In the nineteenth century, heredity was imagined as the blending of two parents' traits like fluids — pink from red and white flowers, medium height from tall and short. But this "blending" view could not solve a puzzle: if mixing were irreversible, over generations all variation would dissolve into a single average. Even Darwin could not resolve this.

Gregor Mendel (1822–1884), a friar at the Augustinian monastery in Brünn (today Brno, Czechia), sought the answer in the monastery garden. Over eight years he systematically crossed peas from which he had bred pure lines, tracking single traits one at a time: seed colour (yellow/green), seed shape (round/wrinkled), plant height (tall/short). He counted some 28,000 plants — this was not gardening but a quantitative experiment.

The results showed a startling regularity. When tall and short peas were crossed, the entire first generation was tall; shortness seemed to have vanished. But in the second generation shortness returned — and in a persistent ratio of 3:1. Mendel drew the conclusion: heredity does not blend; each trait is carried by two discrete "factors", one dominant and one recessive, which pass unchanged from generation to generation and pair at random in fertilization. The recessive trait is hidden but not erased.

Mendel presented his findings in 1865 and published them in 1866 as "Experiments on Plant Hybrids". The biologists of the day failed to grasp their importance; the paper sat on library shelves for thirty-five years. In 1900 three separate botanists independently rediscovered the same laws and recognised Mendel's priority. What he called a "factor" was named the "gene"; in 1953 the structure of DNA gave these discrete units their chemical form. Heredity was not the mixing of a fluid but the copying of discrete messages.