny of the hybrids of two
differentiating characters.
The observation made by Gärtner, Kölreuter, and others, thathybrids are inclined to revert to the parental forms, is also confirmedby the experiments described. It is seen that the number of the hybrids
which arise from one fertilization, as compared with the number offorms which become constant, and their progeny from generation to
generation, is continually diminishing, but that nevertheless they
could not entirely disappear. If an average equality of fertility in allplants in all generations be assumed, and if, furthermore, each hybrid
forms seed of which one–half yields hybrids again, while the otherhalf is constant to both characters in equal proportions, the ratio of
numbers for the offspring in each generation is seen by the following
summary, in which A and a denote again the two parental characters,and Aa the hybrid forms. For brevity’s sake it may be assumed thateach plant in each generation furnishes only four seeds.
Generation
1
2
3
4
5
n

A
1
6
28
120
496

Aa
2
4
8
16
32

a
1
6
28
120
496

RatiosA : Aa : a
1 : 2 : 1
3 : 2 : 3
7 : 2 : 7
15 : 2 : 15
31 : 2 : 31nn
2 –1 : 2 : 2 –1

In the tenth generation, for instance, 2n – 1 = 1,023. There result,
therefore, in each 2,048 plants which arise in this generation 1,023
with the constant dominant character, 1,023 with the recessive
character, and only two hybrids.

THE OFFSPRING OF HYBRIDS IN WHICH SEVERALDIFFERENTIATING CHARACTERS ARE ASSOCIATED.
In the experiments above described plants were used which
differed only on one essential character. The next task consisted inascertaining whether the law of development discovered in theseapplied to each pair of differentiating characters when several diverse
characters are united in the hybrid by crossing.
As regards the form of the hybrids in these cases, the experiments
showed throughout that this invariably more nearly approaches to that
one of the two parental plants which possesses the greater number of
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dominant characters. If, for instance, the seed plant has a short stem,
terminal white flowers, and simply inflated pods; the pollen plant, on
the other hand, a long stem, violet–red flowers distributed along thestem, and constricted pods; the hybrid resembles the seed parent onlyin the form of the pod; in the other characters it agrees with the pollen
parent. Should one of the two parental types possess only dominantcharacters, then the hybrid is scarcely or not at all distinguishable
from it.
Two experiments were made with a considerable number ofplants. In the first experiment the parental plants differed in the formof the seed and in the color of the albumen; in the second in the formof the seed, in the color of the albumen, and in the color of the seed–coats. Experiments with seed characters give the result in the simplest
and most certain way.
In order to facilitate study of the data in these experiments, thedifferent characters of the seed plant will be indicated by A, B, C,
those of the pollen plant by a, b, c, and the hybrid forms of thecharacters by Aa, Bb, and Cc.
Expt. 1. —AB, seed parents
A, form round
B, albumen yellow

ab, pollen parentsa, form wrinkledb, albumen green

The fertilized seeds appeared round and yellow like those of theseed parents. The plants raised therefrom yielded seeds of four sorts,
which frequently presented themselves in one pod. In all, 556 seeds
were yielded by 15 plants, and of these there were:
315
101
108
32

round and yellow,
wrinkled and yellow,
round and green,
wrinkled and green.

All were sown the following year. Eleven of the round yellow
seeds did not yield plants, and three plants did not form seeds. Among
the rest:
38
65
60
138

had round yellow seeds
round yellow and green seeds
round yellow and wrinkled yellow seeds
round yellow and green, wrinkled yellow
and green seeds

AB
ABb
AaB
AaBb

From the wrinkled yellow seeds 96 resulting plants bore seed, of
which:CLASSICAL GENETICS16

GREGOR MENDEL (1865)
aB
aBb

28 had only wrinkled yellow seeds
68 wrinkled yellow and green seeds

From 108 round green seeds 102 resulting plants fruited, of which:AbAab35 had only round green seeds
67 round and wrinkled green seeds

The wrinkled green seeds yielded 30 plants which bore seeds all of
like character; they remained constant ab.
The offspring of the hybrids appeared therefore under ninedifferent forms, some of them in very unequal numbers. When these
are collected and coordinated we find:
38
35
28
30
65
68
60
67
138

plants with the sign
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"
"

AB
Ab
aB
ab
ABb
aBb
AaB
Aab
AaBb

The whole of the forms may be classed into three essentiallydifferent groups. The first includes those with the signs AB, Ab, aB,
and ab: they possess only constant characters and do not vary again in
the next generation. Each of these forms is represented on the average
33 times. The second group includes the signs ABb, aBb, AaB, Aab:these are constant in one character and hybrid in another, and vary in
the next generation only as regards the hybrid–character. Each ofthese appears on any average 65 times. The form AaBb occurs 138
times: it is hybrid in both characters, and behaves exactly as do thehybrids from which it is derived.
If the numbers in which the forms belonging to these classes
appear be compared, the ratios of 1:2:4 are unmistakably evident. The
numbers 33, 65, 138 present very fair approximations to the ratio
numbers of 33, 66, 132.
The development series consists, therefore, of nine classes, ofwhich four appear therein always once and are constant in bothcharacters; the forms AB, ab, resemble the parental forms, the twoothers present combinations between the conjoined characters A, a, B,
b, which combinations are likewise possibly constant. Four classes
appear always twice, and are constant in one character and hybrid in
the other. One class appears four times, and is hybrid in both

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17

characters. Consequently, the offspring of the hybrids, if two kinds ofdifferentiating characters are combined therein, are represented by theexpression
AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb

This expression is indisputably a combination series in which thetwo expressions for the characters A and a, B and b are combined. We
arrive at the full number of the classes of the series by the
combination of the expressions:A + 2Aa + a
B + 2Bb + bExpt. 2. —ABC, seed parents
A, form round
B, albumen yellow
C, seed coat grey–brown

abc, pollen parentsa, form wrinkledb, albumen greenc, seed coat whiteThis experiment was made in precisely the same way as theprevious one. Among all the experiments it demanded the most time
and trouble. From 24 hybrids 687 seeds were obtained in all: thesewere all either spotted, gray–brown or gray–green, round or wrinkled.
From these in the following year 639 plants fruited, and as further
investigation showed, there were among them:
8 plants ABC
Abc
14
"
AbC
9
"
Abc
11
"
aBC
8
"
aBc
10
"
abC
10
"
abc
7
"

22 plants ABCc
AbCc
17
"
aBCc
25
"
abCc
20
"
ABbC
15
"
Abbc
18
"
aBbC
19
"
aBbc
24
"
AaBC
14
"
AaBc
18
"
AabC
20
"
Aabc
16
"

45 plants ABbCc
aBbCc
36
"
AaBCc
38
"
AabCc
40
"
AaBbC
49
"
AaBbc
48
"

78

"

AaBbCcThe whole expression contains 27 terms. Of these eight are
constant in all characters, and each appears on the average ten times;
twelve are constant in two characters, and hybrid in the third; eachappears on the average 19 times; six are constant in one character andhybrid in the other two; each appears on the average 43 times. One
form appears 78 times and is hybrid in all of the characters. The ratios
10:19:43:78 agree so closely with the ratios 10:20:40:80, or 1:2:4:8
that this last undoubtedly represents the true value.
CLASSICAL GENETICS18

GREGOR MENDEL (1865)

The development of the hybrids when the original parents differ
in three characters results therefore according to the following
expression:
ABC + ABc + AbC + Abc + aBC + aBc + abC + abc + 2ABCc +2AbCc + 2aBCc + 2abCc + 2ABbC + 2ABbc + 2aBbC + 2aBbc +2AaBC + 2AaBc + 2AabC + 2Aabc + 4ABbCc + 4aBbCc +4AaBCc + 4AabCc + 4AaBbC + 4AaBbc + 8AaBbCc.

Here also is involved a combination series in which theexpressions for the characters A and a, B and b, C and c, are united.
The expressions:A + 2Aa + a
B + 2Bb + b
C + 2Cc + cgive all the classes of the series. The constant combinations which
occur therein agree with all combinations which are possible between
the characters A, B, C, a, b, c; two thereof, ABC and abc, resemble
the two original parental stocks.
In addition, further experiments were made with a smaller
number of experimental plants in which the remaining characters by
twos and threes were united as hybrids: all yielded approximately thesame results. There is therefore no doubt that for the whole of thecharacters involved in the experiments the principle applies that theoffspring of the hybrids in which several essentially different
characters are combined exhibit the terms of a series of combinations,in which the developmental series for each pair of differentiating
characters are united. It is demonstrated at the same time that therelation of each pair of different characters in hybrid union is
independent of the other differences in the two original parental
stocks.
If n represent the number of the differentiating characters in thentwo original stocks, 3 gives the number of terms of the combinationnnseries, 4 the number of individuals which belong to the series, and 2
the number of unions which remain constant. The series therefore
4
contains, if the original stocks differ in four characters, 3 = 8144classes, 4 = 256 individuals, and 2 = 16 constant forms: or, which isthe same, among each 256 offspring of the hybrids are 81 different
combinations, 16 of which are constant.
All constant combinations which in Peas are possible by the
combination of the said seven differentiating characters were actually
7
obtained by repeated crossing. Their number is given by 2 = 128.

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Thereby is simultaneously given the practical proof that the constantcharacters which appear in the several varieties of a group of plants
may be obtained in all the associations which are possible according
to the laws of combination, by means of repeated artificial
fertilization.
As regards the flowering time of the hybrids, the experiments are
not yet concluded. It can, however, already be stated that the timestands almost exactly between those of the seed and pollen parents,
and that the constitution of the hybrids with respect to this character
probably follows the rule ascertained in the case of the othercharacters. The forms which are selected for experiments of this classmust have a difference of at least 20 days from the middle flowering
period of one to that of the other; furthermore, the seeds when sownmust all be placed at the same depth in the earth, so that they may
germinate simultaneously. Also, during the whole flowering period,
the more important variations in temperature must be taken into
account, and the partial hastening or delaying of the flowering which
may result there from. It is clear that this experiment presents manydifficulties to be overcome and necessitates great attention.
If we endeavor to collate in a brief form the results arrived at, wefind that those differentiating characters, which admit of easy andcertain recognition in the experimental plants, all behave exactly alikein their hybrid associations. The offspring of the hybrids of each pairof differentiating characters are, one–half, hybrid again, while theother half are constant in equal proportions having the characters of
the seed and pollen parents respectively. If several differentiating
characters are combined by cross–fertilization in a hybrid, theresulting offspring form the terms of a combination series in which
the combination series for each pair of differentiating characters areunited.
The uniformity of behavior shown by the whole of the characters
submitted to experiment permits, and fully justifies, the acceptance ofthe principle that a similar relation exists in the other characters whichappear less sharply defined in plants, and therefore could not be
included in the separate experiments. An experiment with peduncles
of different lengths gave on the whole a fairly satisfactory results,
although the differentiation and serial arrangement of the forms could
not be effected with that certainty which is indispensable for correct
experiment.

CLASSICAL GENETICS20

GREGOR MENDEL (1865)

THE REPRODUCTIVE CELLS OF THE HYBRIDS
The results of the previously described experiments led to further
experiments, the results of which appear fitted to afford someconclusions as regards the composition of the egg and pollen cells of
hybrids. An important clue is afforded in Pisum by the circumstancethat among the progeny of the hybrids constant forms appear, and that
this occurs, too, in respect of all combinations of the associatedcharacters. So far as experience goes, we find it in every caseconfirmed that constant progeny can only be formed when the egg
cells and the fertilizing pollen are of like character, so that both areprovided with the material for creating quite similar individuals, as is
the case with the normal fertilization of pure species. We must
therefore regard it as certain that exactly similar factors must be atwork also in the production of the constant forms in the hybrid plants.
Since the various constant forms are produced in one plant, or even in
one flower of a plant, the conclusion appears logical that in theovaries of the hybrids there are formed as many sorts of egg cells, andin the anthers as many sorts of pollen cells, as there are possible
constant combination forms, and that these egg and pollen cells agree
in their internal compositions with those of the separate forms.
In point of fact it is possible to demonstrate theoretically that thishypothesis would fully suffice to account for the development of thehybrids in the separate generations, if we might at the same time
assume that the various kinds of egg and pollen cells were formed in
the hybrids on the average in equal numbers. In order to bring theseassumptions to an experimental proof, the following experiments were
designed. Two forms which were constantly different in the form of
the seed and the color of the albumen were united by fertilization.
If the differentiating characters are again indicated as A, B, a, b,
we have:
AB, Seed parents
A, form round
B, albumen yellow

ab, Pollen parentsa, form wrinkledb, albumen green

The artificially fertilized seeds were sown together with several
seeds of both original stocks, and the most vigorous examples werechosen for the reciprocal crossing. There were fertilized:
1.
2.
3.

The hybrids with the pollen of AB
The hybrids with the pollen of ab
AB with the pollen of the hybrids.
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4.

21

ab with the pollen of the hybrids.

For each of these four experiments the whole of the flowers on
three plants were fertilized. If the above theory be correct, there must
be developed on the hybrids egg and pollen cells of the forms AB, Ab,
aB, ab, and there would be combined:
1.
2.
3.
4.

The egg cells AB, Ab, aB, ab with the pollen cells AB.
The egg cells AB, Ab, aB, ab with the pollen cells ab.
The egg cells AB with the pollen cells AB, Ab, aB, andab.
The egg cells ab with the pollen cells AB, Ab, aB, andab.

From each of these experiments there could then result only thefollowing forms:
1.
2.
3.
4.

AB, ABb, AaB, AaBb
AaBb, Aab, aBb, ab
AB, ABb, AaB, AaBb
AaBb, Aab, aBb, ab

If, furthermore, the several forms of the egg and pollen cells of
the hybrids were produced on an average in equal numbers, then ineach experiment the said four combinations should stand in the sameratio to each other. A perfect agreement in the numerical relations
was, however, not to be expected since in each fertilization, even in
normal cases, some egg cells remain undeveloped or subsequently die,and many even of the well–formed seeds fail to germinate when sown.
The above assumption is also limited in so far that while it demands
the formation of an equal number of the various sorts of egg andpollen cells, it does not require that this should apply to each separate
hybrid with mathematical exactness.
The first and second experiments had primarily the object ofproving the composition of the hybrid egg cells, while the third andfourth experiments were to decide that of the pollen cells. As is shownby the above demonstration the first and third experiments and thesecond and fourth experiments should produce precisely the samecombinations, and even in the second year the result should bepartially visible in the form and color of the artificially fertilized seed.
In the first and third experiments the dominant characters of form andcolor, A and B, appear in each union, and are also partly constant and

CLASSICAL GENETICS22

GREGOR MENDEL (1865)

partly in hybrid union with the recessive characters a and b, for which
reason they must impress their peculiarity upon the whole of theseeds. All seeds should therefore appear round and yellow, if thetheory be justified. In the second and fourth experiments, on the other
hand, one union is hybrid in form and in color, and consequently theseeds are round and yellow; another is hybrid in form, but constant in
the recessive character of color, whence the seeds are round andgreen; the third is constant in the recessive character of form but
hybrid in color, consequently the seeds are wrinkled and yellow; thefourth is constant in both recessive characters, so that the seeds arewrinkled and green. In both these experiments there wereconsequently four sorts of seed to be expected; namely, round andyellow, round and green, wrinkled and yellow, wrinkled and green.
The crop fulfilled these expectations perfectly. There wereobtained in the
1st Experiment, 98 exclusively round yellow seeds;
3rd"
94 exclusively round yellow seeds
In the 2nd Experiment, 31 round and yellow, 26 round and green,
22 wrinkled and yellow, 26 wrinkled and green seeds.
In the 4th Experiment, 24 round and yellow, 25 round and green,
22 wrinkled and yellow, 27 wrinkled and green.
There could scarcely be now any doubt of the success of theexperiment; the next generation must afford the final proof. From theseed sown there resulted for the first experiment 90 plants, and for thethird 87 plants which fruited: these yielded for the
1st Exp. 3rd Exp.
20
25
round yellow seeds
23
19round yellow and green seeds
25
22round and wrinkled yellow seeds
22
21round and wrinkled green and yellow seeds

AB
ABb
AaB
AaBb

In the second and fourth experiments the round and yellow seeds
yielded plants with round and wrinkled yellow and green seeds,
AaBb.
From the round green seeds plants resulted with round andwrinkled green seeds, Aab.
The wrinkled yellow seeds gave plants with wrinkled yellow andgreen seeds, aBb.
From the wrinkled green seeds plants were raised which yieldedagain only wrinkled and green seeds, ab.

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Although in these two experiments likewise some seeds did not
germinate, the figures arrived at already in the previous year were notaffected thereby, since each kind of seed gave plants which, as regards
their seed, were like each other and different from the others. There
resulted therefore from the2nd. Expt.
31
26
27
26

4th Expt.
24
25
22
27

plants of the form AaBb
plants of the form Aabplants of the form aBbplants of the form ab

In all the experiments, therefore, there appeared all the forms
which the proposed theory demands, and they came in nearly equal
numbers.
In a further experiment the characters of flower–color and lengthof stem were experimented upon, and selection was so made that in
the third year of the experiment each character ought to appear in halfof all the plants if the above theory were correct. A, B, a, b serveagain as indicating the various characters.
A, violet–red flowers.
B, axis long.

a, white flowers.
b, axis short.

The form Ab was fertilized with ab, which produced the hybrid
Aab. Furthermore, aB was also fertilized with ab, whence the hybridaBb. In the second year, for further fertilization, the hybrid Aab wasused as seed parent, and hybrid aBb as pollen parent.
Seed parent, Aab.
Possible egg cells, Ab, ab.

Pollen parent, aBb.
Pollen cells, aB, ab.

From the fertilisation between the possible egg and pollen cells
four combinations should result, namely:
AaBb + aBb + Aab + ab

From this it is perceived that, according to the above theory, inthe third year of the experiment out of all the plants,
Half should have violet–red flowers""" white flowers""" a long axis""
" a short axis

(Aa), Classes 1,
(a)
"
2,
(Bb)
"
1,
(b)
"
3,

3
4
2
4

From 45 fertilizations of the second year 187 seeds resulted, of
which only 166 reached the flowering stage in the third year. Amongthese the separate classes appeared in the numbers following:
CLASSICAL GENETICS

24

GREGOR MENDEL (1865)
Class
1
2
3
4

Flower Color
violet–red
white
violet–red
white

Stem
long
long
short
short

47 times
40 times
38 times
41 times

There subsequently appeared,
The
"
"
"

violet–red flower color
white flower–color
long stem
short stem

(Aa)
(a)
(Bb)
(b)

in
in
in
in

85 plants
81 plants
87 plants
79 plants

The theory adduced is therefore satisfactorily confirmed in thisexperiment also.
For the characters of form of pod, color of pod, and position of
flowers, experiments were also made on a small scale and results
obtained in perfect agreement. All combinations, which were possible
through the union of the differentiating characters duly appeared, andin nearly equal numbers.
Experimentally, therefore, the theory is confirmed that the pea
hybrids form egg and pollen cells which, in their constitution,
represent in equal numbers all constant forms which result from the
combination of the characters united in fertilization.
The difference of the forms among the progeny of the hybrids, aswell as the respective ratios of the numbers in which they areobserved, find a sufficient explanation in the principle above deduced.
The simplest case is afforded by the developmental series of each pairof differentiating characters. This series is represented by theexpression A+2Aa+a, in which A and a signify the forms withconstant differentiating characters, and Aa the hybrid form of both. It
includes in three different classes four individuals. In the formation ofthese, pollen and egg cells of the form A and a take part on the
average equally in the fertilization; hence each form [occurs] twice,since four individuals are formed. There participate consequently inthe fertilization
The pollen cells:
The egg cells:

A + A + a + a,
A + A + a + a.

It remains, therefore, purely a matter of chance which of the two
sorts of pollen will become united with each separate egg cell.
According, however, to the law of probability, it will always happen,on the average of many cases, that each pollen form A and a will unite

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equally often with each egg cell form A and a, consequently one of
the two pollen cells A in the fertilization will meet with the egg cell A
and the other with the egg cell a, and so likewise one pollen cell awill unite with an egg cell A, and the other with the egg cell a.
Pollen cells

A

A

a

a

Egg cells

A

A

a

a

The result of the fertilization may be made clear by putting thesigns for the conjoined egg and pollen cells in the form of fractions,those for the pollen cells above and those for the egg cells below theline. We then have
A
—
A

+

A
—
a

+

a
—
A

+

a
— .
a

In the first and fourth term the egg and pollen cells are of like
kind, consequently the product of their union must be constant,namely A and a; in the second and third, on the other hand, there
again results a union of the two differentiating characters of thestocks, consequently the forms resulting from these fertilizations are
identical with those of the hybrid from which they sprang. Thereoccurs accordingly a repeated hybridization. This explains thestriking fact that the hybrids are able to produce, besides the twoparental forms, offspring which are like themselves A/a and a/A bothgive the same union Aa, since, as already remarked above, it makes
no difference in the result of fertilization to which of the two
characters the pollen or egg cells belong. We may write then
A
—
A

+

A
—
a

+

a
—
A

+

a
—
a

=

A

+

2Aa

+ a .

This represents the average result of the self–fertilization of thehybrids when two differentiating characters are united in them. In
individual flowers and in individual plants, however, the ratios inwhich the forms of the series are produced may suffer notinconsiderable fluctuations. Apart from the fact that the numbers inwhich both sorts of egg cells occur in the seed vessels can only be
regarded as equal on the average, it remains purely a matter of chance
which of the two sorts of pollen may fertilize each separate egg cell.
For this reason the separate values must necessarily be subject to

CLASSICAL GENETICS

26

GREGOR MENDEL (1865)

fluctuations, and there are even extreme cases possible, as were
described earlier in connection with the experiments on the forms of
the seed and the color of the albumen. The true ratios of the numbers
can only be ascertained by an average deduced from the sum of as
many single values as possible; the greater the number the more are
merely chance effects eliminated.
The developmental series for hybrids in which two kinds ofdifferentiating characters are united contains among 16 individuals
nine different forms, viz.:
AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb.

Between the differentiating characters of the original stocks Aa andBb, four constant combinations are possible, and consequently thehybrids produce the corresponding four forms of egg and pollen cells:
AB, Ab, aB, ab, and each of these will on the average figure fourtimes in the fertilization, since 16 individuals are included in theseries. Therefore, the participators in the fertilization are
Pollen cells: AB + AB + AB + AB + Ab + Ab + Ab + Ab + aB + aB + aB +aB + ab + ab + ab + ab.
Egg cells:

AB + AB + AB + AB + Ab + Ab + Ab + Ab + aB + aB + aB +aB + ab + ab + ab + ab.

In the process of fertilization each pollen form unites on anaverage equally often with each egg cell form, so that each of the fourpollen cells AB unites once with one of the forms of egg cell AB, Ab,
aB, ab. In precisely the same way the rest of the pollen cells of theforms Ab, aB, ab, unite with all the other egg cells. We obtain
therefore:
AB
AB
AB
AB
Ab
+
+
+
+
+
AB
Ab
aB
ab
AB

Ab
+
Ab

Ab
Ab
+
+
aB
ab

aB
aB
aB
aB
ab
+
+
+
+
+
AB
Ab
aB
ab
AB

ab
+
Ab

ab
ab
+
aB
ab

or
AB + ABb + AaB + AaBb + ABb + Ab + AaBb + Aab +AaB + AaBb + aB + aBb + AaBb + Aab + aBb + ab =AB + Ab + aB + ab + 2ABb + 2aBb + 2AaB + 2Aab + 4AaBb

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27

In precisely similar fashion is the developmental series of hybrids
exhibited when three kinds of differentiating characters are conjoinedin them. The hybrids form eight various kinds of egg and pollen cells:
ABC, ABc, AbC, Abc, aBC, aBc, abC, abc, and each pollen form
unites itself again on the average once with each form of egg cell.
The law of combination of different characters which governs thedevelopment of the hybrids finds therefore its foundation andexplanation in the principle enunciated, that the hybrids produce egg
cells and pollen cells which in equal numbers represent all constantforms which result from the combinations of the characters brought
together in fertilization.

EXPERIMENTS WITH HYBRIDS OF OTHER SPECIES OF
PLANTS
It must be the object of further experiments to ascertain whether
the law of development discovered for Pisum applies also to thehybrids of other plants. To this end several experiments were recently
commenced. Two minor experiments with species of Phaseolus havebeen completed, and may be here mentioned.
An experiment with Phaseolus vulgaris and Phaseolus nanus
gave results in perfect agreement. Ph. nanus had together with thedwarf axis, simply inflated, green pods. Ph. vulgaris had, on the other
hand, an axis 10 ft. to 12 ft. high, and yellow colored pods, constricted
when ripe. The ratios of the numbers in which the different forms
appeared in the separate generations were the same as with Pisum.
Also the development of the constant combinations resulted according
to the law of simple combination of characters, exactly as in the caseof Pisum. There were obtained:
Constant
Combinations
1
2
3
4
5
6
7
8

Axis
long
"
"
"
short
"
"
"

Color of theunripe pods
green"
yellow"green"
yellow"

CLASSICAL GENETICS

Form of theunripe pods
inflated
constrictedinflated
constrictedinflated
constrictedinflated
constricted

28

GREGOR MENDEL (1865)

The green color of the pod, the inflated forms, and the long axis
were, as in Pisum, dominant characters.
Another experiment with two very different species of Phaseolus
had only a partial result. Phaseolus nanus, L., served as seed parent, aperfectly constant species, with white flowers in short recemes andsmall white seeds in straight, inflated, smooth pods; as pollen parent
was used Ph. multiflorus, W., with tall winding stem, purple–red
flowers in very long racemes, rough, sickle–shaped crooked pods, andlarge seeds which bore black flecks and splashes on a peach–blood–
red ground.
The hybrids had the greatest similarity to the pollen parent, but
the flowers appeared less intensely colored. Their fertility was very
limited; from 17 plants, which together developed many hundreds offlowers, only 49 seeds in all were obtained. These were of medium
size, and were flecked and splashed similarly to those of Ph.
multiflorus, while the ground color was not materially different. The
next year 44 plants were raised from these seeds, of which only 31
reached the flowering stage. The characters of Ph. nanus, which had
been altogether latent in the hybrids, reappeared in various
combinations; their ratio, however, with relation to the dominant
plants was necessarily very fluctuating owing to the small number oftrial plants. With certain characters, as in those of the axis and theform of pod, it was, however, as in the case of Pisum, almost exactly
1:3.
Insignificant as the results of this experiment may be as regards
the determination of the relative numbers in which the various forms
appeared, it presents, on the other hand, the phenomenon of aremarkable change of color in the flowers and seed of the hybrids. In
Pisum it is known that the characters of the flower– and seed–color
present themselves unchanged in the first and further generations, andthat the offspring of the hybrids display exclusively the one or theother of the characters of the original stocks. It is otherwise in theexperiment we are considering. The white flowers and the seed–colorof Ph. nanus appeared, it is true, at once in the first generation in one
fairly fertile example, but the remaining 30 plants developed flower–colors which were of various grades of purple–red to pale violet. The
coloring of the seed–coat was no less varied than that of the flowers.
No plant could rank as fully fertile; many produced no fruit at all;others only yielded fruits from the flowers last produced, which did
not ripen. From 15 plants only were well–developed seeds obtained.
The greatest disposition to infertility was seen in the forms withpreponderantly red flowers, since out of 16 of these only four yielded
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ripe seeds. Three of these had a similar seed pattern to Ph. multiflorus,
but with a more or less pale ground color; the fourth plant yielded
only one seed of plain brown tint. The forms with preponderantly
violet–colored flowers had dark brown, black–brown, and quite black
seeds.
The experiment was continued through two more generations
under similar unfavorable circumstances, since even among theoffspring of fairly fertile plants there came again some which wereless fertile and even quite sterile. Other flower–and seed–colors thanthose cited did not subsequently present themselves. The forms whichin the first generation contained one or more of the recessive
characters remained, as regards these, constant without exception.
Also of those plants which possessed violet flowers and brown or
black seed, some did not vary again in these respects in the next
generation; the majority, however, yielded together with offspringexactly like themselves, some which displayed white flowers andwhite seed–coats. The red flowering plants remained so slightly fertilethat nothing can be said with certainty as regards their further
development.
Despite the many disturbing factors with which the observations
had to contend, it is nevertheless seen by this experiment that thedevelopment of the hybrids, with regard to those characters which
concern the form of the plants, follows the same laws as in Pisum.
With regard to the color characters, it certainly appears difficult toperceive a substantial agreement. Apart from the fact that from theunion of a white and a purple–red coloring a whole series of colors
results, from purple to pale violet and white, the circumstance is a
striking one that among 31 flowering plants only one received therecessive character of the white color, while in Pisum this occurs on
the average in every fourth plant.
Even these enigmatic results, however, might probably be
explained by the law governing Pisum if we might assume that thecolor of the flowers and seeds of Ph. multiflorus is a combination of
two or more entirely independent colors, which individually act likeany other constant character in the plant. If the flower–color A were a
combination of the individual characters A1+ A2 + ..... which produce
the total impression of a purple coloration, then by fertilization withthe differentiating character, white color, a, there would be produced
the hybrid unions A1a + A2a +..... and so would it be with thecorresponding coloring of the seed–coats. According to the above
assumptions, each of these hybrid color unions would be independent,

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GREGOR MENDEL (1865)

and would consequently develop quite independently from the others.
It is then easily seen that from the combination of the separate
developmental series a complete color–series must result. If, forinstance, A = A1 + A2, then the hybrids A1a and A2a form thedevelopmental series:
A1 + 2 A1a + a
A2 + 2 A2a + a

The members of this series can enter into nine different
combinations, and each of these denotes another color:
1 A1A2
2 A1A2a
1 A1a

2 A1aA2
4 A1aA2a
2 A1aa

1 A2a
2 A2aa
1 aa

The figures prescribed for the separate combinations also indicate
how many plants with the corresponding coloring belong to the series.
Since the total is 16, the whole of the colors are on the average
distributed over each 16 plants, but, as the series itself indicated, in
unequal proportions.
Should the color development really happen in this way, we could
offer an explanation of the case above described, namely that of thewhite flowers and seed–coat color only appeared once among 31
plants of the first generation. This coloring appears only once in theseries, and could therefore also only be developed once in the averagein each 16, and with three color characters only once even in 64
plants.
It must, nevertheless, not be forgotten that the explanation here
attempted is based on a mere hypothesis, only supported by the very
imperfect result of the experiment just described. It would, however,be well worth while to follow up the development of color in hybrids
by similar experiments, since it is probable that in this way we mightlearn the significance of the extraordinary variety in the coloring of
our ornamental flowers.
So far, little at present is known with certainty beyond the factthat the color of the flowers in most ornamental plants is an extremely
variable character. The opinion has often been expressed that thestability of the species is greatly disturbed or entirely upset bycultivation, and consequently there is an inclination to regard thedevelopment of cultivated forms as a matter of chance devoid ofrules; the coloring of ornamental plants is indeed usually cited as an
example of great instability. It is, however, not clear why the simple
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31transference into garden soil should result in such a thorough andpersistent revolution in the plant organism. No one will seriously
maintain that in the open country the development of plants is ruledby other laws than in the garden bed. Here, as there, changes of typemust take place if the conditions of life be altered, and the species
possesses the capacity of fitting itself to its new environment. It is
willingly granted that by cultivation the origination of new varieties isfavored, and that by man’s labor many varieties are acquired which,under natural conditions, would be lost; but nothing justifies theassumption that the tendency to formation of varieties is so
extraordinarily increased that the species speedily lose all stability,
and their offspring diverge into an endless series of extremely variable
forms. Were the change in the conditions the sole cause of variability
we might expect that those cultivated plants which are grown for
centuries under almost identical conditions would again attain
constancy. This, as is well known, is not the case since it is precisely
under such circumstances that not only the most varied but also themost variable forms are found. It is only the Leguminosae, like Pisum,
Phaseolus, Lens, whose organs of fertilization are protected by thekeel, which constitute a noteworthy exception. Even here there havearisen numerous varieties during a cultural period of more than 1,000
years under most various conditions; these maintain, however, underunchanging environments a stability as great as that of species
growing wild.
It is more than probable that as regards the variability ofcultivated plants there exists a factor which so far has received little
attention. Various experiments force us to the conclusion that ourcultivated plants, with few exceptions, are members of various hybrid
series, whose further development in conformity with law is varied
and interrupted by frequent crossings inter se. The circumstance mustnot be overlooked that cultivated plants are mostly grown in great
numbers and close together, affording the most favorable conditions
for reciprocal fertilization between the varieties present and species
itself. The probability of this is supported by the fact that among thegreat array of variable forms solitary examples are always found,which in one character or another remain constant, if only foreign
influence be carefully excluded. These forms behave precisely as do
those which are known to be members of the compound hybrid series.
Also with the most susceptible of all characters, that of color, itcannot escape the careful observer that in the separate forms theinclination to vary is displayed in very different degrees. Amongplants which arise from one spontaneous fertilization there are often
CLASSICAL GENETICS32

GREGOR MENDEL (1865)

some who offspring vary widely in the constitution and arrangementof the colors, while that of others shows little deviation, and among agreater number solitary examples occur which transmit the color of
the flowers unchanged to their offspring. The cultivated species ofDianthus afford an instructive example of this. A white–flowered
example of Dianthus caryophyllus, which itself was derived from awhite–flowered variety, was shut up during its blooming period in agreenhouse; the numerous seeds obtained therefrom yielded plants
entirely white–flowered like itself. A similar result was obtained froma sub–species, with red flowers somewhat flushed with violet, and one
with flowers white, striped with red. Many others, on the other hand,which were similarly protected, yielded progeny which were more orless variously colored and marked.
Whoever studies the coloration which results in ornamental plantsfrom similar fertilization can hardly escape the conviction that here
also the development follows a definite law which possibly finds its
expression in the combination of several independent color
characters.

CONCLUDING REMARKS
It can hardly fail to be of interest to compare the observations
made regarding Pisum with the results arrived at by the two
authorities in this branch of knowledge, Kölreuter and Gärtner, in
their investigations. According to the opinion of both, the hybrids inoutward appearance present either a form intermediate between theoriginal species, or they closely resemble either the one or the othertype, and sometimes can hardly be discriminated from it. From theirseeds usually arise, if the fertilization was effected by their own
pollen, various forms which differ from the normal type. As a rule, themajority of individuals obtained by one fertilization maintain the
hybrid form, while some few others come more like the seed parent,
and one or other individual approaches the pollen parent. This,
however, is not the case with hybrids without exception. Sometimes
the offspring have more nearly approached, some the one and somethe other of the two original stocks, or they all incline more to one or
the other side; while in other cases they remain perfectly like thehybrid and continue constant in their offspring. The hybrids ofvarieties behave like hybrids of species, but they possess greater

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variability of form and more pronounced tendency to revert to theoriginal types.
With regard to the form of the hybrids and their development, as arule an agreement with the observations made in Pisum is
unmistakable. It is otherwise with the exceptional cases cited. Gärtner
confesses even that the exact determination whether a form bears agreater resemblance to one or to the other of the two original species
often involved great difficulty, so much depending upon thesubjective point of view of the observer. Another circumstance could,
however, contribute to render the results fluctuating and uncertain,
despite the most careful observation and differentiation. For theexperiments, plants were mostly used which rank as good species andare differentiated by a large number of characters. In addition to the
sharply defined characters, where it is a question of greatly or lesssimilarity, those characters must also be taken into account which areoften difficult to define in words, but yet suffice, as every plant
specialist knows, to give the forms a peculiar appearance. If it beaccepted that the development of hybrids follows the law which isvalid for Pisum, the series in each separate experiment must containvery many forms, since the number of terms, as is known, increases
with the number of the differentiating characters as the powers of
three. With a relatively small number of experimental plants theresults therefore could only be approximately right, and in single
cases might fluctuate considerably. If, for instance, the two original
stocks differ in seven characters, and 100 – 200 plants were raised
from the seeds of their hybrids to determine the grade of relationshipof the offspring, we can easily see how uncertain the decision must
become since for seven differentiating characters the combination
series contains 16,384 individuals under 2,187 various forms; now one
and then another relationship could assert its predominance, justaccording as chance presented this or that form to the observer in a
majority of cases.
If, furthermore, there appear among the differentiating charactersat the same time dominant characters, which are transmitted entire or
nearly unchanged to the hybrids, then in the terms of thedevelopmental series that one of the two original parents which
possesses the majority of dominant characters must always be
predominant. In the experiment described relative to Pisum, in which
three kinds of differentiating characters were concerned, all thedominant characters belonged to the seed parent. Although the termsof the series in their internal composition approach both original
parents equally, yet in this experiment the type of the seed parent
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GREGOR MENDEL (1865)

obtained so great a preponderance that out of each 64 plants of thefirst generation 54 exactly resembled it, or only differed in one
character. It is seen how rash it must be under such circumstances todraw from the external resemblances of hybrids conclusions as to theirinternal nature.
Gärtner mentions that in those cases where the development was
regular among the offspring of the hybrids the two original species
were not reproduced, but only a few individuals which approachedthem. With very extended developmental series it could not in fact beotherwise. For seven differentiating characters, for instance, among
more than 16,000 individuals –– offspring of the hybrids –– each of
the two original species would occur only once. It is therefore hardly
possible that these should appear at all among a small number ofexperimental plants; with some probability, however, we might
reckon upon the appearance in the series of a few forms whichapproach them.
We meet with an essential difference in those hybrids which
remain constant in their progeny and propagate themselves as truly as
the pure species. According to Gärtner, to this class belong theremarkably fertile hybrids Aquilegia atropurpurea canadensis,
Lavatera pseudolbia thuringiaca, Geum urbanorivale, and some
Dianthus hybrids; and, according to Wichura, the hybrids of theWillow family. For the history of the evolution of plants thiscircumstance is of special importance, since constant hybrids acquire
the status of new species. The correctness of the facts is guaranteed byeminent observers, and cannot be doubted. Gärtner had an opportunityof following up Dianthus Armeria deltoides to the tenth generation,
since it regularly propagated itself in the garden.
With Pisum it was shown by experiment that the hybrids form
egg and pollen cells of different kinds, and that herein lies the reasonof the variability of their offspring. In other hybrids, likewise, whose
offspring behave similarly we may assume a like cause; for those, on
the other hand, which remain constant the assumption appears
justifiable that their reproductive cells are all alike and agree with thefoundation–cell of the hybrid. In the opinion of renowned
physiologists, for the purpose of propagation one pollen cell and one
egg cells unite in Phanerogams* into a single cell, which is capable by*

In Pisum it is placed beyond doubt that for the formation of the new embryo aperfect union of the elements of both reproductive cells must take place. How could
we otherwise explain that among the offspring of the hybrids both original types
reappear in equal numbers and with all their peculiarities? If the influence of theegg cell upon the pollen cell were only external, if it fulfilled the role of a nurse
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assimilation and formation of new cells to become an independent
organism. This development follows a constant law, which is foundedon the material composition and arrangement of the elements whichmeet in the cell in a vivifying union. If the reproductive cells be of thesame kind and agree with the foundation cell of the mother plant, thenthe development of the new individual will follow the same law whichrules the mother plant. If it chance that an egg cell unites with adissimilar pollen cell, we must then assume that between thoseelements of both cells, which determine opposite characters some sortof compromise is effected. The resulting compound cell becomes thefoundation of the hybrid organism the development of which
necessarily follows a different scheme from that obtaining in each of
the two original species. If the compromise be taken to be a complete
one, in the sense, namely, that the hybrid embryo is formed from twosimilar cells, in which the differences are entirely and permanently
accommodated together, the further result follows that the hybrids,
like any other stable plant species, reproduce themselves truly in theiroffspring. The reproductive cells which are formed in their seedvessels and anthers are of one kind, and agree with the fundamental
compound cell.
With regard to those hybrids whose progeny is variable we may
perhaps assume that between the differentiating elements of the egg
and pollen cells there also occurs a compromise, in so far that theformation of a cell as the foundation of the hybrid becomes possible;but, nevertheless, the arrangement between the conflicting elements isonly temporary and does not endure throughout the life of the hybrid
plant. Since in the habit of the plant no changes are perceptible during
the whole period of vegetation, we must further assume that it is only
possible for the differentiating elements to liberate themselves fromthe enforced union when the fertilizing cells are developed. In theformation of these cells all existing elements participate in an entirely
free and equal arrangement, by which it is only the differentiating
ones which mutually separate themselves. In this way the production
would be rendered possible of as many sorts of egg and pollen cells as
there are combinations possible of the formative elements.

only, then the result of each fertilization could be no other than that the developed
hybrid should exactly resemble the pollen parent, or at any rate do so very closely.
This the experiments so far have in nowise confirmed. An evident proof of thecomplete union of the contents of both cells is afforded by the experience gained on
all sides that it is immaterial, as regards the form of the hybrid, which of the original
species is the seed parent or which the pollen parent.

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GREGOR MENDEL (1865)

The attribution attempted here of the essential difference in thedevelopment of hybrids to a permanent or temporary union of thediffering cell elements can, of course, only claim the value of an
hypothesis for which the lack of definite data offers a wide scope.
Some justification of the opinion expressed lies in the evidence
afforded by Pisum that the behavior of each pair of differentiating
characters in hybrid union is independent of the other differences
between the two original plants, and, further, that the hybrid produces
just so many kinds of egg and pollen cells as there are possible
constant combination forms. The differentiating characters of twoplants can finally, however, only depend upon differences in thecomposition and grouping of the elements which exist in thefoundation–cells of the same in vital interaction.
Even the validity of the law formulated for Pisum requires still to
be confirmed, and a repetition of the more important experiments isconsequently must to be desired, that, for instance, relating to thecomposition of the hybrid fertilizing cells. A differential may easily
escape the single observer, which although at the outset may appear to
be unimportant, yet accumulate to such an extent that it must not beignored in the total result. Whether the variable hybrids of other plant
species observe an entire agreement must also be first decided
experimentally. In the meantime we may assume that in material
points an essential difference can scarcely occur, since the unity in thedevelopmental plant of organic life is beyond question.
In conclusion, the experiments carried out by Kölreuter, Gärtner,and others with respect to the transformation of one species intoanother by artificial fertilization merit special mention. Particularimportance has been attached to these experiments and Gärtner
reckons them “among the most difficult of all in hybridization.”
If a species A is to be transformed into a species B, both must be
united by fertilization and the resulting hybrids then be fertilized withthe pollen of B; then, out of the various offspring resulting, that form
would be selected which stood in nearest relation to B and once more
be fertilized with B pollen, and so continuously until finally a form isarrived at which is like B and constant in its progeny. By this process
the species A would change into the species B. Gärtner alone has
effected 30 such experiments with plants of genera Aquilegia,
Dianthus, Geum, Lavatera, Lynchnis, Malva, Nicotiana, andOenothera. The period of transformation was not alike for all species.
While with some a triple fertilization sufficed, with others this had tobe repeated five or six times, and even in the same species
fluctuations were observed in various experiments. Gärtner ascribes
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this difference to the circumstance that “the specific power by which a
species, during reproduction, effects the change and transformation of
the maternal type varies considerably in different plants, and that,consequently, the periods with which the one species is changed into
the other must also vary, as also the number of generations, so that thetransformation in some species is perfected in more, and in others infewer generations”. Further, the same observer remarks “that in thesetransformation experiments a good deal depends upon which type andwhich individual be chosen for further transformation”.
If it may be assumed that in these experiments the constitution of
the forms resulted in a similar way to that of Pisum, the entire processof transformation would find a fairly simple explanation. The hybrid
forms as many kinds of egg cells as there are constant combinations
possible of the characters conjoined therein, and one of these isalways of the same kind as that of the fertilizing pollen cells.
Consequently there always exists the possibility with all such
experiments that even from the second fertilization there may result aconstant form identical with that of the pollen parent. Whether thisreally be obtained depends in each separate case upon the number ofthe experimental plants, as well as upon the number of differentiating
characters which are united by the fertilization. Let us, for instance,
assume that the plants selected for experiment differed in threecharacters, and the species ABC is to be transformed into the otherspecies abc by repeated fertilization with the pollen of the latter; thehybrids resulting from the first cross form eight different kinds of egg
cells, namely:
ABC, ABc, AbC, aBC, Abc, aBc, abC, abc

These in the second year of experiment are united again with thepollen cells abc, and we obtain the series
AaBbCc + AaBbc + AabCc + aBbCc + Aabc + aBbc + abCc + abc

Since the form abc occurs once in the series of eight terms, it isconsequently little likely that it would be missing among theexperimental plants, even were these raised in a smaller number, andthe transformation would be perfected already by a second
fertilization. If by chance it did not appear, then the fertilization must
be repeated with one of those forms nearest akin, Aabc, aBbc, abCc.
It is perceived that such an experiment must extend the farther thesmaller the number of experimental plants and the larger the numberof differentiating characters in the two original species; and that,furthermore, in the same species there can easily occur a delay of one
CLASSICAL GENETICS38

GREGOR MENDEL (1865)or even of two generations such as Gärtner observed. The
transformation of widely divergent species could generally only becompleted in five or six years of experiment, since the number ofdifferent egg cells which are formed in the hybrid increases as thepowers of two with the number of differentiating characters.
Gärtner found by repeated experiments that the respective periodof transformation varies in many species, so that frequently a species
A can be transformed into a species B a generation sooner than can
species B into species A. He deduces therefrom that Kölreuter’s
opinion can hardly be maintained that “the two natures in hybrids are
perfectly in equilibrium”. Experiments which in this connection werecarried out with two species of Pisum demonstrated that as regards the
choice of the fittest individuals for the purpose of further fertilization
it may make a great difference which of two species is transformed
into the other. The two experimental plants differed in five characters,
while at the same time those of species A were all dominant and thoseof species B all recessive. For mutual transformation A was fertilizedwith pollen of B, and B with pollen of A, and this was repeated withboth hybrids the following year. With the first experiment, B/A, there
were 87 plants available in the third year of experiment for selectionof the individuals for further crossing, and these were of the possible
32 forms; with the second experiment, A/B, 73 plants resulted, which
agreed throughout perfectly in habit with the pollen parent; in theirinternal composition, however, they must have been just as varied as
the forms in the other experiment. A definite selection wasconsequently only possible with the first experiment; with the second
the selection had to be made at random, merely. Of the latter only aportion of the flowers were crossed with the A pollen, the others wereleft to fertilize themselves. Among each five plants which wereselected in both experiments for fertilization there agreed, as thefollowing year’s culture showed, with the pollen parent:
1st Experiment2 plants
3 plants——————

2nd Experiment————2 plants
2 plants
1 plant

in
"
"
"
"

all characters
4
"
3
"
2
"
1 characterIn the first experiment, therefore, the transformation wascompleted; in the second, which was not continued further, two more
fertilizations would probably have been required.

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Although the case may not frequently occur in which thedominant characters belong exclusively to one or the other of theoriginal parent plants, it will always make a difference which of thetwo possesses the majority of dominants. If the pollen parent has the
majority, then the selection of forms for further crossing will afford aless degree of certainty than in the reverse case, which must imply adelay in the period of transformation, provided that the experiment isonly considered as completed when a form is arrived at which not
only exactly resembles the pollen parent in form, but also remains asconstant in its progeny.
Gärtner, by the results of theses transformation experiments, was
led to oppose the opinion of those naturalists who dispute the stabilityof plant species and believe in a continuous evolution of vegetation.
He perceives in the complete transformation of one species intoanother an indubitable proof that species are fixed with limits beyond
which they cannot change. Although this opinion cannot be
unconditionally accepted we find on the other hand in Gärtner’s
experiments a noteworthy confirmation of that supposition regarding
variability of cultivated plants which has already been expressed.
Among the experimental species there were cultivated plants,
such as Aquilegia atropurpurea and canadensis, Dianthus
caryophyllus, chinensis, and japonicus, Nicotiana rustica andpaniculata, and hybrids between these species lost none of theirstability after four or five generations.

CLASSICAL GENETICS