What is incomplete dominance, and how is it different from codominance?

Focus on the heterozygote—the individual with two different alleles. Is its phenotype between the two homozygote phenotypes, or are both allele-associated features expressed at once?

In incomplete dominance, the heterozygote’s phenotype is intermediate between the contrasting homozygous phenotypes. OpenStax gives red-flowered and white-flowered snapdragons producing pink-flowered heterozygotes as an example. In codominance, both alleles are expressed simultaneously in the heterozygote. [S1]

Here’s the contrast in a compact comparison:

Incomplete dominance — The heterozygote has an intermediate phenotype.

Codominance — The heterozygote expresses both alleles simultaneously.

The key is the phenotype shown, not just the allele letters. Neither pattern should be assumed for every trait; use what the problem or diagram actually states. [S1]

Imagine a fictional classroom plant with two alleles for a flower-color trait, R and W. The diagram labels RR as red, WW as white, and RW as pink. Since the heterozygote RW is shown as intermediate between the two homozygotes, this fictional diagram illustrates incomplete dominance.

Now imagine a separate, explicitly fictional diagram with RR shown as red, WW as white, and RW shown with distinct red and white areas at the same time. If the diagram represents both features as expressed together, that would illustrate codominance—not an intermediate color.

The allele labels and plant are placeholders, not claims about a real organism. When solving your own problem, describe only what the diagram depicts. Don’t infer that an appearance is intermediate or simultaneous if its labels or legend do not make that clear.

First, identify which combination is heterozygous: it has two different allele labels, such as RW in our fictional example. Then compare its picture with the two homozygous combinations, where the allele labels match, such as RR and WW.

Ask what the picture communicates. Does the heterozygote appear between the two homozygote phenotypes, like the fictional pink example? Or does it show both features at once, like separate red and white areas? Those point to different patterns. If the picture is too small or ambiguous, consult the diagram’s key, caption, or accompanying text rather than guessing.

Keep genotype and phenotype distinct while reading: the letters show an allele combination; the pictured trait is the phenotype. If the labels or picture do not provide enough evidence, say what is unclear instead of forcing a conclusion.

You can adapt this explanation, replacing the bracketed parts with details from your diagram:

“[Allele combination] is heterozygous because it contains two different alleles. Its phenotype is [intermediate between the two homozygote phenotypes / shows both features together]. Therefore, this diagram illustrates [incomplete dominance / codominance]. The diagram supports this conclusion because [describe what it shows].”

For the fictional example, you could write: “RW is heterozygous. Its pink phenotype is intermediate between the red RR and white WW phenotypes, so this example illustrates incomplete dominance.” That conclusion follows the fictional diagram; it is not a claim about a real plant.

Don’t label a pattern from the letters alone. Explain how the heterozygote’s displayed phenotype compares with both homozygotes. These patterns apply to particular cases, not every trait. [S1]