The Gene: An Intimate History - Siddhartha Mukherjee Page 0,23

the hothouse outside. The abbot had acquiesced. He drew the line at mice, but didn’t mind giving peas a chance.

By the late summer of 1857, the first hybrid peas had bloomed in the abbey garden, in a riot of purple and white. Mendel noted the colors of the flowers, and when the vines had hung their pods, he slit open the shells to examine the seeds. He set up new hybrid crosses—tall with short; yellow with green; wrinkled with smooth. In yet another flash of inspiration, he crossed some hybrids to each other, making hybrids of hybrids. The experiments went on in this manner for eight years. The plantings had, by then, expanded from the hothouse to a plot of land by the abbey—a twenty-foot-by-hundred-foot rectangle of loam that bordered the refectory, visible from his room. When the wind blew the shades of his window open, it was as if the entire room turned into a giant microscope. Mendel’s notebook was filled with tables and scribblings, with data from thousands of crosses. His thumbs were getting numb from the shelling.

“How small a thought it takes to fill someone’s whole life,” the philosopher Ludwig Wittgenstein wrote. Indeed, at first glance, Mendel’s life seemed to be filled with the smallest thoughts. Sow, pollinate, bloom, pluck, shell, count, repeat. The process was excruciatingly dull—but small thoughts, Mendel knew, often bloomed into large principles. If the powerful scientific revolution that had swept through Europe in the eighteenth century had one legacy, it was this: the laws that ran through nature were uniform and pervasive. The force that drove Newton’s apple from the branch to his head was the same force that guided planets along their celestial orbits. If heredity too had a universal natural law, then it was likely influencing the genesis of peas as much as the genesis of humans. Mendel’s garden plot may have been small—but he did not confuse its size with that of his scientific ambition.

“The experiments progress slowly,” Mendel wrote. “At first a certain amount of patience was needed, but I soon found that matters went better when I was conducting several experiments simultaneously.” With multiple crosses in parallel, the production of data accelerated. Gradually, he began to discern patterns in the data—unanticipated constancies, conserved ratios, numerical rhythms. He had tapped, at last, into heredity’s inner logic.

The first pattern was easy to perceive. In the first-generation hybrids, the individual heritable traits—tallness and shortness, or green and yellow seeds—did not blend at all. A tall plant crossed with a dwarf inevitably produced only tall plants. Round-seeded peas crossed with wrinkled seeds produced only round peas. All seven of the traits followed this pattern. “The hybrid character” was not intermediate but “resembled one of the parental forms,” he wrote. Mendel termed these overriding traits dominant, while the traits that had disappeared were termed recessive.

Had Mendel stopped his experiments here, he would already have made a major contribution to a theory of heredity. The existence of dominant and recessive alleles for a trait contradicted nineteenth-century theories of blending inheritance: the hybrids that Mendel had generated did not possess intermediate features. Only one allele had asserted itself in the hybrid, forcing the other variant trait to vanish.

But where had the recessive trait disappeared? Had it been consumed or eliminated by the dominant allele? Mendel deepened his analysis with his second experiment. He bred short-tall hybrids with short-tall hybrids to produce third-generation progeny. Since tallness was dominant, all the parental plants in this experiment were tall to start; the recessive trait had disappeared. But when crossed with each other, Mendel found, they yielded an entirely unexpected result. In some of these third-generation crosses, shortness reappeared—perfectly intact—after having disappeared for a generation. The same pattern occurred with all seven of the other traits. White flowers vanished in the second generation, the hybrids, only to reemerge in some members of the third. A “hybrid” organism, Mendel realized, was actually a composite—with a visible, dominant allele and a latent, recessive allele (Mendel’s word to describe these variants was forms; the word allele would be coined by geneticists in the 1900s).

By studying the mathematical relationships—the ratios—between the various kinds of progeny produced by each cross, Mendel could begin to construct a model to explain the inheritance of traits.I Every trait, in Mendel’s model, was determined by an independent, indivisible particle of information. The particles came in two variants, or two alleles: short versus tall (for height) or white versus violet (for flower color) and so forth. Every plant