Gregor Mendel and Heredity

Gregor Mendel (1822-1884) was an Austrian monk who is now known as the "father of genetics", although in his own lifetime almost nobody noticed his work. By patiently growing thousands of pea plants in the garden of his monastery in Brno (now in the Czech Republic), Mendel worked out the basic rules of how features are passed from parent to offspring, over 30 years before anyone else came close. His discoveries were ignored for almost 35 years, then suddenly rediscovered in 1900 and became the foundation of modern genetics.

• Born1822, HeinzendorfNow in the Czech Republic
• ProfessionAugustinian monkAt St Thomas's Abbey in Brno
• Experiment plantPea plant (Pisum sativum)Easy to control and cross-breed
• Pea plants grownapprox. 28,000Over 7 years of careful experiments
• Published1866In a small Moravian science journal
• Rediscovered190016 years after his death

Who was Mendel?

Gregor Mendel was born Johann Mendel on 20 July 1822 in a small village in what is now the Czech Republic. He came from a poor farming family but was a bright student, and at 21 he joined the Augustinian monastery in Brno as a way to keep studying. He took the religious name Gregor. The monastery was an unusual place: it was one of the leading centres of scientific learning in central Europe at the time, and the monks were strongly encouraged to do research alongside their religious duties.

Mendel briefly studied physics and biology at the University of Vienna in his 30s, then returned to the monastery where he became a teacher and eventually the abbot. From 1856 to 1863 he carried out his famous experiments in the monastery garden, growing thousands of pea plants and carefully recording the features of each one.

Why pea plants?

Mendel chose pea plants because they had several useful features for genetics experiments:

• They grew fast and easily in a garden.
• They had clear-cut features that came in only two versions, easy to count: tall vs short, smooth vs wrinkled peas, green vs yellow peas, purple vs white flowers.
• They could be cross-pollinated by hand (Mendel just brushed pollen from one plant onto the female parts of another, then bagged the flower to prevent any other pollen getting in).
• They normally self-pollinated, so it was easy to get pure-breeding lines.

Mendel grew around 28,000 pea plants over 7 years, carefully cross-breeding them and recording every result. This patient counting was the secret of his success.

What Mendel discovered

Mendel started by taking pure-breeding lines (always tall, or always short, for example) and crossing them. The first generation of offspring were ALL tall: the short feature seemed to have vanished. But when Mendel then let those first-generation offspring self-pollinate to make a second generation, the short feature came back. Roughly 3 out of every 4 of the second-generation plants were tall, and 1 out of every 4 was short.

From this Mendel worked out:

• Each plant carries two "factors" (we now call them alleles) for every feature, one from each parent.
• One factor can be dominant (its effect shows up even with only one copy) and the other recessive (only shows up with two copies).
• When pollen and eggs are formed, the two factors separate, so each pollen or egg only carries one of them.
• The chances of which factor a particular offspring gets are entirely random.

Together these became known as Mendel's laws of inheritance, and they are still the foundation of genetics today.

Why nobody noticed

Mendel published his results in 1866 in a small science journal of the Brno Natural History Society. He sent copies of his paper to several leading biologists around Europe (including Darwin), but almost nobody read it carefully, and those who did mostly did not understand it. The maths-heavy approach of counting peas in ratios was completely new and seemed strange to biologists of the time. Darwin himself had a copy in his library; the pages were still uncut (unread) when he died.

Mendel was disappointed but kept his good humour. He famously said: "My time will come." He died in 1884 still essentially unknown as a scientist. Then in 1900, his paper was independently rediscovered by three different European botanists (Hugo de Vries, Carl Correns and Erich Tschermak) who had been doing similar experiments and were astonished to find that a monk had worked it all out 35 years earlier. Mendel was finally recognised as the founder of a whole new science: genetics.

Fact Mendel's exact 3:1 ratio of tall to short pea plants is famously a bit too exact. When the great 20th century statistician Ronald Fisher reanalysed Mendel's numbers in 1936, he calculated that the results were so close to the predicted ratios that they had only about a 1-in-30,000 chance of happening by random luck. Many historians now think Mendel (or his assistants) may have been a little selective about which plants to count, but his fundamental discoveries are entirely correct.

Mendel's legacy

Mendel's rules turned out to apply not just to peas but to almost every living thing on Earth, from bacteria to humans. The "factors" he predicted in 1865 turned out to be real physical things called genes, made of DNA, sitting on chromosomes inside the cells. Modern genetics, the entire fields of plant and animal breeding, medical genetics and the Human Genome Project all trace back to one bright monk in Brno carefully counting peas.

Did you know? Mendel was also a beekeeper and an enthusiastic meteorologist: for over 20 years he kept careful daily weather records for Brno and was the first person to forecast tornadoes in the area. He once accidentally bred a strain of stingless bees by crossing different bee species: useful, but unfortunately the resulting bees were so aggressive he had to destroy them all. Sometimes science does not go to plan.

Deeper dive: what Mendel could not know

Mendel worked out a complete and accurate theory of inheritance based on the patterns he saw in his pea plant data. Yet he had no way of knowing what his "factors" actually were, or where in the plant they lived. He never used the word "gene" (it was coined in 1909, 25 years after his death). He had no idea that the factors sat on structures called chromosomes inside the nucleus of every cell. He could not have imagined that they were made of a particular chemical called DNA, or that the DNA was written in a 4-letter code, or that this code was the same in every living thing on Earth.

The 20th century filled in all of those details bit by bit. In 1903 the American biologist Walter Sutton noticed that chromosomes behave in cells exactly the way Mendel's factors behave in his pea plants, and proposed that genes sit on chromosomes. In the 1940s, Oswald Avery showed that the inheritance information itself was carried by DNA. In 1953 Watson, Crick, Franklin and Wilkins worked out the DNA double helix. In 2003 the entire human genome was sequenced.

Every one of those discoveries built on the foundation of Mendel's 28,000 pea plants. He had no idea what he had really discovered, but every modern geneticist still works in the building that he laid the first brick of.

For modern inheritance, see inheritance. For what genes really are, see genes and chromosomes.