Which is a phenotype?

The measured height of a bean plant. Exactly. A measured trait is phenotypic; genotype refers to genetic makeup, while environmental inputs help shape the result.

Two plants have different heights. What follows from that observation alone?

Their phenotypes differ, but the cause is not yet established. Exactly. Different outcomes can reflect inheritance, environment, or their interaction.

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.

From the archive · November 2, 2026

What you see is an outcome

Why doesn't a visible difference tell you what caused it?

01 · Word

Phenotype

Pronounced FEE-noh-type

noun

An organism's observable traits, produced through the interaction of its genetic makeup and its environment.

Examples

Leaf size is part of a plant's phenotype.
Two plants can differ in phenotype even when their inherited makeup is very similar.

Origin

The Danish botanist Wilhelm Johannsen introduced phenotype alongside genotype into the vocabulary of heredity in 1909. Phenotype describes observed traits; genotype describes genetic makeup.

02 · Idea

The result is not the recipe

Two plants may differ in height because of their genes, the light they received, or how their genes affect their response to light. Height alone cannot tell you which explanation fits. You need to compare their genes and growing conditions.

Botanist Wilhelm Johannsen distinguished observed traits from inherited makeup. Modern genetics studies how genes and environments produce those traits.

Different heights do not necessarily mean different genes.

Limits

Some traits have a strong relationship to particular genetic variants. The distinction does not imply that all traits are equally changeable or that genetic influences are unimportant.

03 · Moment

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.

The connection

Observable traits result from development involving both inheritance and environment.