In the garden of a monastery in the city of Brno, in what is now the Czech Republic, an Augustinian friar spent the better part of a decade doing something no one had quite done before: counting. Between roughly 1856 and 1863, Gregor Mendel grew and bred an enormous number of pea plants — by his own account around 28,000 of them — and painstakingly tallied the traits of their offspring. Out of that patient bookkeeping came the fundamental laws of inheritance, though almost no one noticed at the time.
Mendel chose the common edible pea, Pisum sativum, for good reasons. It grows quickly, is easy to cultivate, and comes in clearly distinguishable varieties. He focused on seven characteristics that each appeared in two contrasting forms: seeds that were round or wrinkled, pods that were green or yellow, flowers positioned along the stem or bunched at the tip, tall plants or short ones, and so on. Crucially, peas normally fertilise themselves, but Mendel could also cross-pollinate them by hand, giving him precise control over which plants bred with which.
The three-to-one ratio
When Mendel crossed a purebred round-seeded plant with a purebred wrinkled-seeded one, the entire first generation of offspring had round seeds. The wrinkled trait had seemingly vanished. But when he let that generation breed among itself, the wrinkled seeds reappeared in the next generation in a strikingly consistent proportion: roughly three round seeds for every one wrinkled seed. He found the same three-to-one pattern across all seven traits.
From this Mendel deduced that inheritance is carried by discrete, particulate units — what we now call genes — passed from parents to offspring. Each plant carries two copies of each unit, one from each parent. Some versions are "dominant" (round) and mask the presence of a "recessive" version (wrinkled), which only shows itself when a plant inherits two recessive copies. This was a radical break from the prevailing idea that parental traits simply blended in the offspring like mixed paints — a notion that could not explain how a hidden trait could disappear for a generation and then re-emerge intact.
Mendel presented his results to the Natural History Society of Brno in 1865 and published them in the society's journal in 1866. The paper was distributed to libraries and to individual scientists, but it sank almost without trace. The reasons are much debated: the journal was obscure, the mathematical style was unfamiliar to biologists of the day, and the wider scientific world was preoccupied with Darwin's newly published theory of evolution — to which Mendel's work was in fact the missing complement, explaining exactly how traits could be passed on. Mendel himself rose to become abbot of his monastery and, burdened by administrative duties and a dispute over taxation, largely gave up his research. He died in 1884, his great discovery unrecognised.
Rediscovery
The story turned in 1900. Independently and almost simultaneously, three botanists — Hugo de Vries in the Netherlands, Carl Correns in Germany and Erich von Tschermak in Austria — arrived at similar results and, on searching the literature, found that a Moravian friar had beaten them to it by thirty-five years. Their rediscovery finally launched the science of genetics, and the British biologist William Bateson became a fervent champion of Mendel's work, coining the word "genetics" a few years later.
Mendel had, in fact, uncovered two principles that still bear his name. His law of segregation holds that the two copies of each hereditary unit separate when reproductive cells form, so each offspring receives one at random from each parent. His law of independent assortment holds that different traits are inherited independently of one another — the colour of a seed says nothing about whether it will be round or wrinkled. Later biology would qualify the second rule, once it was found that genes sitting close together on the same chromosome tend to be inherited as a package, but as a first approximation Mendel's laws remain the bedrock on which classical genetics is built.
There is one lingering controversy. In the 1930s the great statistician and geneticist Ronald Fisher analysed Mendel's published figures and concluded that they fit the expected ratios almost too perfectly — closer to the ideal than random chance should normally allow. Some have taken this as evidence that Mendel, or an assistant, unconsciously tidied the data. Others argue the discrepancy can be explained by the way Mendel classified borderline plants, or by his stopping counts once a clear pattern emerged. Whatever the truth, the underlying laws have been confirmed countless times over. A quiet monk with a garden and a talent for arithmetic had, it turned out, uncovered one of the deepest rules of life.
Quiz nuggets
- Gregor Mendel was an Augustinian friar who conducted his heredity experiments at a monastery in Brno, in present-day Czech Republic.
- He worked with the pea plant Pisum sativum, studying seven contrasting traits across roughly 28,000 plants.
- Crossing purebred plants and letting the offspring self-fertilise produced a characteristic ratio of about three to one for dominant versus recessive traits.
- Mendel published his results in 1866, but they were ignored for about 35 years.
- His work was rediscovered around 1900 by Hugo de Vries, Carl Correns and Erich von Tschermak, founding modern genetics.