Mendel's pea plants

In 1865 a monk in Brno reported crosses of nearly 30,000 pea plants, and the same 3 to 1 ratio kept turning up.

Why peas

In Mendel's time most scientists believed in blending inheritance: offspring were thought to be a smooth mix of their parents, the way red and white paint make pink. If that were true, a trait could never vanish and then come back. Mendel picked an organism that let him test the idea cleanly. Garden peas normally fertilise themselves inside a closed flower, so a line of plants that always gives the same flower colour or seed shape, called true-breeding, is easy to keep. Peas grow to maturity in one season, and he could raise thousands at once, so chance could not explain his results.

He also chose traits that come in clear either-or forms: violet or white flowers, round or wrinkled seeds, yellow or green seeds, tall or dwarf plants. Seven characteristics in all. To cross two plants he removed the pollen-making parts of one flower before they matured, then brushed on pollen from the other variety by hand. The parent plants are called the P generation, their offspring the F1 generation, and the offspring of F1 plants left to self-fertilise the F2 generation.

A trait that hides and comes back

When Mendel crossed true-breeding violet plants with true-breeding white ones, every F1 plant had violet flowers. Blending would have predicted pale violet. Then he let the F1 plants fertilise themselves, and white flowers came back: 705 violet and 224 white in the F2, a ratio of 705 ÷ 224 ≈ 3.15 to 1. The other six traits behaved the same way. One form disappeared in F1 and returned in about a quarter of F2. He called the form that showed in F1 dominant and the form that hid recessive.

The table, from OpenStax Biology, lists Mendel's F2 counts; together they total 19,959 observations. None of the ratios is exactly 3:1, and that is what real data look like. The larger the count, the closer it tends to sit to 3. The seed traits, with thousands of seeds, land at 2.96 and 3.01, while pod colour, with 580 plants, lands at 2.82.

Mendel's explanation

Mendel proposed that each plant carries two copies of a "unit factor" for each characteristic, that each parent passes on one of its two copies at random, and that the copies stay separate rather than blending. An F1 plant from violet × white holds one violet factor and one white factor; the violet factor masks the white one. When two such plants are crossed, each passes on violet or white with equal chance, so a quarter of the offspring get two white factors and show white flowers. Today the unit factors are called genes, and the different versions of a gene are called alleles. Use the square below to check the 3:1 for yourself, then press Grow and compare a random count with Mendel's.