Unravelling the Double Helix

by Gareth Williams

  This tantalisingly constant relationship led Mendel to construct a mathematical theory which explained the whole baffling sequence. He suggested that every plant carries two versions of each trait and that the combination determined what the plant looked like. He called the version which blotted out the other form in the first hybrid generation ‘dominant’; its submissive alter ego, which melted away in that generation but reappeared in one quarter of the progeny when these hybrids were self-fertilised, he termed ‘recessive’. For the trait governing height, the tall variant (designated T) was dominant and produced a tall plant even if the recessive short variant (designated t) was also present. Therefore, both pure-bred TT and hybrid Tt were tall, whereas short plants could only be tt.

  What happened during germ-cell formation and fertilisation? The only logical explanation was that the germ cells (pollen grains or egg-cells) somehow ended up with just one version of the trait, so that the embryo formed by the fusion of pollen and egg-cell had the full complement of two copies, one from each parent. On the assumption that the allocation of the copies to each germ cell was random, Mendel’s theory predicted that the ratio of tall short plants in the second hybrid generation would be 3 to 1 – which fitted his experimental data as snugly as any glove can do (Figure 4.2).

  Having proved that his theory held for each of the seven traits, Mendel went on to test whether the inheritance of a particular trait was influenced by the others. He found no evidence of such interference. For example, the magical 3:1 ratio of yellow to green peas in the self-fertilised hybrids remained the same, irrespective of whether the plants were tall or short.

  These were undoubtedly Mendel’s happiest years. The Realschule kept him busy with classes of over a hundred up to twenty times each week, but he enjoyed teaching and the respect and affection of his pupils. He was also gaining status beyond the abbey. In the summer of 1862, he joined the Moravian deputation to the Second International Exhibition in London, directing a display on crystallography from the Realschule. And in the following year, together with Abbot Napp, Mendel founded the Brünn Natural History Society.

  At this time, he was very much a man about town, usually dressed as a teacher in frock coat and tall hat rather than as a priest. A typical sighting described him as ‘medium height, broad-shouldered and already slightly corpulent, with a big head, high forehead and friendly blue eyes that twinkled through his gold-rimmed glasses’.

  Figure 4.2 Mendel’s cross-fertilisation studies in peas. In this case, tallness (T) is dominant and shortness (t) is recessive. Hybrids (Tt) are therefore tall; only plants with the genotype tt are short.

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  Finally, in early 1865, Mendel was ready to break the news of his discovery. He started close to home, by preaching to the converted: the members of the Brünn Natural History Society. Forty or so of the Society’s worthies turned up at the Neuschule on the evening of 8 February to hear Part 1 of Mendel’s presentation; Part 2 followed at the Society’s next meeting on 8 March. Mendel’s view that eight years’ work could not be compressed into one evening appeared to be justified. Part 1 was a blow-by-blow account of his pea studies, and Part 2 his interpretation of the results, together with a résumé of his follow-up experiments on various varieties of bean – which unfortunately were not as clear-cut.

  This was the first time that the world heard about what came to be known as Mendel’s Laws, let alone the first grains of absolute truth about the mechanisms of heredity. Not surprisingly, some have been tempted to infuse the occasion with high drama, usually overlaid with the tragedy of a prophet unheeded in his own land. Hugo Iltis, who pulled off a notable feat of resurrection by writing Gregor Mendel’s Autobiography forty years after Mendel’s death, portrayed him as nervous and intense, bludgeoning his uncomprehending audience with equations and statistics; at least Iltis did not mention the scornful laughter which someone else later dubbed in. In fact, Mendel was a gifted communicator with a well-pitched sense of humour, who routinely held a class of a hundred restless young minds in the palm of his hand (‘He aroused me with a lust and love for natural science’, as one eminent former pupil later said). The local paper reported an ‘interesting meeting, with demonstrations and lively participation’; Mendel himself noted ‘divided opinions’, but interpreted this as a healthy sign that an informed audience had run up against something new and startling. There was just one sad note: Abbot Napp, now elderly and frail, was confined to his quarters and did not hear his protégé talk about the project in which both had invested so much.

  There is little doubt, however, that the members of the Brünn Natural History Society – mostly self-taught, and schoolteachers rather than professional scientists – completely missed the wider significance of what they heard. But then so did Mendel. He had set out with the narrow aim of finding practical guidance for plant breeders; like his audience, he was unaware that he had unearthed fundamental principles of heredity which apply to all living things.

  *

  Afterwards, Mendel tried to spread the word about his findings. As was usual for these meetings, he wrote up his talks for publication in the Society’s Transactions. His forty-four-page paper appeared in 1866, under the inscrutable title, Versuchen über Pflanzehybriden (Studies of plant hybridisation). It not for the mathematically faint-hearted, and diligent weeding is needed to reveal what all the fuss is about.

  Throughout his paper, Mendel was careful not to speculate about what was going on deep inside the pea plants, even though his botany lectures at Vienna had brought him up to date with the concept of the cell and the nucleus, then thirty-three years after its discovery by Brown. However, the nucleus was still uncertain territory, and nearly twenty years would elapse before Walther Flemming trained his microscope on his ‘nuclear threads’. Mendel therefore had no physical structures inside the cell on which to hang his ideas. Instead, he used the gloriously vague term ‘element’ to denote whatever magical property manifested itself as T or t.

  The Society routinely sent out 120 copies of the Transactions to learned institutions across Europe, including the universities of Cambridge and Geneva and the Royal Society in London. Mendel also ordered forty reprints with which to impress some of the big names in botany and heredity.* Only one of Mendel’s reprints scored a palpable hit, and the outcome was not what he had hoped for. The target was Carl von Nägeli, Professor of Botany at Munich, an expert on evolution and the classification of hybrids, especially of the dandelion-like Hieracium (hawkweed). Nägeli had a formidable reputation and was widely regarded (and by himself) as a leading botanist of the day. Mendel must have steeled himself on New Year’s Eve 1866 to write a covering letter that accompanied the reprint destined for Nägeli. ‘Highly Esteemed Sir!’ he began, ‘The acknowledged pre-eminence that Your Honour enjoys . . . makes it my agreeable duty to submit for your kind consideration the description of some experiments in artificial fertilisation.’

  If Mendel had claimed that he had discovered fundamental laws of heredity which explained the behaviour of hybrids – and even the maddening throwback to the grandparental form – Nägeli might have replied promptly and more positively. Instead, he took two months to write back, in a letter which made it crystal-clear that he was short of time and fully committed to his own projects. He told Mendel to finish off his studies ‘which were far from being completed’ and to do better than ‘following in the footsteps of your celebrated predecessors’. The eminent professor evidently failed to grasp that Mendel had already done that. Nonetheless, Nägeli’s reply was comprehensive and comes across as something dashed off by a man in a hurry, rather than a put-down; he also enclosed five of his own reprints about hawkweed. Thus began a fitful correspondence which lasted seven years – nearly as long as Mendel’s pea experiments.

  As their relationship evolved, Mendel relaxed and his letters, although still respectful, lost the servility of his opening shot. ‘Highly Esteemed Sir!’ mutated into ‘Highly Esteemed Friend!’, while ‘with t
he greatest esteem and respect for Your Honour’ became ‘Your devoted friend’. Neither was an assiduous correspondent, but Nägeli more often responded quickly and continued to provide specimens of hawkweed – even when his previous offerings had come to grief in Brünn.

  Mendel gave the impression (undoubtedly accurate) of a man constantly tripped up by life’s little obstacles. He told Nägeli all about the abbey gardener who fatally overwatered the specimens that Nägeli had sent (‘I hope none of the species was lost in its entirety’) and the self-inflicted eye-strain brought on by trying to make out the intimate anatomy of hawkweed. Not forgetting the weight gain (‘an excess of avoirdupois’) which made it increasingly difficult for Mendel to collect specimens in the wild and was especially tiresome ‘as a consequence of the law of general gravitation, when climbing mountains’.

  These letters also charted Mendel’s decline and fall as an experimenter. Nägeli strong-armed him into studying hawkweed hybrids; flattered, Mendel tried to repeat his pea experiments, but Hieracium refused to behave. The hybrids produced were unpredictable, forcing Mendel to confess to Nägeli that he felt ‘greatly deceived’ and could offer no explanation – no doubt painfully aware that the fiasco had undermined his original findings and may have turned Nägeli even further against his ‘incomplete’ studies.

  In May 1868, Mendel wrote with momentous news. Following the recent death of Abbot Napp, ‘my unimportant self’ had been elected ‘lifelong head by the chapter of the abbey to which I belong’. The newly appointed abbot confessed that this was ‘a sphere which appears strange to me, and it will take some time and effort before I feel at home in it’. That prediction was correct; by contrast, his promise to ‘devote more time and attention’ to the puzzle of the hawkweed hybrids quickly turned out to be hopelessly optimistic.

  Mendel’s last letter to Nägeli began with an overdue apology: ‘Despite my best intentions, I was unable to keep my promise . . .’ The hawkweed plantation in the abbey, to which Mendel had squeezed in ‘only a few hurried visits’ had ‘withered again’, and he had made no further progress in understanding the plant’s idiosyncrasies. And with that admission of defeat, and ‘the expression of my greatest admiration and esteem’, Abbot Gregor Mendel signed himself off, ‘Yours very devotedly’.

  Ten letters that sprawled across seven years – and containing no hint that Nägeli, one of the age’s sharpest brains in botany, understood what Mendel had discovered, or that he showed any interest in that discovery.

  A greater calling

  Mendel’s research career had already peaked when the Bishop of Brünn installed him as abbot in April 1868, just two years after the appearance (and disappearance) of his paper. His vain attempts to make Nägeli’s favourite plants behave like peas immediately lost ground to his new duties, but he managed to produce one paper on hawkweed hybrids (in German, possibly heartfelt: Hieracium-Bastarde). Published in 1870, this conspicuously failed to recapture the mathematical neatness of his experiments with peas.

  Mendel was undoubtedly frustrated by being an ex-scientist, especially as most of his new administrative duties were tedious and energy-sapping. One of the few bright spots was his appointment of a brilliant young composer as organist to the abbey – Leoš Janáček, whose later opera The Cunning Little Vixen was supposedly inspired by the abbot’s pet fox.† Mendel was also imprisoned by his worsening obesity and declining health. He managed to escape only occasionally, no longer to wild places in pursuit of plants, but to events such as the Association of German Beekeepers in Kiel in 1871.

  By now, the well-ordered pea plantation had become history, ravaged by natural disasters that swept in like Old Testament scourges: first pea weevils, then something that looked suspiciously like an act of God. One autumn morning, as Mendel watched, a tornado scythed through the abbey gardens and flattened the greenhouse which Napp had built for him. A lesser man might have interpreted this as the Almighty’s way of telling him to concentrate on being a priest, but Mendel reacted like a true scientist. He collated all the available information, including his own observations of ‘a hellish symphony’, the clockwise rotation of the tornado’s funnel, and the roof slate that was hurled across his desk, and wrote up the definitive account of ‘The whirlwind of the 13th of October 1870’.

  C.W. Eichling’s visit in 1878 found Mendel apparently at peace with his role as abbot, but the firmness with which he steered conversation away from the ‘little trick’ with the peas and the story that would have taken too long to tell suggests that this was still a sore point. We can only guess at what he was reluctant to share. Perhaps the speed and completeness with which his paper had slipped into oblivion: no flicker of interest from any of the 120 learned institutions which were sent the Transactions of the Brünn Natural History Society, let alone 39 recipients of the reprints. The only response was from Nägeli, who had dismissed his experiments as incomplete and then recruited him as a research assistant on the disastrous Hieracium-Bastarde project – which left Mendel looking like a fool, and probably gnawed by suspicions that his peas had told him lies.

  Mendel’s last forays into botany included the satisfying demonstration that even the greatest living scientists could be wrong. Armed with a fine paintbrush and a microscope, the poor-sighted abbot proved that a single grain of pollen was enough to fertilise an ovum – something that Charles Darwin had insisted was impossible. He also experimented on the occupants of his fifty beehives, attempting to map body colour, flying behaviour and fondness for stinging, just as he had done for the characteristics of pea plants. When bees joined the list of species that were too complicated to study, Mendel had the abbey’s telescope adapted to show the sun in all its glory, and tried to find relationships between sunspots and meteorological conditions in Brünn. That too led nowhere, leaving him with the undemanding research ritual which he had begun in 1856. Three times each day, he recorded the weather.

  Had C.W. Eichling returned to Brünn during Gregor Mendel’s last years, he would have found a changed man: fat and morose, chain-smoking cigars on the orders of his doctor to try to lose weight, and ground down by a running battle with an authority that set itself above God. By decree of Otto von Bismarck, the government had pushed through a new tax on religious establishments. Mendel refused to pay, even after the government began seizing the abbey’s assets. The whole unhappy confrontation could have been easily defused by negotiation and a modicum of compromise – as indeed it was by his successor – but Mendel turned it into a point of principle to which he clung obstinately, and which eventually poisoned his life.

  By now, his health was failing and, just as had happened to Abbot Cyrill Napp twenty years earlier, the walls of the abbey steadily closed in around him. Towards the end of 1883, Mendel knew what the future held. In December, he wrote a last letter to the pupil whose ‘lust and love for natural history’ he had aroused while teaching at the Realschule, and who was now an eminent meteorologist: ‘Since we are not likely to meet again in this life, let me take the opportunity of wishing you farewell and of invoking on your head all the blessings of the meteorological deities.’

  Mendel managed to keep up his thrice-daily weather records until the morning of 4 January 1884, but was too ill to get out of bed that afternoon; two days later, he was dead. The post-mortem examination, attended on Mendel’s instructions by his nephew Alois, showed shrivelled kidneys and a flabby, dilated heart. His funeral on 9 January was a lavish affair. Set to music by Janacek, it was attended by a ‘huge concourse’ of friends, admirers and one-time adversaries: his students, people of faith (Protestant and Jewish as well as Catholic), townsfolk of Brünn and farmers from across Moravia, the inhabitants and fire brigade of Heinzendorf (whose fire station had been built at Mendel’s expense) – and the government lackeys who had helped to push him into an early grave at the age of sixty-two.

  In the spirit of ‘The King is dead, long live the King’, the abbot’s chamber had already been cleared for Mendel’s successor.
Mendel’s books and official letters were carefully transferred to the library, where they remain in a position of honour to this day. But all his notebooks, experimental records and calculations were carted away and dumped on a bonfire behind the abbey gardens. We can only speculate as to whether this was done on the orders of the new abbot – or his predecessor.

  * It has been claimed that Charles Darwin headed Mendel’s hit list, and that Darwin’s failure to read the reprint represented one of the greatest missed opportunities in the history of evolutionary biology. Poignant though this would be, there is no evidence that it ever happened.

  † A more likely candidate was the pictorial epic about ‘Vixen Sharp-Ears’, serialised in Brünn’s daily paper during the spring of 1920.

  5

  OF GRASSHOPPERS AND FLIES

  Some time after Studies of Plant Hybridisation had sunk into the abyss of indifference, Gregor Mendel announced to one of his Brothers that ‘Mein’ Zeit wird schon kommen’ (‘My time will surely come’). Subsequent events must have sapped his confidence: the frustrations of Nägeli’s Hieracium-Bastarde and the wrangle with the government which was still hanging over the abbey like the Sword of Damocles when, in early January 1884, Mendel’s time came in the definitive sense. He might well have left the world believing that he had wasted his time experimenting on peas.