Mendel and the Birth of Genetics

A monk, a garden of pea plants, and a quiet act of counting that revealed inheritance is not a smooth blend but a shuffle of discrete things.

In the years before Mendel, the prevailing picture of inheritance was something like mixing paint. A tall parent and a short parent would produce children of intermediate height; a red flower crossed with a white would yield pink. Traits blended, smoothed out, averaged. This was so obvious it barely required argument. It was also wrong.

The man who proved it wrong was an Augustinian friar named Gregor Mendel, working in a monastery garden in Brno. Between 1856 and 1863 he grew, by his own count, around 28,000 pea plants. He kept ledgers. He counted offspring. And out of that careful, almost meditative bookkeeping he extracted a fact that would not be properly understood for another half-century: inheritance is particulate. Whatever passes from parent to child comes in discrete units, like coins, not like fluids.

The monk and his peas

Mendel chose Pisum sativum, the garden pea, for reasons that look obvious in hindsight and were brilliant in practice. Peas have flowers that normally self-pollinate, sealing them off from interference. They have several traits that come in two clearly distinguishable forms — seeds that are round or wrinkled, yellow or green, flowers purple or white, pods full or constricted, plants tall or dwarf. Nothing in between. The traits are discrete, which means they can be counted, and counting is what Mendel did.

He began by establishing pure-breeding lines. Plants whose round-seeded ancestors had only ever produced round-seeded children, for generations. Then he did something that anyone could have done, and nobody had bothered to do at scale: he crossed a pure round-seeded plant with a pure wrinkled-seeded one and looked at every single offspring.

All of them were round. Not a smoothed average. Not slightly rounder. Round. The wrinkled trait had vanished.

Then he let those first-generation hybrids self-pollinate. In the second generation the wrinkled trait reappeared — in roughly one plant out of four. He counted 5,474 round seeds and 1,850 wrinkled. The ratio was 2.96 to 1. Across seven different traits, the same pattern. Three to one.

Three laws hidden in the ratios

From those numbers Mendel inferred something nobody had inferred before. Each plant must carry two heritable factors for each trait, one from each parent. Some factors are dominant — they mask the presence of the other — and some are recessive, only showing themselves when paired with another copy of themselves. When a plant makes its gametes (its pollen or its eggs), the two factors separate, and each gamete carries only one.

That is Mendel's first law — the law of segregation. The 3:1 ratio is its fingerprint. Cross two hybrid plants, each carrying one dominant R and one recessive r, and the four equally likely combinations in their offspring are RR, Rr, rR, rr. Three of those four show the dominant trait. One in four shows the recessive. Exactly what he counted.

His second law follows from a different experiment: crossing plants that differed in two traits at once — say, round-yellow with wrinkled-green seeds. The two traits, he found, were inherited independently. The chance of getting a round seed had no bearing on whether it would be yellow or green. The factors assort independently, like coins flipped one after the other. This is the law of independent assortment, and the ratio it produces in the second generation is 9:3:3:1, which Mendel also counted.

The third law, implicit in the experiments, is the law of dominance: when two different factors meet in the same plant, one of them — the dominant — determines what the plant looks like, while the other is carried silently, waiting to reappear in a later generation. The wrinkled trait was never lost. It was hidden.

A paper that no one read

In 1865 Mendel presented his results to the Brno Natural History Society. The next year they were published in its proceedings. The paper sat there, in a journal that almost no one outside Brno read, for thirty-five years. Darwin, who would have found in it the missing mechanism his theory of natural selection desperately needed, never came across it. Mendel sent reprints; the recipients either failed to grasp the significance or politely shelved them. He moved on to administrative work, took over as abbot of the monastery, and died in 1884 known mostly as a horticulturist with an unusual interest in statistics.

“My time will come.” — attributed to Mendel, late in life

It did, in 1900. Three botanists working separately — Hugo de Vries in Holland, Carl Correns in Germany, Erich von Tschermak in Austria — each independently rediscovered the same ratios in their own breeding experiments. Each, in the course of searching the prior literature, turned up Mendel's paper and realised he had got there first, with cleaner data and clearer reasoning. Within a few years, the new science of genetics (a word coined in 1905) had a founder.

From factors to genes

Mendel did not know what his “factors” were physically. He treated them as abstract bookkeeping units. The genius of his framework is that it works without any commitment to mechanism — you can do all the algebra of inheritance without knowing whether the units live in the nucleus, in the cytoplasm, or anywhere at all. But the units had to be somewhere, and the twentieth century slowly filled in the physical picture.

Walter Sutton and Theodor Boveri noticed, around 1902, that chromosomes behave during cell division exactly the way Mendel's factors behave during inheritance: paired, separating, reuniting. Thomas Hunt Morgan, working with fruit flies at Columbia, showed that genes sit at specific loci on chromosomes and can be mapped. Avery, MacLeod, and McCarty showed in 1944 that the molecule carrying genes is DNA. Watson and Crick, in 1953, showed what DNA looked like. The arc from Mendel's pea seeds to the double helix took less than a century.

What Mendel got right, before there was any reason to believe it, is that biology runs on discreteness. The information that builds a body comes packaged in countable units. Sex shuffles those units. Most of modern biology — Mendelian disease, population genetics, evolutionary theory after the “modern synthesis,” the entire architecture of the human genome project — rests on that one observation, made by a man counting peas in a garden, who died not knowing he was right.