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Copyright 0 1992 by the Genetics Society of America
Perspectives
Anecdotal, Historical and Critical Commentaries on Genetics
Edited by James F. Crow and William F. Dove
What Did GREGOR
MENDELThink He Discovered?
Daniel L. H a d * and Vitezslav Orel+
*Department of Genetics, Washington University School of Medicine, St. Louis,Missouri 631 10-1095, and +Museum of the
Mendelianum, Brno, Czechoslovakia
REGOR MENDEL is accorded a special place in
the history of genetics. His experiments, beautifully designed, were the first to focus on the numerical relationships among traits appearing in the progeny of hybrids; and his interpretation, clear and concise, was based on material hereditary elements that
undergo segregation and independent assortment.
Poignantly overshadowing the creative brilliance of
MENDEL’S work
is the fact that it was virtually ignored
for 34 years. Only after the dramatic rediscovery in
1900-16 years after MENDEL’Sdeath-was MENDEL
rightfully recognized as the founder of genetics.
Mendelian mythmaking?This orthodox interpretation of MENDEL’Scontribution has recently been
challenged as
G
. . . a myth created by the early geneticists to reinforce the
belief that the laws of inheritance are obvious to anyone
who looks closely enough at the problem (BOWLER
1989, p.
103).
This opinion reflects revisionistviews of MENDELand
his intentions that have received considerable prominence in journals dealing with the history of science.
The new perspectives come from several directions.
One influential view is that MENDELwas not interested
in heredity, as such, but in the role of hybrids in the
generation ofnewspecies.
This viewof MENDEL’S
work
. . . strips it of inflated Whiggish interpretations and places
it squarely within the context of mid-nineteenth century
biology (OLBY1979, p. 53).
T h e iconoclasticconclusion is that ifyou
define a
Mendelian
. . . as one who subscribes explicitly to the existence of a
finite number of hereditary elements which in the simplest
case is two per hereditary trait, only one of which mayenter
a germ cell, then Mendel was clearly no Mendelian (OLBY
1979, p. 68).
Genetics 131: 245-253 uune, 1992)
A second view echoes the emphasis on MENDELas
hybridist
. . . concerned with the formation and development of hybrids . . . using empirical methods . . . [who did] not explain
his results by employing invisible particulate determiners,
paired or otherwise (MONAGHAN
and CORCOS
1990, p. 268).
A third view imputes to MENDEL a dark motive in
performing his experiments: that he had actually set
out to prove that hybrids gave invariant progeny
(essentially anantidarwinian rejection of descent with
modification) and that his work with Hieracium was
not undertaken to demonstrate agreement withhis
results with Pisum but rather to demonstrate the existence ofconstant (nonsegregating)hybrids (CALLENDER 1988). A fourth view goes well beyond the others
in claiming that
[Mendel’s monohybrid] experiments are fictitious in the
sense that they have been carried out only on paper: the
numerical data relative to them have been obtained by
progressively disaggregating those from polyhybrid crosses.
In otherwords, Mendel never carried outthese experiments
in the garden,but rather only on the pages ofhis notebooks
(DI TROCCHIO
1991, p. 515).
We believe that it would be worthwhile to examine
these claims in a forum accessible to geneticists. Most
instructors in genetics, and most genetics textbooks,
still put forth the orthodox interpretation of MENDEL’S work. STERNand SHERWOOD
(1966), in the
foreword to their translation of MENDEL’SVersuche
uber P$anzen-Hybriden, point out that
. . . in the history of Mendelism the eternal is strongly confounded with the ephemeral. There is the neglect of Mendel’s accomplishments during his lifetime and the futility of
his years of writing to Nageli. There is the uneven history
of the rediscovery of his work 34 years after its publication
and its striking reanalysis [by FISHER(1936)l another 36
years later.
246
D. L. Hart1 and V.Orel
Only time will tell whether the revisionist views of
MENDEL’S
work are ephemeral, but if the orthodox
work is seriously flawed
interpretation of MENDEL’S
in any respect, then it ought to be adjusted.
Why the neglect?Although the main themes of this
paper are paraphrases of FISHER’S(1936) searching
questions-What was MENDELtrying to discover? What
did he discover? What did he think he discovered?there are additional
issues concerning MENDEL’S
work
that warrant updating. First,
why was the work neglected for so long? A comprehensive and thoughtful
discussion of various categories of explanation is presented by SANDLER
and SANDLER
(1985). While granting a role to the relativeinaccessibility of the Verhan-
dlungen des naturforschenden Vereines in Briinn (Brno),
in which MENDEL’Spaper was published, to the overshadowingcontemporaryinterest
in organic evolution, and to the strangeness andnovelty of MENDEL’S
experimental approach, SANDLER SANDLER
and
nevertheless conclude that MENDEL’S
paper was simply incomprehensible to his contemporaries since heredity
and development were completely confounded conceptually in the latter half of the 19th century. T h e
transmission of hereditary traits from parents to offspring was considered partof the same process as the
development of traits in the offspring; heredity
was
but a moment in development with no need for a
conceptual framework of its own. Hence, while
Mendel . . . defined his problem in purely genetic terms,
and produced a correct and amazingly completeanswer, [it
was] to an asyet unformulated question! (SANDLER
and
SANDLER
1985, p. 69).
The all too goodness of fit: Another issue is the
excessive goodness of fit of MENDEL’Sratios (FISHER
1936; EDWARDS
1986). Itis unfortunate that FISHER’S
. . . painstaking analysis and his defense of Mendel’s integrity havesometimesbeenincorrectly
reported ashaving
exposed a scientific fraud of major proportions. . . (EDWARDS
1986, p. 296).
T h e truth is that, while the datashow a persistentlack
of extreme segregations, as if there had been some
trimming of highly deviant values (EDWARDS 1986),
the bias toward expectation is slight (WRIGHT 1966).
Considering the entirety of the evidence, EDWARDS
(1 986) agrees
with DOBZHANSKY’S
conclusion:
Few experimenters are lucky enough to have no mistakes
or accidents happen in any of their experiments, and it is
only common sense to have such failures discarded. The
evident danger is ascribing to mistakes
and expunging from
the record perfectly authentic experimental results which
do not fit one’s expectations. Not having been familiar
with
chi-squares and other statistical tests, Mendel may have, in
perfect conscience, thrown out some crosses which he suspected to involve pollen contamination or other accident
(DOBZHANSKY
1967, p. 1589).
More recently, MENDELhas also been accused of a
1991).
more subtle kind of deception (DI TROCCHIO
From his analysis of the Versuche, DI TROCCHIO
supposes that MENDELdid not actually do the crosses he
reported.What he actuallydid,accordingtothis
reconstruction, was to carry out no fewer than 484
crosses representing all pairwise reciprocal combinations of the 22 varieties of Pisum that MENDELhad
available. Most of these crosseswouldhaveshown
either no segregationor segregation for several traits.
Among the latter,MENDELpooled the marginal totals
and reported them as monohybrid segregations.Even
if MENDEL followed this procedure, the marginal totals were obtained in realcrosses and could hardly be
regardedas“fictitious.”Furthermore,it
is hard to
believe that MENDEL actually followed thisprocedure
without saying so, since he roundly criticized GARTNER for not describing his experiments in sufficient
detail to allow MENDELto repeat them:
The results which Gartner obtained in his experiments are
known to me; I have repeated his workand have reexamined
it carefullyto find, if possible, an agreement with those laws
of development which I found to be true for my experimental plant. However, try as I would, I was unable to follow
his experiments completely, not in a single case! It is very
regrettable that this worthy man did not publish a detailed
description of his individual experiments . . . However, in
most cases, it can at least be recognized that the possibility
of an agreement with Pisum is not excluded (letter to NACELI,December 31, 1866, P. 57). [In the present account,
all citationsto MENDEL’Swritings refer to the page numbers
in STERNand SHERWOOD
(1966).]
MENDELas a hybridist: In considering MENDEL’S
contributions, the greatest weight must be placed on
his own writing o n hybridization, which is unfortunately limited to two papers (1866 and 1870) and 10
letters to CARL NAGELI (1866-1873). T h e principal
source of information is the Versuche (1966), which
was the text of MENDEL’S
public lectures of February
8 and March 8, 1865, to fellow members of the Brno
Natural History Society. In MENDEL’S
letter of April
18, 1867, to NAGELI,he says,
I made every effort to verify, with other plants, the result
obtained with Pisum . . . I attempted to inspire some control
experiments, and for that reason discussedthe Pisum experiments at the meeting of the local society of naturalists. I
encountered, was
as
to be expected, divided
opinion . . . When, last year [ 18661 I was asked to publish my
lecture in the proceedings of the society, I agreed to do SO,
after having re-examined my records for the various years
of experimentation, and not havingbeenable to find a
source of error. The paper which was submitted to YOU is
the unchanged reprint of the draft of the lecture mentioned;
thus the brevity of the exposition, as is essentialfor a public
lecture (p. 60).
Therefore, while the Versuche is written in remarkably
Perspectives
clear German (STERNand SHERWOOD1966), the account is abbreviated and addressed to a general audience. We have no way of knowing what revisions
MENDELmight have madehad he foreseen that every
word and phrase in the paper would still be parsed
and analyzed morethan125
years afterward.For
(199 1, p. 5 16)readily admits
example, DI TROCCHIO
that his allegation that MENDEL’Smonohybrid experiments were “fictitious” is based almost exclusively on
the occurrence of two “unexpected” words: eintheilung
(“breaking up into several parts”), which appears in
the third subheading of the Versuche (p. 5); and wesentlich (“essential, substantial”), which appears in the
statement that “plants were
used which differed in
only one essential trait” (p. 17).
Did MENDELhave an agenda to disprove descent
with modification that led to his choice of Hieracium,
with its constant hybrids, for his later experiments
(CALLENDER1988)? T h e Darwinian theory waswell
known by the time of MENDEL’Spresentation. The
German translation of On the Origin of Species was
published in 1863, and the monastery copy has marginalnotes in MENDEL’Shandwriting. InJanuary,
1965, at the monthly meeting of the Brno Natural
History Society, A. MAKOWSKY,one of MENDEL’S
friends, lectured on the theory of natural selection.
MENDELgave the nexttwo lectures, and thushe could
easily have focused on the evolutionary angle had he
wished. However, the Versuche mentions evolution on
only three occasions: on page2 in the phrase “the
evolutionary history of organic forms,” on page 4 1 in
the phrase “the evolutionary history of plants,” and
on page 47 where MENDELcites GARTNER’Sview that
“a species has fixed limits beyond which it cannot
change” and says that this statement cannot be accepted unconditionally. Furthermore, a number of
plant species other than Hieracium were chosen in
making “every effort to verify, with other plants, the
result obtained with Pisum” (MENDEL’Sletter to NAGELI, April 18,1867, P. 60). T h e reason MENDEL
chose Hieracium is given in his 1870 paper:
This genuspossessessuch an extraordinary profusion of
distinct forms that no other genus of plants can compare
with it (p. 51).
MENDEL’S
overriding interest was
. . . the question whether and to what extent hybridization
plays a part in the production of this wealth of forms (p. 5 1)
knowing full well that
. . . we may be led into erroneous conclusions if we take
rules deduced from observation of certain other hybrids to
be laws of hybridization,and try to apply them to Heiracium
without further consideration (p. 52).
At the beginning of his Pisum experiments MENDEL
was certainly aware of the Linnean conceptof constant
247
hybrids and thought that they might be important
exceptions tothe laws deducedfor Pisum. Inthe
Versuche he remarked that there is
. . . an essential dafference in those hybrids that remain constant in their progeny and propagate like pure strains (p.
41, emphasis his).
He also understood that
. . . this feature is of particular importanceto the evolutionary history of plants, because constant hybrids attain the
status of new species (p. 41 , emphasis his).
On the other hand, he had
. . . proved experimentallythat in Pisum hybrids form d i e r ent kinds of germinal and pollen cells and that this is the
reason for the variabilityof their offspring (p.4 1 , emphasis
his).
Far from providingthe constant hybrids that MENDEL
was allegedly seeking, theexperiments with Hieracium, as recounted in the letters to NACELI,were one
long chronicle of failure and frustration.
On the other hand, there
is no disputing that MENDEL was in the tradition of the plant hybridists, like
KOLREUTERand GARTNER,whose work, among that
of others, he cites in the Versuche. The question is
whether he was so engrossed in this tradition that the
process of heredity itself was either unrecognized or
of little interest (OLBY1979; MONACHANand CORCOS
1990). OLBY(1 979,p. 67) claims that
Mendel’s overridingconcern was with the role of hybrids in
the genesis of new species . . . The laws of inheritance were
only of concern to him in so far as they bore on his analysis
of the evolutionary role of hybrids.
It is certainly true that MENDEL’Smain interest was in
hybrids. The answer to the question “What was MENDEL trying to discover?” is clearly given in the introductory remarks in the Versuche, inwhich MENDEL
describes the experimental program necessary to discover a
. . . generally applicable law of the formation and development of hybrids (p. 2)
which he considered as alaw whose
. . . significancefor the evolutionary history of organic
forms must not be underestimated (p.2).
On the other hand, in the very first sentence of the
Versuche, MENDELexplains that the experiments were
based on
. . . artificial fertilization undertaken on ornamental plants
to obtain new color variants (p. I),
which seems also to imply a significant interest in
heredity for its role in practical breeding. MENDEL
also had little concern for the systematic status of his
pea plants. He remarks that,in the opinion of experts,
248
and D. L. Hart1
most of the plants belong to thespecies Pisum sativum,
but
. . . the rank assigned to them in a classification system is
completely immaterial to the experiments
in question (p.5 ) .
One will never know exactly what MENDELwas thinking, but one might reasonably have expected a less
cavalier attitude toward species classification on the
part of an authorwhose primary concern was the role
of hybridization in the formation of newspecies.
(MENDELwas not alone in regarding the distinction
between species and varieties as purely arbitrary. The
viewwas commonamong 19th century biologists,
including DARWIN,and almost universal among the
botanists. It was to cause MENDELgreat trouble after
1866 when he began making hybrids between what
would now be regarded as distinct species. We are
indebted to ERNST MAYR for pointing this out in a
letter of December 17, 199 1, to DLH.)
The Moravianconnection: If, as SANDLERand
SANDLER
(1 985) pointout, therewas no clear conceptual distinction between heredity and development,
then how could MENDELhave been interested in heredity for its own sake? Even before MENDEL’Stime,
there had been great interest in Moravia in the practical breeding of sheep, fruit trees, and vines, partly
through the leadership of the naturalist, C . C. A N D R ~
(1 763-1 83 l), who in 1806, through the patronage
of
COUNTSALM-REIFFERSCHEID
(1776-1 836), had organized in Brno the Royal and Imperial Moravian and
Silesian Society for the Improvement of Agriculture,
Natural Science and Knowledge of the Country (OREL
and MATALOV;~1983). Manyof the monks of the
monastery of St. Thomas were active in this society,
including MENDEL’Spredecessor as abbot, F. C. NAPP
(1 792-1 867), and MENDEL himself. NAPP had organized the Brno Natural History Society as one of the
sections in thelargerorganizationuntil
it become
independent in 186 1. If MENDEL had a mentor in
Brno, it was NAPP:he had supported the teaching of
agriculture in thecontext of natural science, and
MENDELattended these lectures in 1846; and he had
MENDELsent to the university in Vienna in 18511853to study with, amongothers,theprominent
plant physiologist F. UNGER and the physicistC. J.
DOPPLER
(of the DOPPLER
effect). On the other hand,
NAPP’S role
in shaping MENDEL’Soutlook and research
agenda is uncertain. What is clear is that NAPP,motivated by issues in practical breeding, was interested in
heredity as a problem in itself. dn 1836 he attended a
meeting of the Sheep Breeders Association and summarized the discussion of breeding methodsby noting
that the crucial questions were: What is inherited?
How is it inherited? (OREL 1984). Later,
in 1840, ata
meeting of agriculturalists, NAPP defended hybridization as a method of obtaining new varieties of fruit
trees and drew attention to the element of chance in
V.Orel
this process (OREL 1983).That MENDEL himself was
interested in practical breeding can hardly
be
doubted, since he chose to memorialize these interests
in ceiling paintings on therenovated monastery reception room as well as on hisown tomb (MATALOV;~
1983). MENDELeven ate thefruits of his pea breeding
efforts:
In 1859 I obtained a very fertile descendant with large,
tasty,seedsfrom a firstgenerationhybrid.Since, in the
following year,its progeny retained the desirable characteristics and were uniform, the variety was cultivated in our
vegetable garden, and many plants were raised every year
up to 1865 (MENDELletter of April 18, 1867 to NAGELI,p.
61).
What did MENDELdiscover?If MENDELwas trying
to discover a “generally applicable law of the formation and development of hybrids,” what did he actually discover? In the traditional view, MENDELdiscovered that hereditary traits are determined by cellular elements, now called genes, that exist in pairs,
undergo segregation andindependent assortment,
and persist unchanged through successive generations
of hereditary transmission. A key passage in the Vers u c h that is relevant to this interpretation is as follows
(p. 29ff):
The difference of forms among the progeny of hybrids, as
well as the ratios in which they are observed, find an adequate explanation in the principle [of segregation] just deduced. The simplest case is given by the series for one pair
of differing traits. It is shown that this series is described by
the expression: A A a a, in which A and a signify the
forms with constant differing traits, and
A a the form hybrid
+
+
for both. The series contains four individuals in three different terms. In their production, pollen and germinal
cells
of form A and a participate, on theaverage,equallyin
fertilization; therefore each form manifests itself twice, since
four individualsare produced. Participating in fertilization
are thus:
Pollen cells
A + A + a + a
Germinal cells
A
+A +a +a
It is entirely a matter of chance which of the two kinds of
pollen combines with each single germinal
cell. However,
according to the laws of probability, in an average of many
cases it will always happen that every pollen form A and a
will unite equally often with every germinal-cellform A and
a ; therefore, in fertilization, one of the two pollen cells A
will meet a germinal cell A, the other a germinal cell a, and
equally, onepollencell
a will becomeassociatedwith
a
germinal cell A , and the othera.
I’ollenzcllen
A
A
a
A
a
f
Keirnzellen
A
a
!
a
The result of fertilization can be visualized by writing the
designations for associated germinal and pollen cells in the
form of fractions, pollen cells above the line, germinalcells
Perspectives
below. In the case under discussion one obtains
A/A + A / a + a / A + ala
In the first and fourth terms germinal and pollen cells
are alike; therefore the products of their association must
be constant, namely A and a; in the secondand third,
however, a union of the two differing parental traits takes
place again, therefore the forms arising from such fertilizations are absolutely identical with the hybrid
from which
theyderive. Thus, repeatedhybridization t a b s place. The
striking phenomenon, that hybridsare able to produce, in
addition to the two parental types, progeny that resemble
themselvesis thus explained. Ala and a/A both give the
same association Aa, since, as mentioned earlier, it makes
no difference to the consequence of fertilization which of
the two traits belongs to the pollen and which to the germinal cell. Therefore
A/A + A/a + a / A a/a = A + 2 Aa + a
This represents the average course of self-fertilization of
hybrids when two differingtraits are associated in them.In
individual flowers and individual plants, however, the ratio
in which themembers of the series are formed maybe
subject to not insignificant deviations.
+
In our opinion, this passage is strikingly perceptive
and hardly betrays befuddlement or deep confusion
as to whether the formative elements are paired in
the plants and segregate in thereproductive cells.
Although the passage does not use any of the words
gene, genotype, phenotype, homozygous,heterozygous, or
segregation, its meaning to modern geneticists is unmistakable. The only fault, which occurs on several
occasions in the Versuche, is that the term trait (Merkmal) is used to mean either phenotype or allele, depending on the context, which indicates that the distinction may not always have been clear or entirely
sharp in MENDEL’Sown mind.
Particulate inheritance:At this point itis necessary
to discuss several issues bearing on the level of MENDEL’S understanding of hisown work (OLBY1979;
MEIJER 1983; MONACHAN and C o ~ c o s1990; DI
TROCCHIO
1991). One issue is whether MENDEL
thought of his “potentially formative elements (bildungsfuhigenElemente)”(Versuche, p. 43) as particles
rather thanas fluids or emulsions (MEIJER1983; MONACHAN and CoRcos 1990). Nowhere in the Versuche
is the physical nature of die Elemente discussed in
enough detail to infer how MENDELmight have imagined them. On page 42, in the concluding remarks,
MENDELrefers to “the material composition and arrangement of the elements . . . in the cell” (“in der
materiellen Beschaffenheit undAnordnungderElemente. . . inderZelle”). This phrase would at least
seem to imply that he was thinking in terms of some
sort of material entities, but even this interpretation
is disputed by KALMUS(1983), whodismisses it as “an
afterthought” (p. 6 1)and arguesinstead that MENDEL
must have been thinking of die Elemente in terms of
nonmaterialistic scholastic metaphysics and invoking
249
the Aristotelian concept of the potential. But this
opinion seems flatly contradicted by MENDEL’Sunselfconsciously materialistic statement that
. . . the distinguishing traits of two plants can, after all, be
caused only by differences in the composition and grouping
their
of theelementsexisting in dynamicinteractionin
primordial cells (p. 43).
It does not matter whether MENDELwas thinking in
terms or particles or fluids, since he emphasized repeatedly the key point that differing elements emerge
unchanged from theirassociation. For example,in the
letter of April 18, 1867, to NAGELI,MENDELwrote,
The course of development consists simply in this; that in
each generation the two parental traits appear, separated
and unchanged, andthere is nothing to indicatethat one of
them has either inherited or taken over anything from the
other (p. 62).
(This passage also contains MENDEL’Sonly known use
of the term inherited.)
The use of A instead ofA A : MENDELroutinely used
the symbol A in genetic formulas in which a modern
geneticist would use AA, and this notation has been
interpreted as meaning that MENDELdid not believe
that hereditary elements occur in pairs (OLBY 1979;
MONAGHAN
and CORCOS1990). However,throughout
most of the Versuche, MENDELused the symbols A and
a in a very different sense than used in modern
genetics with reference to genes. In his usage, A refers
to a plant that breeds true for the dominant trait, and
similarly with a for the true-breeding recessive. Occasionally he used the same symbols to refer to the
hereditary determinants, as in the quotationabove
from page 29ff, and in the context of this particular
discussion it is quite clear that the expression
A/A
+ A / a + a / A + a l a = A + 2 Aa + a
summarizes the expected genetic constitutions of the
progeny on theleft and gives their physical and breeding characteristics on the right. There are also other
interpretations of MENDEL’Ssymbolism, including that
of MEIJER (1983), who argues that MENDEL’Ssymbol
A means “the potentialforcreating
thedominant
trait,” and hence the use of AA for the homozygote
would be redundant, since the two A’s are logically
equivalent. Lest we be too harsh in our anachronous
criticism of MENDEL’Ssomewhat inconsistent use of
symbols, it is worthwhile to bear in mind that modern
Drosophila geneticists routinely use unpaired symbols
when referring to homozygous recessives; for example, in referring to Drosophila strains, the symbol a1
means the genotype allal, and cn bw means the genotype cn bwlcn bw.
Segregation in AA and aa genotypes: The segregation issue provides one of OLBY’S (1 979)
principal
arguments that Mendel was no Mendelian:
V.Orel
250
and
D. L. Hart1
[Mendel] did not conceive of pairs of elements in the cell
representing and determiningthe pairs of contrasted characters. If he had this conception he would have allowed a
separation between like membersof such pairs as well as between
unlike members (OLBY1979, p. 66, emphasis his).
T h e strongest evidence in the Versuche in support of
OLBY’S
view is the following passage:
[The] differing elements [in hybrids] succeed in escaping
from the enforced association only
at the stage atwhich the
reproductive cells develop. In the formation of these cells
all elements present participatein completely free and uniform fashion, and only those that differ separate from each
other (p. 43).
FIELD 1985). However, by definition, alleles must be
different:
Allele . . . one of two or more alternate forms of a gene
occupying the same
locus
on
a particular
chromosome . . . and differing fromother alleles of that locus . . . .
(RIEGER,MICHAELIS
and GREEN1968, p. 11);
Allele . . . one of a series of possible alternative formsof a
given gene . . , differing in DNA sequence . . . , (KING and
STANSFIELD
1985, p. 14).
Hence, MENDEL’Sview of segregation occurring only
in the heterozygotes (i.e., with different alleles) could
easily be defended as
being completely consistent even
with the modernuse of the term.Moreover, this usage
provided MENDELwith the opportunity tosummarize
in the following way what he clearly regarded as his
main result:
O n the other hand, the context
of this passage clearly
refers tohybrids that produce variable progeny.
MENDEL never specifically addresses the question whether
segregation occurs in homozygous genotypes, and we
[Pea] hybrids form germinal and pollen cells that in their
have noway of knowing how he might have responded composition correspond in equal numbers to all the constant
forms resulting from the traits united through fertilization (p.
if queried directly about the matter. MENDEL’S seem29, emphasis his).
ing indifference to segregation in homozygotes was
entirely consistent with his primary interestin hybrids
This summary is the subject of further discussion
and the principle that the hereditary determinants
below.
emerge unchanged after their association together in
The P h a s e o h issue: In the Versuche MENDEL also
hybrids. As ERNST MAYR has noted, “The homozycommented on his results with flower color in Phaseolus multijlorus, in which the F2 generation from an
gotes, not being hybrids,simply did not interest him”
original cross of white X crimson consisted of a whole
(personal communicationto DLH, October
28, 199 1).
range of colors from purple to pale violet and white,
Also against OLBY’S
argument is MENDEL’Sformula
in theratio 30 coloredto 1 white, instead of the
expected 3: 1. He then wrote,
A / A A / a a / A a/a = A 2 Aa a
+
+
+
+
+
which clearly implies that the homozygous forms A
and a each contain two hereditary determinants. In
addition, MENDELwrites (p. 41),
. . . it seems permissible to assume that the germcellsof
those [plants] that remain constant are identical, and also
like the primordialcell of the hybrid.
T h e key question is whether the word identical (gleichartig) is intended to mean “identical in number” or
“identical in type.” We presume that MENDEL meant
identical in both senses, otherwise some qualification
would have been included, which would imply that
the germ cells from homozygotes must contain only
one of each of the paired determinants, since this is
the case in “the primordial cell of the hybrid.”
In a wider sense, whether or not segregation may
be said to occur in homozygous genotypes is largely a
matter of semantics. In the contemporary mind segregation is often confused with the process of chromosome separation during meiosis, but MENDELknew
nothing about chromosomes. Furthermore, in a precise technical sense, chromosomes undergo disjunction, not segregation. Segregation is a formal genetic
phenomenon in which alleles are separated from one
another and distributed into different germ
cells (RIEGER, MICHAELISand GREEN 1968; KING and STANS-
But thesepuzzlingphenomena,too,couldprobablybe
explained by the law valid for Pisum if one might assume
that in Ph. multzjlorus the colorofflowersandseeds
is
composed of two or more totally independent colors that
behave individually exactly like any other constant trait in
the plant. With blossom color A composed of independent
traits A I + An . . . , which produce the overall impression
of crimson coloration, then, through fertilization with the
differing trait of white color a, hybrid associations Ala +
Ana + . . . would have to be formed . . . (p. 35).
+
Hence, considering only A , and A2, the
. . . terms of the [FPprogeny] seriescan enter into 9 different
combinations, eachof which represents the designationfor
another color:
1 AI A2
2 Ala AP
1 AP a
2 AI Ana
4 A l a Ana
2 Ana
a
1 AI a
2 Ala
l a
a
a
The numbers preceding the individual combinations indicate how many plants of corresponding coloration belong
to the series (p36).
OLBY(1979) discusses this examplein some detail and
wonders
. . . why [Mendel] made no apology for putting both
A I and
AP with the same contrasted charactera . . . The chief rea-
Perspectives
25 1
son for this obscurity was. . .that Mendel was thinking in
terms of the white d o u r when he wrote down a . . . It may.
of course. be objectedthatthe way Mendelset out his
Phaseolw series [the matrix above] showsthat he was thinking of t w o gene loci for white, hencethegenotype
u o . . .[However, the layout of the table] makes clcar the
derivation of the c 1 a . w of offspring fromthe multiplication
of terms andno more ( 0 t . w 1979. p. 6Off).
As is clear from QLRY'S discussion, there is plenty of
room for disagreement about MENDEL'S intentions in
summarizing the Phaseolus situation in this manner.
Although MENDEL undoubtedly associated the symbol
u with the color white, he also clearly stated that the
two or more colors are 'totally independent," that
they "behave individually exactly like any other constant trait in the plant," and that 'hybrid associations
Ala
AM
. . ." would be formed by fertili7ation
with germ cells from a white plant. These comments,
and particularly the latter formula. clearly suggest
that he had independent factorsin mind, and that the
u written as a partner for AI is different from the a
written as a partner for An. MENDEL'S matrix and the
symbol a u makes perfect sense if he were using positional notation; otherwise, consistency would require
the white plants to be symbolized simply as a.
At the risk of seeming Whiggish, we will point out
herethat positional notation is commonly used in
modern Drosophila genetics. although the wild-type
alleles, rather than the mutant alleles. are designated
For example, to a
with the single symbol. namely
modern Drosophila geneticist, the symbol+ + +/y w f
clearly means the triple heterozygote for y. w , and/,
without any ambiguity, and w is the double recombinant chromosome with the wild-type alleles ofy
andf. We believe that it was this kind of positional
notation that MENDEL had in mind when he wrote a a
in his table of F? progeny in Phaseolus.
The Notizblatt argument:A good deal of discussion
hascentered on interpreting what appear to be genetic
symbols in MENDEL'S handwriting found on a document called the MENDEL Kotizblatt (Figure 1). which
hears theroughdraft
of a letter on the overleaf
containing material that dates the letter (though possibly not the putative genetic symbols) to about 1875,
o r some seven years after MENDEL became abbot and
two years after his last crossing experiments (RICHTER
1924). I t is not at all clear what the symbols in the
Notizblatt refer to: results from I,ina+ia, Phaseolus,
and Pisum have all been suggested (HEIMANS 1968).
T h e Pisum suggestion comes from HEIMANS( I 968)
and is based on the observation that the number of
individuals in the class Win the Notizblatt (166)agrees
with the total for white seed coat color in the trifactorial cross in the Vtrsuche (MENDEL, p. 21). T h e
agreement could be coincidental, since the totals d o
not agree (601 progeny recorded in the Notizblatt vs.
639 in the trifactorial cross). Whatever the case. the
+
I
U P
+
+.
+ +
L
FIGUREI .--The ~IEWEI.Sotirhlarr.containing apprcnt KC
netic s y ~ ~ h nin
l s \ f F . w m . ' s llandwri~ing.dl14IO a1n-1~11875.
Notizblatt suggests that MENDEL was playing with the
data.apparentlytrying
various wavs to groupthe
phenotypes into classes. and he flirts briefly with a
7:3:2 ratio in the middle of the page(Figure 1).
However, by the bottom of the page he has gotten to
the ratio'/4:'/R:'/lfi:'/?:'/Ifi,
which isa perfectly respectable Mendelian ratio found among the 147 theoretically possible ratios in the F? progeny of a dihybrid
cross (HARI'L and MARUYAMA1968).
I t should also be noted that 7:2:3 is quite close to
9:3:4, which is one of the conventional examples of a
modified 9:3:3: 1. Nevertheless, much has been made
of the 7:2:3 ratio. and OLRY
(1979, p. 62) develops a
model of what MENDEL might have been thinking
when he wrote the markings and explains the model
in a paragraph that OLRYimagines MENDEL might
have written werethis the model that 1MmmLactu;dIv
had in mind. However, MEIJER( 1 983) points out that
OLRY'S
model does not really explain the i:2:3 ratio
anyway. These disagreements only serve to highlight
the main problem with the Sotizblatt: it is an unpuh
lished page of markings pertaining to unknown traits
in an unidentified organism witten at an undetcrmined time and apparently of little importance to
MENDEI. himself since he used the page as scrap for
drafting a letterabout monastery business. (;iven
these uncertainties, the Notizblatt is hardly the sort
of thing to rely on in making inferences about MENDEL'S understanding of his work in Pisum.
The issue of constant hybrids: In his argument
252
V.Orel
and D. L. Hart1
that MENDEL was an opponent of the fundamental
principle of descent with modification, CALLENDER
(1988) also puts great emphasis on MENDEL’Sdiscussion of “constant” hybridsas evidence that he did not
regard his findings as generally applicable to other
plant species. Indeed, in what CALLENDER
(1988, p.
56) regards as MENDEL’S“most important reference
to constant hybrids,” MENDELwrote:
We encounter an essentialdifference in those hybrids that
remain constant in their progeny and propagate like pure
highly fertile
strains. Accordingto Gartner these include the
hybrids Aquilegia atropurpurea-canadensis, Lauatera pseudolbia-thuringiaca, Geum urbano-riuale, and some Dianthus hybrids . . . The correctness of these observations is vouched
for by the eminent observers and cannot
be doubted (p.41).
However, later in the text of the Versuche, MENDEL
also wrote:
Finally, the experiments performedby Kolreuter, Gartner,
and others on transformation of onespecies into another by
artijicial fertilization deserves special mention . . . Gartner
himselfhas carried out 30 experiments ofthiskind with
plants from genera Aquilegia, Dianthus, Geum, Lauatera,
Lychnis, Malua, Nicotiana, and Oenothera . . . If one may
assume that the development of forms proceeded in these
experiments in a manner similar to that forPisum, then the
entire process of transformation w‘ould havea rather simple
explanation (p.44).
It is importantto emphasize that, in this passage,
MENDELpointedly and specifically includes all of the
genera referred to earlier as containing constant hybrids as amongthose genera in whichone may assume
that the laws for Pisum hold.
While MENDELmay have been uncertain about the
universal applicability of the laws for Pisum, particularly with regard to species with constant hybrids, he
had already convinced himself that they were probably valid at least for all variable hybrids, since he
asserted:
Whethervariablehybridsof
other plantspeciesshow
complete agreementin behavior also remains to be decided
experimentally; one might assume, however, that no basic
difference couldexist in important matters sinceunity in the
plan of development of organic life is beyond doubt (p. 43,
emphasis his).
What did MENDELthink he discovered?Whatever
MENDELthought he had discovered, hewas certainly
under the impression that it was important. In his
letter of April 18, 1867, to NACELIhe says,
I knew that the results I obtained werenot easily compatible
with our contemporaryscientific knowledge, andthat under
the circumstances publication of one such isolated experiment was doubly dangerous; dangerous for the experimenter and for the cause he represented (p.60).
In a paper containingas much dataand interpretation
as the Versuche, how should one decide what the
author regardedas the main message? In our opinion,
the key is found in the fact thatthe Versuche was
written for oral presentation, and in an oral presentation the principal technique for emphasis is repetition. Thus, in order todiscern what MENDELregarded
as the main message, one might examine the Versuche
in order to identify any particular ideas or passages
that are repeated. We have examined the document
with this goal in mind, and have identified one idea
that is expressed repeatedly in almost identical language. One version of the idea is as follows:
. . . pea hybrids form germinal and pollen cells that in their
compositioncorrespond in equalnumbers to all the constant
forms resulting from the combination of traits united through
fertilization (p. 29, emphasis MENDEL’S).
This statement, or close variants of it, is repeated no
fewer than six times in the Versuche, on pages 24, 29,
32, twice on page 43, and 44. Furthermore, since
MENDEL refersto this formulation as the “law of
combination of differingtraitsaccordingto
which
hybrid development proceeds” (p. 32), we may infer
that he considered this statement as summarizing the
“generally applicable law of the formation and development of hybrids” (p.2),the elucidation of which he
clearly regarded as his main goal of his experiments.
That MENDELregarded this law as widely applicable
is supported by his statement that it
. . remains more than probable that a factor that so far has
received little attention is involved in the variability of
cultivated plants. . . [Our]cultivated plants,with few exceptions, are members of difjferenthybridseries whose developis altered and retarded
by frequent
ment along regular lines
intraspecific crosses (p. 37ff, emphasis his).
T h e context of this paragraph clearly points to segregation as “the factor that so far has received little
attention.”
MENDEL’Sown one-sentence summary of what he
thought he discovered is remarkably general and concise. It containsboth the lawsof segregation and
independent assortment in a form that is generally
applicable to hybrids of any numberofunlinked
genes. Even amodern
geneticist would be hard
pressed to drafta sentenceof comparable clarity without using terminology unavailable to MENDEL.
Conclusions: Our review of the issues and evidence
regarding MENDEL as a Mendelian leads us to the
following conclusions.
MENDELshould certainly be regarded as a hybridist. He was in the hybridist tradition,and this is
apparently how heregarded himself. However, his
interest in hybridization didnot blind him tothe
recognition of heredity as a process distinct from
evolution. His outlook was conditioned by being in
Brno,where there was aparticularinterest
in the
results of artificial fertilization for the improvement
253
Perspectives
of sheep, fruit trees, and vines, in which colleagues of
MENDEL,such as ABBOTNAPP,were leading participants, and conditioned also by MENDEL’Sown admitted interest in practical breeding as well as the colorMENation of ornamental flowers. On the other hand,
DEL’S hybridist tradition did result inhis failure to
emphasize the genetic composition of the constant
(homozygous)forms, although the symbolism he used
in the Versuche
A/A
+ A / a + a / A + a/a = A + 2 Aa + a
implies to us that he was quite aware that theconstant
forms must contain paired hereditary determinants.
Nowhere did MENDELstipulate that thehereditary
determinants are particulate, as opposed to fluid, vapor, colloid, gel, plasma, or whatever. He did, however, assume that they are material entities, rather
than metaphysical “potentials,” and asserted that the
distinguishing traits of plants are caused only by differences in the composition and grouping of these
hereditary elements.
MENDELclearly considered segregation as a phenomenon that characterizes the hybrids. On the other
hand, the issue whether segregation of genes, as abstract entities, canbesaid to occur inhomozygous
genotypes is largely a matter of semanticsand can be
debated even today. For MENDEL,the clear cut segregation in heterozygotes carried the critical implication that the hereditary elements remain unchanged
by their association together in heterozygous genotypes.
The laws of inheritance for Pisum were regarded
by MENDELas quite generally applicable.He proposed
explicitly that they would applyto all forms of variable
(segregating) hybrids, and he also suggestedthat they
might very well explain why genera that include constant (nonsegregating) hybridscan be transformed
into new forms by meansof artificial fertilization.
According to MENDEL’S ownwords,hisstudiesof
Hieracium and otherspecies wereundertaken in order
to “verify, with other plants, the result obtained with
Pisum” (MENDEL’Sletter to NAGELI
of April 18, 1867,
p. 60).
The key point of MENDEL’Sdiscovery is contained
in a sentence that he repeated with minor variations
no less than six times in the Versuche:
. . . pea hybrids form germinal and pollen cells that in their
compositioncorrespond in equalnumbers to all the constant
forms resulting from the combination of traits united through
fertilization (p. 29, emphasis his).
This is the kind of “law of the formation and development ofhybrids” (p. 2) that he hadset out to
discover, it is a remarkably concise statement of what
he actually did discover, and it certainly represents
the distillation of what he thought he discovered. We
conclude that MENDEL understood very clearly what
his experiments meant for heredity. He deserves, not
only the eponymous credit for Mendelism, but also
the historical credit and a considerable measure of
respect and admiration for his remarkable insights.
We are very grateful to ERNSTMAYRfor his advice, encouragement, and numerous helpful letters throughout the course of this
study, and to MEL GREEN,ELENALOZOVSKAYA,
and DANKRANE
for their comments and suggestions.
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