The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics

by Robin Marantz Henig

Klácel was no doubt a radicalizing influence on his impressionable young friend. By the time Mendel met him in 1843, Klácel was in many ways a beaten man. Within a few years he would be relieved of his teaching duties as a philosophy professor in Brünn because of his writings in defense of Naturphilosophie, a German philosophy that combined evolutionary thought, a belief in purposeful activity in nature, and a view that the material world was a projection of a deeper spiritual reality. Among the most prominent German adherents of Naturphilosophie, which originated with the thinkers F. W. J. von Schelling and Lorenz Oken, were the poet Johann Wolfgang von Goethe and the philosopher G. W. F. Hegel.

Through most of the eighteenth and nineteenth centuries, scientists believed that the adaptations of every organism on earth were evidence of a divine plan, a plan that, for most, ended and began with man. People tended to indulge in a human-oriented view of life. Many believed, for instance, that horses’ backs were shaped simply to give man a comfortable seat for riding; that seawater contained alkaline substances like magnesia and lime so sailors could clean their clothes without using soap; that tides existed just to help the ships come in.

This was a comforting view of the universe. Everything was orderly, everything was preordained, and everything existed for the aid and benefit of mankind. This view was seriously challenged in the middle of the nineteenth century, when the physicist Rudolf Clausius discovered entropy. Nature was not comforting and orderly at all, he said; objects tended to move in the direction of extreme disorder. The second law of thermodynamics, which Clausius formulated, also called the law of entropy, stated that every isolated system becomes more disordered over time. In this construction an orderly system is as regulated as a crystal, with each atom in precisely the right spot, while a disordered system is like vegetable soup, arranged at random without any clear pattern or organization. And the flow of nature is toward greater randomness, patternlessness, and disorganization—an unsettling view for people who saw God’s logic in every flutter of a butterfly.

This was a risky decision, since Vienna, despite its reputation as a festival of scientific and cultural confections, allowed for such brazen pursuit of pleasure that one of its favorite sons, the playwright Franz Grillparzer, called it “the Capua of the mind.” It was in the wealthy Italian town of Capua that Hannibal’s troops had abandoned themselves to reckless overindulgence, rendering them unfit for further warmongering and eventually bringing an end to the Second Punic War.

As Mendel would later explain, large numbers are necessary, “because with a smaller number of experimental plants . . . very considerable fluctuations may occur.” To deduce “true numerical ratios,” he said, requires “the greatest possible number of individual values; and the greater the number of these the more effectively will mere chance be eliminated.”

Mendel, though aware of the brilliance of his predecessor, was bold enough in designing his own experiments to recognize the flaws in Gärtner’s. He showed the sensibility of a modern scientist, whose métier is replication of prior results, when he wrote: “It is very regrettable that this worthy man did not publish a detailed description of his individual experiments, and that he did not diagnose his hybrid types sufficiently.” This imprecision meant that he could not reproduce Gärtner’s results even “in a single case!” In addition, Gärtner seemed to have had no idea whether the plants he used as the parental strains were pure-breeding or were themselves hybrids. Nor did he carry his crosses into the second and third generations—an essential step if results of further hybrid-hybrid crosses were to be detected.

Also, Gärtner had focused on the wrong unit of experimentation. Mendel always considered his object of study to be the plant’s individual parts, its character traits. Gärtner, like so many of his contemporaries, was interested in the plant as a whole, which he considered the expression of all its parts. The thought of examining each part individually did not fit into his overall view. The holistic conception of inheritance, common even into the late nineteenth century, led to the widespread acceptance of

“blending inheritance,” the theory that offspring were a combination of traits and thus roughly mi...

But Mendel found seven different traits in peas, easy to identify with the naked eye, that occurred in an either-or configuration. They never blended; they were always inherited separately and intact. “Transitional forms were not observed in any experiment,” he said emphatically. In the “height” character, for example, a plant was either very tall—some six feet or more and needing to be staked—or very short, no more than twenty inches high. No traits—at least of the seven Mendel chose—were in between or blended. He called the traits “pairs of differentiating characters.” If he could identify these differentiating characters in his peas and...

Mendel used two different words that are both usually translated as “character trait.” But in German these words have slightly different shades of meaning. The first, Merkmal, implies a quality you can see and recognize, something we usually call a “trait.” The second, Elemente—which Mendel used only in the plural form—is close to its English cognate, “elements,” the unknown substances that might account for

an organism’s Merkmale. Mendel used the word Merkmal more than 150 times, compared to just 10 for Elemente, which he used only in the conclusion of his paper, where he deduced units, or elements, from the way the traits he had been observing were passed on from generation to generation.

The men who translated his paper into English in the early 1900s might have been overly generous in their assumptions of what Mendel understood. In their translations Merkmal often became “unit” or “factor” or “determinant.” With twentieth-century hindsight, the translators’ readiness to ascribe to Mendel all sorts of prescient views of heredity through the use of these modern words may...

Each of Mendel’s seven traits appeared on a different chromosome—or, in one case, on two distant ends of the same chromosome. This reduced the chances that these traits would be coupled through a process called linkage—which would have seriously muddied the monk’s results.

For one thing, he could differentiate between the two traits by calling one “dominating” and the other “recessive.” The dominating trait—which we now call “dominant”—was the one that showed up in all the F1 hybrids and three-quarters of the F2’s. The recessive trait was the one that seemed to disappear entirely in the first filial generation, but arose again in one-quarter of the second.

Mendel was the first to use double letters for his hybrids—indicating that he understood that a hybrid carried two differentiating character traits, one of which was hidden from view until it chanced to reappear in subsequent generations in a different combination. This made it seem as though Mendel knew that each individual would possess two of these traits, or Merkmale, one from each parent. Yet in the pure-breeding plants, either pure yellow or pure green, he used only a single letter to represent the full complement.

One of Mendel’s most revolutionary insights was that the combinations of Merkmale were essentially random. Four kinds of germ cells were available for mixing—A pollen, a pollen, A egg, and a egg—and each had an equal chance of joining with each of the others. The A pollen was just as likely to pair with the A egg as with the a egg; similarly, the a pollen had an equal chance of pairing with an egg from either A or a. Mendel’s calculations allowed him to go one step beyond his simple 3:1 ratio. When he thought through how the next year’s peas would look, he understood that among the F2 peas arrayed on his table in the orangery, there were really two types of yellows. Of all the yellow peas (which made up three-quarters of his entire cache), one-third would “breed true” in the next generation, producing only yellow seeds. Two-thirds would behave just as their parents had and produce two different colors, yellow and green, in the 3:1 ratio again.

It is hard to believe that Mendel could have envisioned this notion clearly in the autumn of 1857. It is easier to imagine that he reconstructed the idea in hindsight, after he saw how the crops of 1858 and 1859 and maybe even 1860 actually looked. But the explanation he finally arrived at, long before he formalized his thoughts in the two-part lecture delivered in early 1865, was this: that for every four seeds resulting from self-fertilization of a yellow-green hybrid, one of the yellow peas could be designated as A, or pure yellow; two were yellow but also carried the potential to produce green peas, which could be designated as Aa; the last pea, the 1 in the 3:1 ratio, would be green, or a. In this way Mendel made the first tentative step toward a concept that would not be fully elucidated for another fifty years: the difference between phenotype (the way something looks) and genotype (the particular combination of genes that explains those looks). If a pea’s phenotype is one of Mendel’s dominating alternatives—round-seeded rather than angular, or yellow-seeded rather than green, or smooth-podded, or tall—you could not tell just by looking what its genotype was. (This is not true for peas with recessive phenotypes, which are always pure recessives, since dominant traits cannot hide in the genotype but are always revealed in the way a plant or animal looks.) In other words, you could not tell just by looking whether a yellow pea carried only the yellow Merkmal for pea c...

Mendel eventually carried out his crosses in this particular set of experiments for four more generations, so he had a total of six generations derived by self-fertilizing hybrids. In each generation the seed traits broke down into three types: pure dominants (meaning their descendants were all dominant themselves), pure recessives (meaning their descendants were all recessive), and a group that revealed themselves to be hybrids only after giving rise to both kinds of offspring, always in that same 3:1 ratio. With the new information derived from successive generations, Mendel could refine the 3:1 ratio h...

If Darwin had known about Mendel’s work, he would have had a rebuttal to the swamping argument. Recessive traits do not get blended away in Mendel’s scheme; they disappear in hybrids that also carry the dominating trait, but reemerge in later generations when the gametes rearrange themselves and form a few double-recessive offspring. Mendel’s experiments, which revealed the random and independent transmittal of different traits, threw notions of blending inheritance into question. Character traits cannot be inherited separately—a condition that would later become known as “segregation”—and at the same time also blend.

Yes, existence is a struggle, Darwin agreed. Almost every living thing produces more offspring than can possibly survive, given the limitations of the food supply and of a parent’s ability to protect its young from predators. Some guiding principle must be involved in determining which of the offspring live and which die. Maybe that was adaptation. Darwin knew there were variations in nature, though he could

not explain why. Now, with Malthus’s phrase, he could make the next logical leap: that favorable variations tend to be preserved, and unfavorable variations, in the context of the struggle for existence, tend to be destroyed. Darwin’s next step was to offer natural selection as the mechanism that winnowed out the favorable from the unfavorable. He arrived there through an analogy to artificial selection, the breeding of plants and animals. In artificial selection, the intelligence of the breeder pushed the changes in a particular predetermined direction. In natural selection, however, Darwin envisioned no such overriding intelligence. He believed that changes occurred without an end in sight, without any supernatural agent in control—a

Mendel took the first steps toward the foundation of modern genetics by demonstrating, at least if one reads between the lines, that he understood the difference between a plant’s appearance and its underlying makeup. He did not have the vocabulary to explain it; that would not come along until nearly fifty years later, when scientists understood more about the cell, the nucleus, and the gene. But he was nonetheless revealing the distinction between what we now call phenotype and genotype. To Mendel, it was the logical extension of the law of dominance. Because dominating traits are able to hide recessive ones, not every round or yellow pea is the same underneath.

each parent is Aa—in other words, hybrid for the characteristic symbolized by A—the product of (Aa) X (Aa) is the combination series A + 2Aa + a; that is, one dominant offspring for every two hybrids for every one recessive.

First came his observation of the quality that was later called segregation. He began by outlining his idea that some factors in the germ cells, still unidentified, were able to pass on traits from parent to offspring. Whatever they were, he deduced, they separated while getting ready to pass to the next generation through the gametes of the parents. Mendel also deduced that the sex cells somehow changed from having a double dose of hereditary factors to having only one.

A related observation, which would come to be called the law of independent assortment, was that each factor that is passed from parent to offspring is passed alone, independent of any other factor.

Flower color was especially perplexing, he said. When he crossed the white bush bean with the red scarlet runner bean (P. multiflorus), his results were not what he had expected. “Apart from the fact that from the union of a white and a purple-red coloring a whole series of colors results, from purple to pale violet and white,” he said, “the circumstance is a striking one that among thirty-one flowering plants only one received the recessive character of the white color, while in Pisum this occurs on the average in every fourth plant.” It was to explain such anomalies, in fact, that he presented his lectures in the first place, hoping to encourage some of his colleagues to take on similar experiments and to prove his findings either wrong or right.

“No one will seriously maintain that in the open country the development of plants is ruled by other laws than in the garden bed,” Mendel said. “Here, as there, changes of type must take place if the conditions of life be altered, and the species possesses the capacity of fitting itself to its new environment.” But, he went on, by “fitting itself” he meant that the species undergoes a change in the gametes, through the units that pass on dominating and recessive traits in an evenhanded, random matter.

“Nothing justifies the assumption that the tendency to form varieties increases so extraordinarily that the species speedily lose all stability, and their offspring diverge into an endless series of extremely variable forms,” he said. To the contrary, the tendency is toward stability, with variation being the exception, not the rule.

Indeed, Darwin himself obtained ratios similar to the Mendelian 3:1 ratio—but had no idea what they meant.

Darwin probably had never heard of Mendel. Nor had anyone but Nägeli—until 1881, when Die Pflanzen-Mischlinge by Wilhelm Olbers Focke appeared. The book, known familiarly as Focke, summarized the world’s leading plant experiments and cited Mendel’s work fifteen times. “Mendel believed that he found constant numerical proportions between the types of hybrids,” Focke wrote, adding that the monk’s work followed the tradition of early hybridizers. Like them, Mendel found that hybrids tend to revert to parental form and that old, apparently lost characteristics can reemerge generations later.

Shortly after Mendel’s death, all his personal and scientific papers were burned in a huge bonfire in the monastery courtyard on the very spot where his greenhouse had once stood.

derided the biometricians’ most impressive statistical feat, the correlation table, as a rigid structure in which “the biometrical Procrustes fits his arrays of unanalyzed data.”

But the effort would have been in vain had not the wife of the famous Cambridge philosophy professor Alfred North Whitehead, who was a close friend of Bateson’s, taken the initiative to send him the September issue of the English Illustrated Magazine the following April. Bateson, of course, had never seen it—he was not in the habit of reading women’s magazines—and he instantly dispatched a letter to Miss Durham.

“I have been led to think it possible that you may be willing to see me again,” he wrote in his earnest scrawl. “If it is not so, you tell me and that will be all; but if it is so, will you some day meet me?. . . [I]t has been for a long time my earnest desire to meet you again, if only as one who was once my dear friend, without regard to the future at all.” But the future quickly announced itself anyway. In a matter of weeks the two were engaged, and on June 16, 1896, they married at last.

Punnett’s matrix was the clearest. He called it a checkerboard, setting it up so that the female contribution was shown in the horizontal boxes and the male contribution in the vertical boxes. In each box, then, two gametes intersected—one from the top, one from the side—and it was an easy matter to write in the box which two gametes had paired to form each particular offspring. Punnett’s matrix first appeared in the third edition of his textbook, Mendelism, in 1911. Not until his death in 1967, at the age of ninety-one, was the checkerboard renamed the Punnett square. As a result, Punnett’s name is far more familiar to casual genetics students, and to the general public, than is Bateson’s.

“Soon every science that deals with animals and plants will be teeming with discovery, made possible by Mendel’s work,” Bateson had said in other speeches. “Each conception of life in which heredity bears a part—and which of them is exempt?—must change before the coming rush of facts.”

Scientists who tried, after the rediscovery, to replicate Mendel’s work had found that—with certain refinements made possible by twentieth-century advances—his surprisingly modern approach to data analysis had laid an excellent foundation for an emerging branch of knowledge.

Sutton’s conclusion was that the same chromosomes that lined up in matching pairs at the moment the zygote was formed persisted throughout an organism’s lifespan. Through every cell splitting,

through every reduction division, the chromosomes reproduced themselves, retaining their identity through countless cycles of mitosis and meiosis. “The association of paternal and maternal chromosomes in pairs,” he concluded in 1902, “and their subsequent separation during the reducing division . . . may constitute the physical basis of the Mendelian law of heredity.” This was the first expression of what would come to be known as the chromosome theory of inheritance.

He used multiple fertilization and other laboratory tricks to create embryos with unnatural numbers of chromosomes and found that the only ones that developed into viable sea urchins, with all of their body parts in place, were those with the correct

chromosome complement of thirty-six.

“It is evident that the hypothesis failed when tested,” he concluded, “and must therefore be abandoned.”

Other biologists, such as Bateson’s friend Erwin Baur of Berlin, were also uncovering anomalies—in Baur’s case, in snapdragons—caused by the failure of a single class of zygotes to survive. In other words, according to Castle, Mendel’s law was right, but the specifics of these mice or snapdragons led to a confusing anomaly. This explanation was soon confirmed by other investigators, who dissected the dead embryos of Cuénot’s strain of yellow mice and found that all were double dominants.

The moral here, if there is one, is that sometimes it makes sense to hold tight to a cherished theory, even in the face of some apparent inconsistencies.

All it indicates is that “many characteristics of the organism are specified in the

gametes by means of special conditions, foundations, and determiners which are present in unique, separate, and thereby independent ways—in short, precisely what we wish to call genes.”

“phenotype,” meaning an organism’s appearance; and “genotype,” meaning its genetic makeup.

Using this logic, Morgan concluded that whenever a mutation showed up overwhelmingly in males, it was probably carried on the X chromosome. This allowed him to calculate how often different mutants arose, how often they were associated with other mutations in the newly discovered phenomenon called linkage, and where exactly along the chromosome “string” each genetic “bead” was located. Within a few years of finding the white-eyed fly, Morgan and his associates had created a map for the X chromosome—a map that was the precursor for all subsequent genetic maps, including the gargantuan map now being compiled of the normal locations of all the 30,000 or so genes that make up the total human complement.

But this view, then as now, undermines the brilliance of what Mendel did accomplish. He might not have known, prior to the twentieth-century understanding of the gene, exactly what his findings meant, but he arrived at them in a logical and sophisticated way, making him decidedly the first in a long line of modern genetic investigators.

The world of genetics changed, even in the Soviet bloc, when James Watson, an American, and Francis Crick and Maurice Wilkins, two Englishmen, were awarded the Nobel Prize in 1962. Their discovery of the structure of DNA—the molecule that makes up every gene on earth, arranged in the shape of the now-famous double helix—allowed them, and later their colleagues, to deduce the mechanism by which DNA reproduced itself in generation after generation after generation. The process of DNA reproduction explained, in turn, how genes reproduce and how they maintain themselves endlessly, in precise and predictable order, along the chromosomes that characterize each living species. Watson and Crick’s findings gave form to observations that Mendel could only begin to explain.