Evolutionary Anthropology 17:250À253 (2008)
Evolutionary Anthropology 17:250À253 (2008)
The Construction of Mendel’s Laws
While ‘‘Mendel’s Laws’’ are generally taught as natural facts, they are actually Mendel’s honor, conspicuously
pedagogical constructions, which originated in a series of lectures at Princeton avoided the term ‘‘law’’ in his 1909
in 1916. What accounts for their popularity? Mendel’s Principles of Heredity (and
no longer a ‘‘defence’’ against W. F.
R. Weldon). Indeed, Bateson retained
the explicit denial that (as Weldon
Any modern textbook of genetics, or Mendel’s work, although he drew the had said) Mendel enunciated a ‘‘Law
even of general biology or biological familiar conclusions from the fa- of Dominance’’ (p. 13). Bateson in
anthropology, presents the funda- mous data. But at the dawn of the fact took pains to contrast, once
mentals of heredity in a stereotypical Mendelian era 35 years later, William again, Galton’s Law of Ancestral He-
fashion, beginning with the two laws Bateson2 began his ‘‘Defence’’ of redity against the proper Mendelian
of heredity derived from the work on ‘‘Mendel’s principles of heredity’’ ‘‘scheme,’’ ‘‘principles,’’ ‘‘phenomena,’’
peas by the monk of Brunn, Gregor
¨ with ‘‘the rediscovery and conﬁrma- ‘‘methods,’’ ‘‘analysis,’’ and ‘‘facts.’’
Johann Mendel. These two laws are tion of Mendel’s Law. . .’’(p. 104). Whatever he considered ‘‘Mendel’s
‘‘segregation’’ and ‘‘independent assort- Note the odd-sounding singular in law’’ actually to be in 1911, the Har-
ment,’’ to wit: place of the now-familiar plural. vard geneticist William E. Castle8
These observations of discrete inheri-
Bateson described this law quite condensed it into ‘‘three principles:
tance and the segregation of alleles are simply, stating ‘‘that there are three (1) the existence of unit characters;
collectively known as ‘‘Mendel’s ﬁrst dominants to one recessive’’ in a (2) dominance, in cases where the
law’’ or the ‘‘Law of Segregation’’. . .. mating of two hybrids (p. 10). What parents differ in a unit character;
This phenomenon, known as ‘‘Mendel’s
Bateson proposed is actually an and (3) segregation of the units con-
second law’’ or the ‘‘Law of independent
assortment,’’ means that the alleles of explicit alternative to Galton’s Law of tributed by the respective parents,
different genes get shufﬂed between Ancestral Heredity, which divided a this segregation being found among
parents to form offspring with many person’s heredity equally among the the gametes formed by the offspring’’
different combinations.1 lineal ancestors in any generation. (p. 37). This is repeated in Castle’s9
This formalization into two general Punnett’s3 1905 textbook Mendel- 1916 textbook, in which he also
laws about segregation and inde- ism self-interestedly declared, ‘‘As referred to Hugo de Vries’ ‘‘law of the
pendent assortment is not explicit in our knowledge of heredity clears, splitting of hybrids’’ and observed that
and the mists of superstition are dis- ‘‘the same law had been discovered
pelled, there grows upon us with and clearly stated many years previ-
ever-increasing and relentless force ously by . . . Gregor Mendel, and we
the conviction that the creature is have now come to call this law by his
Jonathan Marks is Professor of Anthro- not made but born’’ (p. 54). But even name, Mendel’s law’’ (p. 82).
pology at the University of North this early expression of what has By now, the modern Mendelian
Carolina at Charlotte. He is the author
of Human Biodiversity (1995), What It come to be known as geno-hype4 lexicon was nearly fully established,
Means to Be 98% Chimpanzee (2002), didn’t mention Mendel’s Two Laws. with Bateson’s ‘‘homozygous,’’ ‘‘heter-
and Why I Am Not a Scientist: Anthro- Neither did other genetics textbooks ozygous,’’ and ‘‘allelomorphs’’ along-
pology and Modern Knowledge (forth-
coming in 2009, from the University of of that ﬁrst Mendelian generation.5,6 side Wilhelm Johannsen’s ‘‘pheno-
California Press). Rather, they all spoke vaguely about type,’’ ‘‘genotype,’’ and ‘‘gene.’’ And
Department of Anthropology, University
of North Carolina at Charlotte, Charlotte,
‘‘Mendel’s Law’’: nothing more than yet, there still were no laws of segre-
NC 28223. E-mail: [email protected] general recognition of the segrega- gation and independent assortment.
tion of unit characters into the germ That formalization would only
cells and their subsequent statistical arise with the research program of
Key words: genetics; history
union with other germ cells, produc- the geneticist who was most inter-
ing offspring in predictable genetic ested in the transmission of genes on
V 2008 Wiley-Liss, Inc.
C proportions. different chromosomes, distinguish-
DOI 10.1002/evan.20192 William Bateson,7 who by now had ing pairs of genes that assort inde-
Published online in Wiley InterScience
(www.interscience.wiley.com). named his youngest son Gregory in pendently of one another from those
ISSUES Construction of Mendel’s Laws 251
that are on the same chromosome, research program, which was focused delian Law/s in three ways. Some
or ‘‘linked.’’ That was the Columbia on the relationship between genes presented it as it had always been
University geneticist Thomas Hunt and chromosomes. Further, at the presented, as a singular ‘‘Law.’’ Thus,
Morgan. He invented Mendel’s Laws time, Morgan’s Columbia group was the 1932 fourth edition of Castle’s13
in 1916. involved in a dispute with William E. Genetics and Eugenics retained the
In 1915, Morgan and his students10 Castle’s Harvard group over whether structure of its earlier editions, dat-
published The Mechanism of Mende- genes always passed across genera- ing to 1916, the year Morgan ﬁrst
lian Heredity, in which they presented tions intact or could be contaminated presented Mendel’s two laws. To
a synthesis of their work, unifying the by the presence of other genes. In that Castle, there was still just ‘‘Mendel’s
genetic research of cytologists and context, the Law of Independent Law,’’ including ‘‘three principles’’ in
breeders. The Mendelian unit charac- Assortment might also have had the the same words as quoted earlier,
ters of the breeders existed as discrete rhetorical effect of de-legitimizing the from the ﬁrst edition of his text.
points on the chromosomes of the contamination argument. The Morgan And while the principle we now call
cytologists. The genes could travel ‘‘independent assortment’’ is discussed,
into the next generation separately, as Castle does not use the phrase.
Mendel had described, if they lay on Indeed, genetics textbooks into the
different chromosomes; or together, if
In the second lecture, 1940s could readily discuss the phe-
they lay on the same chromosome, from a presentation nomenon of independent assortment
although they might still be transmit- without acknowledging its status as
ted separately due to crossing-over,
given on 1 March, 1916, Mendel’s Second Law.14–16
the group’s own crucial discovery. Morgan tells us ‘‘what Others wrote of Mendel’s (sin-
Indeed, this work introduced the fa- Mendel’s law did for gular) Law or (multiple) principles,
mous metaphor of ‘‘chromosomes . . . but conceded that there were indeed
as a linear series of beads’’ (p. 131– heredity’’ and then sets some who presented them in the
132). out for the first time ‘‘his form of two laws. Herbert Walter17
Mendel’s Law, however, is pre- in 1922 continued to write of Men-
sented in the singular. The book first lawÀÀthe law of del’s Law: ‘‘The essential feature of
opens forthrightly: ‘‘Mendel’s law segregation’’.11 A few Mendel’s law is brieﬂy this: hereditary
was announced in 1865. Its funda- characters are usually independent
mental principle is very simple. The pages later, Morgan units which segregate out upon cross-
units contributed by two parents sep- writes, ‘‘The second law ing, regardless of temporary domi-
arate in the germ cells of the off- nance’’ (p. 100), but went on to men-
spring without having had any inﬂu-
of Mendel may be tion, ‘‘This law of segregation, or ‘in-
ence on each other.’’ (p. 1). The pref- called the law of dependent assortment’ as Morgan
ace mentions ‘‘the Mendelian laws’’ prefers to call it. . ..’’ (p. 103). In this
but with no greater subsequent spec-
independent assortment passage, he thus manages to conﬂate
iﬁcity. And the phrase ‘‘independent of different character the two ‘‘laws’’ of Mendel/Morgan in
assortment’’ makes its ﬁrst pedagogi- pairs.’’ precisely the same fashion that
cal appearance on p. 26. would continue to vex introductory
Morgan’s next book, however, pre- biology students for decades to
sented a heuristic change (one is come.
tempted to say ‘‘mutation’’) for Men- group’s explanation for Castle’s data As late as 1939, two former pro-
delian genetics. Based on a series of on coat coloration in hooded rats was teges of Morgan, Sturtevant and Bea-
lectures he gave at Princeton in early in terms of independently assorting dle,18 discussed independent assort-
1916, Morgan11 published A Critique modiﬁer genes, rather than contamina- ment with a parenthetical phrase,
of the Theory of Evolution later that tion of a single gene, a point of sufﬁ- ‘‘. . . (sometimes known as ‘Mendel’s
year. In the second lecture, from a cient gravity to constitute the closing second law,’ segregation being con-
presentation given on 1 March, 1916, argument of the book. In Morgan’s12 sidered the ‘ﬁrst law’). . ..’’ (p. 52).
Morgan tells us ‘‘what Mendel’s law The Physical Basis of Heredity (1919), Still others, however, adopted a
did for heredity’’ and then sets out he devoted separate chapters to third pedagogical strategy: present-
for the ﬁrst time ‘‘his ﬁrst law—the ‘‘Mendel’s First Law—Segregation of ing it more or less Morgan’s way.
law of segregation’’.11 A few pages the Genes’’ and ‘‘Mendel’s Second The enormously successful Principles
later, Morgan writes, ‘‘The second Law—The Independent Assortment of of Genetics by Sinnott and Dunn19
law of Mendel may be called the law the Genes.’’ went through four editions in a quar-
of independent assortment of differ- ter of a century, the last, in 1950,
ent character pairs.’’ (p. 53). THE ‘‘DOMINANCE’’ OF with Theodosius Dobzhansky as
It need hardly be mentioned that third author. In the ﬁrst (1925) edi-
this pedagogical separation and enun-
MORGAN’S APPROACH tion, they explicitly presented stu-
ciation of a ‘‘second law’’ implicitly The next generation of trade books dents with the statement that
privileged Morgan’s own work and and textbooks dealt with the Men- ‘‘Mendel’s Laws of Inheritance . . .
252 Marks ISSUES
include several distinct principles’’
(p. 40). Those principles included
dominance and unit characters, but
also a ‘‘Principle of Segregation’’ in
one chapter, called ‘‘Mendel’s Laws
of Inheritance. I’’ and a ‘‘Principle
of Independent Assortment’’ in the
next chapter, called ‘‘Mendel’s Laws
of Inheritance. II.’’
For their 1932 second edition,
which notably involved jettisoning
the entire chapter that had formerly
extolled eugenics,20 Sinnott and
Dunn rewrote the particular sentence
introducing the laws, to agree more
fully with Morgan: ‘‘Mendel’s Laws
of Inheritance . . . include two major
principles of Segregation and Inde-
pendent Assortment, together with a
number of less fundamental general- Figure 1. A ‘‘memic’’ depiction of the origin and spread of Mendel’s Laws. Mendel’s work
izations’’21 (p. 41). That sentence was published in 1866, but did not affect the field of heredity. The work was rediscovered
in 1900 and precipitated the wide-scale presentation of Mendel’s Law. Thomas Hunt
remained unchanged in subsequent
Morgan divided and named the Two Laws in 1916, and won the Nobel Prize in 1933. By
editions. Dunn had wanted to do mid-century, all textbooks on the subject were presenting Mendel’s results in Morgan’s
his doctorate with Morgan in 1914, fashion.
but Morgan’s facilities were over-
crowded, so Dunn did his work at
Harvard and eventually replaced
at Cal Tech, also now presented awarded the Nobel Prize in 1933,
Morgan at Columbia.22
Mendel’s laws Morgan’s way. which greatly increased his stature
Similarly, Shull’s23 1926 textbook
and may well have given his heuris-
Heredity referred to ‘‘Mendel’s law,’’
CONCLUSION tic presentations of Mendel greater
stating that its ‘‘essential features . . .
weight. Another factor might have
are (1) the segregation of the genes While I generally do not feel as been the perceived value for genetics
. . ., and (2) the assortment of the though ‘‘memes’’ have much value in to formalize its generalizations into
genes of different pairs into the germ understanding or analyzing cultural laws, in accordance with the norma-
cells wholly independently of one evolution, there may be some value in tive contemporary philosophy of sci-
another’’ (p. 134). He noted ambiva- depicting the origin and spread (i.e., ence. Perhaps, then, ‘‘Mendel’s Two
lence about including other features the history) of Mendel’s Laws ‘‘memi- Laws’’ sounded a bit more ‘‘sciencey’’
of the Mendelian corpus as ‘‘diag- cally’’ (Fig. 1). Mendel’s Two Laws of
than do Mendel’s Principles, Men-
nostic of Mendelian heredity.’’(p. Segregation and Independent Assort-
del’s Generalizations, or even Men-
134). In the second edition, however, ment were invented in 1916 and dif-
not only did the indexed citations of del’s (vague) Law.
fused gradually across the ﬁeld of
And third, perhaps the operation
Morgan grow from one to nine, but genetics over the next two decades, so
Shull24 now presented Morgan’s laws of Planck’s Principle.27 A new gener-
that by mid-century they constituted
without ambiguity or accurate attri- ation of geneticists was writing the
the normative, ofﬁcial version of the
bution: ‘‘Mendel’s laws, as derived science of heredity. With the timeline textbooks. When Morgan ﬁrst coined
from his own experiments, involved, imposed, one can identify the formerly ‘‘the two laws’’ in 1916, L. C. Dunn
(1) segregation of the genes of the invisible question: What factors might was 21 years old and a ﬁrst-year
same pair; and (2) independent account for the success, in the latter graduate student; C. H. Waddington
assortment of genes of different half of the twentieth century, of Men- was but 11 years old. Morgan’s con-
pairs’’ (p. 206). The phrase ‘‘Mendel’s del’s Two Laws of Segregation and In- temporaries from the dawn of Men-
law’’ remained only in two end-of- dependent Assortment, in spite of hav- delism in 1900 were thinning out
chapter problems for students. ing been formulated as such neither by and a new generation had been
Colin’s25 Elements of Genetics Mendel nor by the ﬁrst generation of weaned on Morgan’s prestige and
(1941) presented Mendel’s work ex- Mendelians? research program. Moreover, Mor-
plicitly in Morgan’s fashion, as two Obviously, it is difﬁcult to demon- gan was, with the exception of Wil-
named laws. Across the Atlantic, an strate an answer with any degree of liam Bateson, who died in 1926, the
inﬂuential 1938 textbook by Wad- rigor, but it seems possible that three least tainted of his generation by the
dington,26 who had spent some time social factors were acting in parallel. eugenics movement. The wholesale
visiting Morgan’s fruitﬂy laboratory First, Thomas Hunt Morgan was embrace of that movement by the
ISSUES Construction of Mendel’s Laws 253
Nazis cast the entire ﬁeld of genetics 3 Punnett R. 1905. Mendelism. Cambridge: 17 Walter HE. 1922. Genetics: an introduction
Bowes and Bowes. to the study of heredity, 2nd ed. New York:
in an ugly light. Macmillan.
4 Holtzman NA. 1999. Are genetic tests
In short, Morgan was an excellent adequately regulated? Science 286:409. 18 Sturtevant A, Beadle G. 1939. An introduc-
role model. His Mendelian heuristic 5 Herbert S. 1910. The ﬁrst principles of hered- tion to genetics. New York: WB Saunders.
might have been unfamiliar either ity. London: A. and C. Black. 19 Sinnott E, Dunn L. 1925. Principles of
6 Wilson J. 1916. A manual of mendelism. Lon- genetics. New York: McGraw-Hill.
to Mendel himself or to the ﬁrst wave
don: A. and C. Black. 20 Marks J. 1993. Historiography of eugenics.
of his followers, but it was, and Am J Hum Genet 52:650–652.
7 Bateson W. 1909. Mendel’s principles of he-
remains, a good way to present things redity. Cambridge: Cambridge University Press. 21 Sinnott E., Dunn L. 1932. Principles of
and consequently has been universally genetics, 2nd ed. New York: McGraw-Hill.
8 Castle WE. 1911. Heredity in relation to evo-
memorized by students of genetics lution and animal breeding. New York: Appleton. 22 Allen GE. 1978. Thomas Hunt Morgan: the
man and his science. Princeton: Princeton
since at least mid-century. Mendel’s 9 Castle WE. 1916. Genetics and eugenics.
Cambridge: Harvard University Press.
Two Laws, however, are not so much 23 Shull AF. 1926. Heredity. New York:
10 Morgan TH, Sturtevant A, Muller H, Bridges
facts of nature as facts of nature/cul- McGraw-Hill.
C. 1915. The mechanism of Mendelian heredity.
ture.28 New York: Henry Holt. 24 Shull AF. 1938. Heredity, 2nd ed. New York:
11 Morgan TH. 1916. A critique of the theory
25 Colin E. 1941. Elements of genetics: Men-
ACKNOWLEDGMENTS of evolution. Princeton: Princeton University
del’s laws of heredity with special application to
man. Philadelphia: Blackiston.
This essay beneﬁted greatly from 12 Morgan TH. 1919. The physical basis of he-
26 Waddington CH. 1939. An introduction to
comments by Elof Carlson. redity. Philadelphia: JB Lippincott.
modern genetics. London: George Allen & Unwin.
13 Castle W. 1932. Genetics and eugenics, 4th
27 Hull D, Tessner P, Diamond A. 1978.
ed. Cambridge: Harvard University Press. Planck’s principle. Science 202:717–723.
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