Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step of Mendel's reasoning. The garden lab uses real random draws.
Tap the buttons and see what happens.
Mix the paints, then cross the peas twice. Which one blends?
Compare a true blend with the F1 and F2 of a pea cross.
Mix the paint. Then mix the flowers. Which one turns pink?
Tap a trait. Is there ever anything in between?
Each character has two discrete forms. Tap through all seven.
Tap a button. See the two kinds.
Drag the brush to the purple flower, then to the white one.
Transfer pollen from the donor to the emasculated flower.
Drag the brush to the purple flower, then to the white one.
Pick each parent's two factors. How many boxes turn white?
Choose genotypes for both parents and read off the expected ratio.
Grow 10, 100, then 1,000 plants. Where does the share of white settle?
Each plant is a random Pp × Pp draw. How does scatter shrink with sample size?
A simple model: each parent passes P or p with a 50% chance.
Tap a year on the timeline.
Tap each year: experiments, publication, neglect and rediscovery.
Tap a year to hear what happened.
Long ago, a man named Gregor Mendel lived in a stone monastery in a city called Brno. He was a teacher, and he had a big garden full of peas.
People thought children were a mix of their parents, like red and white paint making pink. Mendel wanted to find out if that was true.
Easy to sort. A pea flower is purple or white. A pea is round or wrinkly, yellow or green. Never in between!
Fast to grow. Peas grow up quickly, and you can grow lots and lots of them.
He could pick the parents. He took the pollen out of one flower and brushed on pollen from another one.
Purple and white made only purple flowers. No pink! But the next time, a few white flowers came back. White was hiding inside the purple plants.
Mendel said each plant has two hidden signs, one from each parent. Today we call them genes. They are made of DNA.
What is DNA? → · How did humans evolve? → · Plant cells and animal cells → · ← More explainers
Gregor Mendel chose the garden pea (Pisum sativum) because it made heredity easy to see and easy to count. Peas come in many distinct varieties, they are easy to grow, their pollination can be controlled, and most of their seeds sprout.4 In plain words, peas gave him four big advantages:
With that plant, and about eight years of patient work, he discovered the basic rules of heredity.65
Gregor Mendel was born Johann Mendel on July 20, 1822, in Heinzendorf, a village in the Austrian Empire that is now Hynčice in the Czech Republic.4 In 1843 he joined the Augustinian St Thomas's Abbey in Brünn (today's Brno, in the Czech Republic) and took the name Gregor.5 He was a friar, and he also taught physics and natural science at a secondary school in the city.14
The abbey sent him to the University of Vienna from 1851 to 1853, where he studied physics and mathematics (one of his teachers was Christian Doppler, of the "Doppler effect") and learned about plant anatomy and the microscope.45 That training in maths is a big part of the story. Mendel didn't just look at his plants. He counted them, and he looked for patterns in the numbers.4
Back at the abbey, Abbot Cyril Napp, a keen farmer, let Mendel run a large breeding experiment in the monastery garden.45
Most people thought heredity worked like mixing paint. A child was a blend of its parents, so a purple parent and a white parent should make something in between.21 Even Charles Darwin's idea of heredity was built around blending.5 Traits such as human height really do look blended, because many genes act together on them.1 Mendel's peas, with their either-or traits, showed that something else was going on underneath.
He didn't grab the first seeds he found. Mendel bought 34 more or less distinct pea varieties from seed sellers and grew them for two years, to check that each one bred true.6 He kept 22 of them for his experiments.62 Then he picked seven traits, each with two clear forms:1
| Trait | Dominant form | Recessive form | 2nd-generation ratio he found |
|---|---|---|---|
| Flower colour | Purple (violet) | White | 3.15 : 1 |
| Flower position | Along the stem (axial) | At the tip (terminal) | 3.14 : 1 |
| Plant height | Tall | Short (dwarf) | 2.84 : 1 |
| Seed shape | Round | Wrinkled | 2.96 : 1 |
| Seed colour | Yellow | Green | 3.01 : 1 |
| Pod shape | Inflated | Pinched (constricted) | 2.95 : 1 |
| Pod colour | Green | Yellow | 2.82 : 1 |
Ratios from OpenStax Biology, based on Mendel's 1866 paper.1 Every one of them lands close to 3 to 1.
Because a pea flower keeps its pollen-making stamens and its pollen-catching stigma tucked inside a boat-shaped petal called the keel, it usually pollinates itself before it even opens.61 To make a cross, Mendel had to step in:
Mendel admitted this was "a somewhat elaborate process", but wrote that it "nearly always succeeds".6 If you want to see how flowers normally get pollinated, with bees doing the job, try our explainer on how pollination works.
Take flower colour. When Mendel crossed true-breeding purple plants with true-breeding white ones, every plant in the first generation had purple flowers. No pink, no pale purple.1 Then he let those purple plants pollinate themselves. In the second generation he counted 705 purple and 224 white plants, a ratio of 3.15 to 1.1 The white hadn't been destroyed. It had been hiding.
705 purple : 224 white ≈ 3 : 1
Mendel's explanation was simple and brilliant:12
Write the purple factor as P and the white factor as p. The first-generation plants are all Pp: purple on the outside, carrying a hidden white factor. When two Pp plants are crossed, each parent passes on P or p with equal chances. A grid called a Punnett square shows the four equally likely combinations:
| P | p | |
|---|---|---|
| P | PP (purple) | Pp (purple) |
| p | Pp (purple) | pp (white) |
Three boxes out of four give purple and one gives white: 3 to 1. Mendel even checked the hidden part. He grew on the purple second-generation plants and found that one third of them bred true (PP) while two thirds were hybrids (Pp), just as the grid predicts. So the 3 : 1 is really 1 : 2 : 1.46
Because 3 to 1 is a probability, not a promise. In a family of just four plants you could easily get 4 purple and 0 white, or 2 and 2. Only when you count big numbers does the true ratio show. Mendel reported results from 19,959 second-generation plants alone,1 and over the whole project he grew nearly 30,000 pea plants between 1856 and 1863.15 Britannica points out that earlier plant breeders had missed the pattern because they didn't grow large enough populations or follow single traits separately.4
The "grow a garden" lab above works the same way: every plant is a real random draw with a 1-in-4 chance of being white, so small gardens wobble and big ones settle near 25%.
You can run Mendel's cross with two coins. Each coin is one parent passing on a factor: heads = purple (P), tails = white (p).
This works because each coin has a 1-in-2 chance of tails, and both must land tails: ½ × ½ = ¼.1
In 1936 the statistician Ronald A. Fisher analysed Mendel's numbers and argued that they agreed with the expected ratios more closely than chance would normally allow. He suggested they might have been adjusted, perhaps by an assistant who "knew too well what was expected".7 Later researchers have argued that Fisher's reconstruction of how Mendel ran his experiments was too narrow, and that the data are reasonable.7 Historians still discuss it. Either way, scientists still use the rules Mendel found to explain the basics of inheritance.2
Mendel presented his results at two meetings of the Natural History Society of Brünn, on February 8 and March 8, 1865, and published them in 1866 as "Experiments in Plant Hybridization".35 Almost nobody paid attention. Most scientists still believed in blending, and the paper appeared in a small local journal.15 In 1868 Mendel became abbot of the monastery, and his new duties left little time for experiments.15 He died on January 6, 1884.4
In 1900, three botanists, Hugo de Vries, Carl Correns and Erich von Tschermak, independently reached the same results and brought his paper back to light.51 It was soon translated and read around the world.5
Mendel turned heredity into a science.3 His "factors" are what we now call genes, and the different versions of a gene (like P and p) are called alleles.2 We now know genes are made of DNA; read more in why DNA is important. The way a plant shares out its two copies matches how cells divide to make pollen, eggs and sperm, which you can explore in why cells divide. His rules also filled a gap in the theory of evolution, which you can follow in how humans evolved. Peas are plants, so all of this happens inside plant cells; see how plant and animal cells differ.
Scientists have even found the exact gene behind one of his traits: in 1990 researchers showed that wrinkled peas carry an extra piece of DNA inserted into a gene for a starch-making enzyme.3 Nature Education sums up his biggest contribution as his method: making predictions with maths and then testing them with experiments.2
Pea plants have traits that come in two clear forms with nothing in between (purple or white flowers, round or wrinkled seeds, tall or short plants). They also grow to full size in one season, can be grown in large numbers, normally pollinate themselves so pure 'true-breeding' lines are easy to keep, and can be crossed by hand so the scientist decides exactly who the parents are.
Between 1856 and 1863 he crossed pea plants with different traits and counted their offspring over several generations, nearly 30,000 plants in all. When purple crossed with white gave only purple, and the next generation came out about 3 purple to 1 white, he explained it with pairs of hidden 'factors', one from each parent, some dominant and some recessive. Today we call those factors genes.
About eight years. His paper describes 'eight years' pursuit', and historians usually date the pea experiments from 1856 to 1863. Before that, he spent two years testing 34 pea varieties to be sure they bred true.
Nearly 30,000 pea plants over the whole project, according to the OpenStax biology textbook. For one generation of his main experiments alone (the second generation, or F2), he reported results from 19,959 plants.
Flower colour (purple or white), flower position (along the stem or at the tip), plant height (tall or short), seed shape (round or wrinkled), seed colour (yellow or green), pod shape (inflated or pinched) and pod colour (green or yellow).
He was the first to show, with careful counts and maths, that traits are passed on as separate units that don't blend, and to describe the rules for how they are shared out: dominant and recessive factors, the law of segregation and the law of independent assortment. Those rules are still the starting point of genetics.
He published in 1866 in the journal of a local natural history society, and most scientists at the time believed in blending inheritance. His maths-heavy approach was unusual for biology then. His work was only rediscovered in 1900, 16 years after he died, when three other scientists reached the same results.