Mendel's Pea Experiments (1865): Founding the Science of Genetics

Gregor Mendel’s pea plant experiments, carried out in a monastery garden in Brno between 1856 and 1863 and presented in 1865, revealed the basic rules of heredity. By crossing thousands of pea plants and counting their offspring, Mendel showed that traits are passed on by discrete units in predictable ratios. Ignored for 35 years, his work became the foundation of the modern science of genetics.

Background: heredity before Mendel

In the mid-nineteenth century nobody knew how parents pass characteristics to their offspring. The most common idea was blending inheritance, in which the traits of the two parents mixed like paints, so that offspring were an average of both. Plant breeders had long produced hybrids, but they had not found consistent laws behind the results.

The question mattered for the new theory of evolution. In On the Origin of Species, published in 1859, Charles Darwin argued that natural selection acts on variation between individuals, yet he could not explain how variation was inherited. Under blending, useful differences should be diluted away within a few generations.

The monk in the garden

Johann Mendel was born on 20 July 1822 in Heinzendorf (now Hynčice in the Czech Republic), in Austrian Silesia, to a family of small farmers. To pursue an education he joined the Augustinian Abbey of St Thomas in Brünn (now Brno) in 1843, taking the name Gregor. The abbey was a centre of learning, with members active in science, agriculture and teaching.

From 1851 to 1853 the abbey sent Mendel to the University of Vienna. There he studied physics under Christian Doppler and botany under Franz Unger, and learned the value of careful experiment and statistics. He returned to Brno as a teacher of natural science and began his experiments in the abbey garden.

The experiments

Between about 1856 and 1863 Mendel grew and tested tens of thousands of pea plants (Pisum sativum); commonly quoted estimates are around 28,000 to 29,000. He chose seven traits that came in two clearly different forms: seed shape (round or wrinkled), seed colour (yellow or green), seed coat colour, pod shape, pod colour, flower position and stem length (tall or dwarf).

He first established pure-breeding lines that always produced the same form. Then he cross-pollinated plants with contrasting forms by hand, transferring pollen with a brush and removing the stamens to prevent self-pollination. In the first hybrid generation, all offspring showed only one form, which he called dominant; the other, which he called recessive, seemed to disappear.

When the hybrids were allowed to self-pollinate, the recessive form reappeared in about a quarter of the next generation. For seed shape, for example, he counted 5,474 round and 1,850 wrinkled seeds, a ratio of about 2.96 to 1. Similar 3:1 ratios appeared for all seven traits. For seed colour he counted 6,022 yellow and 2,001 green seeds, a ratio of about 3.01 to 1.

Mendel then followed the plants into further generations. He found that the recessive plants always bred true, while only about a third of the dominant-looking plants did; the other two-thirds behaved like the original hybrids and again produced a 3:1 mixture. The hidden structure behind the 3:1 ratio was therefore 1:2:1, one pure dominant to two hybrids to one pure recessive.

The choice of the pea was crucial to his success. Peas grow quickly, produce many seeds, normally fertilise themselves inside a closed flower and were available from seed merchants in many distinct varieties. Mendel also tested the purity of his starting lines for two years before beginning the crosses, a precaution many earlier hybridisers had not taken.

Mendel’s laws

From these numbers Mendel reasoned that each plant carries two hereditary “factors” (he called them Elemente) for each trait, one from each parent. These factors do not blend: they separate when pollen and egg cells are formed, and each reproductive cell receives only one. This principle is now called the law of segregation.

By following two traits at once, such as seed shape and seed colour, he found that they were inherited independently, giving a ratio of about 9:3:3:1 in the second generation. This became known as the law of independent assortment. Later research showed that it holds only for genes that lie on different chromosomes or far apart on the same one.

Mendel’s use of large numbers, controlled crosses and mathematical ratios was unusual for biology at the time. It treated heredity as a problem of probability, rather than of vague tendencies.

Presentation and silence

Mendel presented his findings in two lectures to the Natural History Society of Brünn on 8 February and 8 March 1865. The paper, “Versuche über Pflanzen-Hybriden” (“Experiments on Plant Hybridisation”), appeared in the society’s proceedings for 1865, published in 1866. Copies reached libraries across Europe, and Mendel sent reprints to several scientists.

The response was almost nothing. Mendel corresponded with the Munich botanist Carl Nägeli, a leading authority on heredity, who was unconvinced and suggested he work with hawkweed (Hieracium). Hawkweed often produces seeds without fertilisation, a fact unknown at the time, so the experiments gave confusing results that seemed to contradict the pea findings.

In 1868 Mendel was elected abbot of St Thomas’s. Administrative duties and a long dispute with the government over taxation of the monastery took up much of his time. He died on 6 January 1884, his scientific work still almost unknown.

Rediscovery in 1900

In 1900 three botanists, Hugo de Vries in the Netherlands, Carl Correns in Germany and Erich von Tschermak in Austria, published hybridisation results that matched Mendel’s and acknowledged his earlier paper. The extent to which each had arrived independently at the same conclusions is still discussed by historians.

The English biologist William Bateson became Mendel’s most energetic champion, translating his work and coining the word “genetics” in 1905. The Danish botanist Wilhelm Johannsen introduced the word “gene” in 1909. Soon Mendel’s factors were linked to chromosomes, and in the 1930s and 1940s Mendelian genetics and Darwinian natural selection were combined in what became known as the modern synthesis.

Myths and facts

A popular story holds that Darwin owned an unread copy of Mendel’s paper. There is no evidence for this, and Darwin’s library does not appear to have contained it. Darwin did know the work of other hybridisers who cited Mendel, but never engaged with his findings.

Another debate concerns Mendel’s data. In 1936 the statistician Ronald Fisher argued that the results were closer to expected ratios than chance would normally allow. Explanations range from an assistant’s bias to unconscious rounding or selective reporting, and some scholars defend the data. The debate does not affect the correctness of Mendel’s conclusions.

Timeline

• 1822, 20 July: Johann Mendel is born in Heinzendorf, Austrian Silesia.
• 1843: He enters St Thomas’s Abbey in Brünn and takes the name Gregor.
• 1851–1853: Studies at the University of Vienna.
• c. 1856–1863: Conducts his pea hybridisation experiments.
• 1865, 8 February and 8 March: Presents his results to the Natural History Society of Brünn.
• 1866: “Experiments on Plant Hybridisation” is published.
• 1884, 6 January: Mendel dies in Brno.
• 1900: De Vries, Correns and Tschermak rediscover his work.

Why it matters

Mendel showed that inheritance follows simple, measurable rules carried by discrete units, solving the puzzle that had troubled Darwin. His approach made heredity a quantitative science and led, through the rediscovery of 1900, to chromosome theory, the structure of DNA and modern genetics, medicine and plant breeding.