Development: Mendel's Legacy to Genetics

IN 1900, Gregor MENDEL's (1866) paper was rediscovered and restored to the scientific community. His work was discussed, criticized, and tested, and within the decade the new discipline of genetics had been launched, on the basis of the principles embodied in that paper. William Castle clearly expressed the views held in common by the early Mendelians when he wrote: "What will doubtless rank as one of the great discoveries in biology, and in the study of heredity perhaps the greatest, was made by Gregor Mendel, an Austrian monk, in the garden of his cloister, some forty years ago" (CASTLE 1903 , p. 535).

In the year 2000, Mendel's paper is still being discussed and criticized, interpreted and reinterpreted, but the harmony that had prevailed in 1900 is no longer in evidence. In the year 2000, Mendel's position as founder of genetics is being seriously challenged. (For a comprehensive discussion of these revisionist views, see HARTL and OREL 1992 .) The traditional story, so long cherished by students of genetics, has been relegated to the status of myth: "... 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 problems" (BOWLER 1989 , p. 103).

Since 1965, this wave of revisionist studies has variously represented Mendel as a Darwinian whose experiments were designed primarily to support Darwinian evolution (DE BEER 1964 ), as a hybridist "whose overriding concern 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" (OLBY 1979 , p. 67), or as a student investigating the properties and behavior of the hybrid per se: "The real objective of the work was the creation of a mathematically precise science of hybridization modelled upon the physical sciences" (MONAGHAN and CORCOS 1990 , p. 289). Although these interpretations differ from each other, they agree on one point—Mendel's principal interest was not the study of heredity.

	Entwicklung and development

TOP Entwicklung and development Development--a world view Mendel on development LITERATURE CITED

When I began to study Mendel's paper, I had no grandiose intention of flying to the defense of Mendel's reputation. Nor was I feeling especially resentful of the revisionist views. After all, the subject of Mendel is still sufficiently rich to accommodate all kinds of new and differing positions. However, instead of rehashing the material from the translations of Mendel's already heavily plundered work, I decided to return to the original text of Mendel's Versuche über Pflanzen-Hybriden. Here I discovered a new Mendel—a Mendel who did not fit any of the revisionist pictures, but neither was he quite the hero of the traditional account. This was a Mendel who had stepped from the pages of another story altogether—a different Mendel, who spoke of a different theme. The theme was Entwicklung, which translates into English as development.

The term Entwicklung appears in the first paragraph of the first page of Mendel's paper and emerges and re-emerges throughout the remaining forty-odd pages with persistent regularity. It occurs as an independent noun (always capitalized, always imposing) and frequently as an element in a compound word—e.g., die Entwicklungsreihe, developmental series; die Entwicklungsgeschichte, the history of development; das Entwicklungs-Gesetz, the law of development.

In whatever form it occurs, however, the cumulative effect of repeated exposure to Entwicklung, in simple or compound form, is undeniable. Entwicklung is recognizably the single most important element in Mendel's thoughts. Its English equivalent does not fare as well. As I recall from my own experience, as often as I had read Mendel in translation, the word development remained an unremarkable term. It was noticed and then dismissed, with little to recommend it as something significant. (Incidentally, there are two translations of Mendel's paper. The first translation appeared in 1902 in William Bateson's Mendel's Principles of Heredity: A Defence; the second was published in 1966 in STERN and SHERWOOD's The Origin of Genetics—A Mendel Source Book.)

Perhaps this reaction to development is prompted in part by the way it is treated in the translations. For example, there are many instances in which Entwicklung appears in the German text, but development is omitted from the translation. It is just possible that the frequency of repetition of the term is needed to make an impression on the reader. Also, in the Stern translation, the phrase the developmental series is introduced just once. The reader is then notified in a footnote (p. 2) that thereafter the phrase is replaced by the word series, even though Mendel only and always uses the phrase the developmental series. Obviously, in the judgment of the translator, development has become an expendable modifier. And in the Bateson translation there is a seemingly minor error, but its repercussions are major. Instead of stating that the purpose of Mendel's experiments was to follow the development of the hybrids in their progeny, the translation reads: "The object was to follow up developments of the hybrids in their progeny" (p. 40). The simple addition of an "s" transforms Mendel's technical term development into a nonscientific word that, in the vernacular, can be interpreted as an "event," an "occurrence," or a "happening," thereby diverting attention from Mendel's original focus on development as a biological process.

There might also have been a far more powerful, silent factor at work. Development may have become the victim of change—the change in meaning that so often occurs in the life of a word. Development in nineteenth century biology embraced both heredity and embryological change. By the time Mendel's paper had been rediscovered and translated, development had taken on a very specialized significance in biology. It referred almost exclusively to ontogenetic change. But Mendel does not discuss ontogenetic change. Thus an unusual situation arises. The word development is unaccompanied by the appropriate and expected underlying process. In these circumstances, development could easily be glossed over, and since it now lacked biological significance, it would be dismissed as irrelevant and unimportant.

Whatever the reason for its neglect, there is no denying that the twentieth century audience has been unresponsive to Mendel's development. How else can one explain the following anomaly? Mendel is described as expressing an interest in evolution, yet the term evolution occurs only twice in his paper. It is suggested that Mendel is really interested in examining the process of hybridization, but the term hybridization appears only once. And then, of course, there is the traditional story—that Mendel's interest resides in the study of heredity; however, the word heredity does not appear at all. On the other hand, although development occurs on practically every page of the paper, no one has ventured to claim that Mendel was motivated by an interest in development.

So if Mendel is not discussing embryological development, but he regularly refers to development, what is he talking about? What is Mendelian development?

	Development—a world view

TOP Entwicklung and development Development--a world view Mendel on development LITERATURE CITED

That Mendel would be interested in development is neither implausible nor idiosyncratic. The concept of development, which had been relatively quiescent in the seventeenth and eighteenth centuries, had a dramatic renaissance in the nineteenth century. In place of the static view of the world that had prevailed, one was now encouraged to think "developmentally"—to see things in terms of their origins and the orderly, irreversible changes that followed. All things were governed by development, a process of unfolding, whether they were animate or inanimate, natural or humanmade. The questions that underlay nineteenth century investigation (scientific and otherwise) was: "How have things come to be what they are? What is their history?" (MERZ 1904 , p. 280).

One of the earliest applications of the principle of development was LaPlace's Nebular Hypothesis of 1796 (BRUSH 1987 , p. 250)—a bold attempt to explain the origin and subsequent unfolding of the solar system by natural means. His approach "powerfully influenced the minds of many students of nature in the direction of a genetic view of phenomena" (MERZ 1904 , p. 285). In 1825, K. E. von Baer published Uber Entwickelungsgeschichte der Thiere (OSPOVAT 1976 )—a history of the development of animal life, describing the changes from the simple homogeneous germ to the specialized, complex heterogeneous form. It was the Law of Development at work. And what was undeniably the grandest attempt to unify all phenomena under the principle of development came from the pen of Herbert SPENCER 1904 :

Abundant proof has been given that the law of organic development, formulated by von Baer, is the law of all development. The advance from the simple to the complex, through a process of successive differentiations, is seen alike in the earliest changes of the Universe to which we can reason our way back and in the earliest changes that we can inductively establish; it is seen in the geologic and climatic evolution of the Earth; it is seen in the unfolding of every single organism on its surface and in the multiplication of kinds of organisms; it is seen in the evolution of Humanity, whether contemplated in the civilized individual or in the aggregate of races; it is seen in the evolution of Society in respect alike of its political, its religious, and its economical organization; and it is seen in the evolution of all those endless concrete and abstract products of human activity that constitute the environment of our daily life.

This, then, is the panoramic view of development. It is likely that anyone preparing to investigate any area of the sciences or philosophy, society, language, or religion would have approached his or her subject developmentally. It is not too difficult to accept that Mendel would also have been so influenced. However, it is possible to locate Mendel even more precisely within the framework of development.

First, recall that Mendel was not only a breeder and hybridist, he was a botanist; and as a botanist he would have been familiar with the textbook written by the leading botanist of the period, M. J. Schleiden (see LORCH 1969 , p. xxv). Schleiden had galvanized botany with his "hands-on, how-to" book, The Principles of Scientific Botany (1849)—and indeed, a well-thumbed copy of the third edition of The Principles was found in Mendel's private collection (ILTIS 1932 , p. 104). Its influence was widespread; its message clear and incisive: "the one and only way to achieve scientific insight, and consequently the sole and indispensable methodological procedure which derives from the very nature of the subject, is the study of development." Now, in urging the study of development, Schleiden was referring not only to the cells of the developing embryo, but also to any changes in the adult body. Unlike the French and English usage, which did restrict development to its narrow scope, a focus on the embryonic cells, the German Entwicklung embraced the study of change in the structure, growth, and division of embryonic and adult cells (MERZ 1904 , p. 197). Schleiden enjoined botanists to study the ordinary cells that complete their cycle of development on their own and those special cells, the germ cells, whose further development was suspended until their union in fertilization. Now, into this swirl of developmental activity came Mendel, who combined an interest in development and an interest in the germ cells. He had a decidedly unique view of development and a decidedly bold view of the role of the germ cell in that development.

	Mendel on development

TOP Entwicklung and development Development--a world view Mendel on development LITERATURE CITED

[All quotations from Mendel, in this section, are taken from the STERN and SHERWOOD 1966 translation.] Mendel introduces development to us in the very first paragraph of the first page of his paper. He informs us that he had previously carried out hybridization experiments designed to produce new color variants in ornamental plants. The crosses had revealed a striking fact: the offspring of the hybrid forms regularly produced the same classes of progeny. It was this regularity that prompted him to undertake this series of experiments "to follow the development of the hybrids in their progeny" (p. 1). Mendel has just provided us with the first clue—development is an intergenerational process. It is begun in the hybrid parent but achieves completion in the progeny.

Mendel then informs us that the hybridization experiments undertaken by others have not produced the information needed to deduce "a generally applicable law of the formation and development of hybrids" (p. 2). His first set of experiments is intended to do precisely that—to give Mendel the data necessary to formulate a law of development. His classic artificial fertilization crosses between varieties bearing contrasting characters (yellow vs. green albumen, tall vs. short stems, etc.) are carried out, and the resulting monohybrid plants are allowed to self-fertilize. Each hybrid gives the same results: "Of the seeds formed by the hybrids with one pair of differing traits, one half again develop the hybrid form, while the other half yield plants that remain constant and receive the dominating and the recessive character in equal shares" (p. 15). Mendel refers to this descriptive statement as the Law of Development. By denoting the dominant constant trait by A, the recessive trait by a, and the hybrid trait by Aa, Mendel gives form to the Law of Development. It is A + 2Aa + a, and he calls this expression the developmental series (p. 16).

These are the three facets of development that become the subject of Mendel's inquiry: the development of the hybrid, the Law of Development, and the developmental series. And now Mendel makes a thoroughly unexpected statement: he will undertake "further experiments whose outcome would throw light on the composition of the seed and pollen cells in the hybrid" (p. 23). But why bring in the germ cells? What is the connection between the composition of the germ cells and development? Mendel tells us directly that the composition of the germ cells provides "an adequate explanation for the difference of forms among the progeny of hybrids as well as the ratios in which they are observed" (p. 29); i.e., the composition of the germ cells explains the production of the development series.

But how does one go about determining the composition of the germ cells at a time when cytologists had barely begun venturing into the nucleus? The task seems daunting at first, if not impossible. Mendel, however, succeeds in what can be described only as a brilliant tour de force. He makes use of one important clue:

... in Pisum constant forms appear among the progeny of hybrids and that they do so in all combinations of the associated traits. In our experience we find everywhere confirmation that constant progeny can be formed only when germinal cells and fertilizing pollen are alike (p. 24).

Thus, if constant progeny can be formed only when the egg and pollen cells are identical, then the composition of the egg and pollen cells can be determined by merely observing the constant classes produced by the hybrid. Thus, for example, the dihybrid AaBb produces constant progeny of the forms AB, Ab, aB, and ab. Therefore, the composition of the germ cells must also be AB, Ab, aB, ab. Thus, Mendel realizes that the germ cells carry all possible combinations of a's and b's, but A and a or B and b never occur in the same germ cells. This is the substance of Mendel's hypothesis—that the germ cells carry only constant combinations of traits. Mendel proceeds to confirm this hypothesis experimentally, and in an appropriate italicized fanfare he makes the following general statement:

[P]ea hybrids form germinal and pollen cells that in their composition correspond in equal numbers to all the constant forms resulting from the combination of traits united through fertilization (p. 29).

HARTL and OREL 1992 rightly describe this achievement as "the keypoint of Mendel's discovery." It is also the key to defining Mendelian development.

Mendel's first step is to demonstrate how the germ cell "explains" the production of the developmental series. He starts with the simplest case—a hybrid that bears the single hybrid trait—Aa. According to his hypothesis, such a hybrid should produce equal numbers of only two kinds of germ cells: A and a. Because, in the simplest case, only four individuals are produced, this means that the hybrid forms four pollen cells and four germinal (egg) cells, described by Mendel as (p. 29):

These germ cells then participate in fertilization, but:

[I]t 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 cell form 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, one pollen cell a will become associated with a germinal cell A, the other with a (p. 30).

Mendel diagrams this chance fertilization as:

He then visualizes each fertilization in the form of a fraction, in which the pollen cell occupies the numerator and the germinal cell occupies the denominator:

Finally, Mendel translates these fertilization fractions into the resultant classes of progeny:

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 second and 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 they derive. Thus, repeated hybridization takes place (p. 30).

Therefore, the fertilization fractions are (p. 30)

These fertilization fractions are none other than the terms of the developmental series.

The mono-, di-, and trihybrids behave in the same way, passing through these four steps to produce their characteristic developmental series:

1. Fertilization brings together pairs of contrasting traits to form the hybrid trait;

2. germ cells that carry only constant combinations of traits, in equal number, are then formed;

3. the germ cells unite randomly in fertilization;

4. the different classes of progeny make their appearance.

These are the steps that demonstrate the role of the germ cells in the production of the developmental series. But does this get us any closer to defining the development of the hybrid? Mendel kindly comes to our aid once again. He informs us that the development of the hybrid also results in the developmental series. One may logically assume that there is only one set of steps in the formation of the developmental series. Since Mendel has already elaborated the four steps that result in the developmental series, and since development of the hybrid also produces the development series (p. 21), we may conclude that those same four steps comprise the development of the hybrid. This process, which originates in the hybrid and finds completion in the progeny, is Mendelian development.

Interestingly, a letter from Mendel to Karl Nageli, dated April 18, 1867 (STERN and SHERWOOD 1966 , p. 62), provides independent corroboration that Mendel viewed development as a process linking the hybrid with the different kinds of progeny it produced. In the letter, Mendel tried to convince Nageli (a believer in blending inheritance) that from a hybrid Aa one could retrieve the parental traits A and a among the progeny of that hybrid:

I am inclined to regard the separation of parental traits in the progeny of hybrids in Pisum as complete and thus permanent. The progeny of hybrids carries one or the other of the parental traits, or the hybrid form of the two; I have never observed gradual transitions between the parental traits or a progressive approach towards one of them. The course of development (italics mine) consists simply in this; that in each generation the two parental traits appear, separated and unchanged, and there is nothing to indicate that one of them has either inherited or taken over anything from the other (STERN and SHERWOOD 1966 , p. 62).

But what exactly is happening in the course of Mendelian development? Mendel is describing the manner in which the hybrid trait is passed to and distributed among the progeny of the hybrid. The hybrid Aa produces progeny that are constant A, constant a, and hybrid Aa once again, in the ratio 1:1:2 (the law of development). Moreover, without recourse to the term heredity, which was not in common use in the nineteenth century, Mendel makes it very plain that heredity is the focus of his interest:

When two plants, constantly different in one or several traits, are crossed, the traits they have in common are transmitted unchanged to the hybrids and their progeny ... a pair of differing traits, on the other hand, are united in the hybrid to form a new trait, which usually is subject to changes in the hybrid's progeny. It was the purpose of the experiment to observe these changes for each pair of differing traits, and to deduce the law according to which they appear in successive generations (p. 5).

KING and STANFIELD's A Dictionary of Genetics (1997, p. 157) defines heredity as a "familial phenomenon wherein biological traits appear to be transmitted from one generation to another." The equivalence between this definition of heredity and Mendel's description of the "course of development" is strikingly apparent. There is no doubt that in his study of the development of the hybrid, Mendel is actively, knowingly, intentionally, dealing with heredity. However, in concluding that Mendelian development is a study in the transmission of traits, have we not returned full circle to the traditional story? Or have we?

Mendel is traditionally personified as the nineteenth century monk who was definitely ahead of his time. Part of his success is attributed to his very modern attitude towards heredity and development. Whereas his contemporaries maintained that heredity was merely a stage in the seamless process of development, Mendel, it is claimed, recognized that the events of transmission could be detached from development and studied separately (SANDLER and SANDLER 1985 ). However, if we keep in mind that Mendel described his work in transmission as a study in the development of the hybrid, he suddenly does not seem to be quite so detached from his nineteenth century intellectual heritage. There is no doubt that Mendel's focus is on the events of transmission, but he continues to think and speak and write about transmission in terms of nineteenth century development. However, the nineteenth century garb cannot disguise his actual achievement. Mendel's intention—"to follow the development of hybrids in their progeny"—is a step-by-step description of the transmission and distribution of hybrid traits between parent and progeny. Is it not fitting that we restore to Mendel his well-deserved title—Father of Genetics?

	FOOTNOTES

¹ From the Editors: This year marks the hundredth year of genetics as a science, while the next two years mark the turning of the millennium. The year 1900 saw the rediscovery of Mendel's laws by DeVries, Correns, and Tschermak, and the revolution that was set forth by this triple confirmation has had few equals in science. Our way of celebrating this anniversary has been to invite another look at Mendel's paper itself. Iris Sandler and her late husband, Larry, were long interested in looking for the deeper meaning of Mendel's experiments. We are pleased to have Iris's essay to celebrate this century of genetics and begin another year of Perspectives. For a personal account of Larry Sandler, see "Larry Sandler: personal recollections" (D. LINDSLEY, 1999, Genetics 151: 1233–1237). J.F.C. and W.F.D.

	ACKNOWLEDGMENTS

I express my thanks to two very special people who never stinted in their support: to my daughter Dianne who, with great effort, taught me to see the computer as a friend and not some mechanical fiend, and to my son Jack, who was my patient and gallant sounding board. A very grateful thank you goes to Barbara Wakimoto and Dan Lindsley, who prodded me into turning some scattered ideas into a coherent whole. There can, however, be only one dedication—for Larry.

	LITERATURE CITED

TOP Entwicklung and development Development--a world view Mendel on development LITERATURE CITED

BATESON, W., 1902 Mendel's Principles of Heredity: A Defence. The Genetics Heritage Press, New Mexico (reprinted in 1996).

BOWLER, P. J., 1989 The Mendelian Revolution. The Johns Hopkins University Press, Baltimore.

BRUSH, S., 1987  The Nebular Hypothesis. Hist. Sci. 25:245-278.

CASTLE, W., 1903  Mendel's Law of Heredity. Proc. Am. Acad. Arts Sci. 38:535-548.

DE BEER, G., 1964 Charles Darwin. Doubleday, Garden City, NY.

HARTL, D. and V. OREL, 1992  What did Gregor Mendel think he discovered? Genetics 131:245-253[Medline].

ILTIS, H., 1932 Life of Mendel Norton, New York (translation of Gregor Johann Mendel: Leben, Werk und Wirkung)

KING, R., and W. STANSFIELD, 1997 A Dictionary of Genetics. Oxford University Press, New York.

LORCH, J., 1969 The Principles of Scientific Botany by M. J. Schleiden, a facsimile of the London 1949 edition, with a new introduction by Jacob Lorch; Johnson Reprint Corporation, NY.

MENDEL, G., 1866  Versuche über Pflanzen-Hybriden. Reprinted in 1951(J. Hered. 42):3-47.

MERZ, J. T., 1904 A History of European Thought in the Nineteenth Century. Dover, New York (reprinted in 1965).

MONAGHAN, F. and A. CORCOS, 1990  The real objective of Mendel's paper. Biol. Philos. 5:267-292.

OLBY, R., 1979  Mendel no Mendelian? Hist. Sci. 17:53-72.

OSPOVAT, D., 1976  The influence of Karl Ernst von Baer's Embryology: 1828–1859. J. Hist. Biol. 9:1-28[Medline].

SANDLER, I. and L. SANDLER, 1985  A conceptual ambiguity that contributed to the neglect of Mendel's paper. Life Sci. 7:3-70.

SPENCER, H., 1904 Progress: its law and cause, pp. 8–62 in Essays: Scientific, Political and Speculative, Vol. 1. D. Appleton, New York (originally published in The Westminster Review for April 1857).

STERN, C., and E. SHERWOOD, 1966 The Origin of Genetics—A Mendel Source Book. W. H. Freeman, San Francisco.

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