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The beans that separated appearance from inheritance

Copenhagen, Denmark; experiments reported in 1903

Wilhelm Johannsen studied lines of self-fertilizing plants descended from individual ancestors. Seeds within a line varied in size, but selecting larger or smaller seeds within that line did not produce the same lasting shift as selecting between different lines. A visible difference was not automatically a hereditary difference.

Researcher
Wilhelm Johannsen
Material
Self-fertilizing bean plants
Comparison
Seeds within and between pure lines

The caveat

Selection can change a population when the selected differences are inherited. Johannsen’s result concerned variation within his pure lines.

The experiments helped him separate visible differences from inherited ones. He developed the terminology of genotype and phenotype in his subsequent writing, including his 1909 book on heredity.

How it connects

Johannsen's pure lines made the distinction between observed variation and inherited variation experimentally useful.

This moment appeared in What you see is an outcome, the Involves connection for November 2, 2026, which asked: Why doesn't a visible difference tell you what caused it?

Check yourself

What was the key distinction in Johannsen's selection experiments?

Variation within a pure line differed from variation between hereditary lines. Exactly. Selecting a visible difference does not ensure a lasting inherited change when that difference is not heritable.

What the sources establish

  • Johannsen's pure-line experiments, reported in 1903, distinguished variation among observed seeds from differences between hereditary lines.

Sources