Incomplete dominance is a classic inheritance pattern where heterozygotes show an intermediate phenotype. Understanding how allele products interact links Mendelian genetics to molecular function and helps interpret offspring ratios that differ from simple dominant–recessive traits.

Core idea: why phenotypes “blend”

What incomplete dominance means

In incomplete dominance, neither allele fully determines the phenotype in a heterozygote; instead, the heterozygote expresses an intermediate (blended) phenotype relative to the two homozygotes.

Pink snapdragon flowers illustrate incomplete dominance in flower-color pigmentation: the heterozygote’s phenotype is intermediate between the two homozygotes. This makes the “blended” phenotype idea concrete while still emphasizing that alleles remain distinct and segregate in gamete formation. Source

This “blending” refers to the phenotype, not the alleles themselves—alleles remain discrete units that segregate during gamete formation.

How it differs from complete dominance

Incomplete dominance contrasts with complete dominance in a specific, testable way:

• Complete dominance: heterozygote phenotype matches one homozygote.

• Incomplete dominance: heterozygote phenotype is distinct from both homozygotes and intermediate along a measurable trait axis (such as pigment intensity).

Because the heterozygote is a third phenotype category, phenotypic ratios can mirror genotypic ratios in certain crosses.

Recognising incomplete dominance in crosses

Genotype–phenotype mapping

A single gene with two alleles can produce three phenotypes under incomplete dominance:

• Homozygote 1 (e.g., AA): phenotype 1

• Heterozygote (Aa): intermediate phenotype

• Homozygote 2 (aa): phenotype 2

A key AP Biology implication is that heterozygotes are not “hidden” by a dominant allele; they are often identifiable by phenotype.

Typical ratio pattern (conceptual)

When two heterozygotes for an incomplete-dominance trait mate, the expected outcomes commonly show:

This diagram set traces snapdragon crosses across generations, showing how heterozygote × heterozygote produces three genotype classes and three corresponding phenotype classes. The key visual takeaway is that the expected outcomes align as a 1:2:1 ratio because each genotype maps to its own phenotype rather than being masked by dominance. Source

• Genotypic ratio: 1 AA : 2 Aa : 1 aa

• Phenotypic ratio: 1 phenotype 1 : 2 intermediate : 1 phenotype 2

This happens because each genotype corresponds to its own phenotype category, unlike complete dominance where two genotypes can share one phenotype.

Biological basis: why an intermediate phenotype occurs

Incomplete dominance often reflects how gene products function in cells. Common mechanisms include:

• Dosage (amount) effects: one functional allele produces less product than two functional alleles, yielding an intermediate trait value (for example, reduced pigment production).

• Protein activity levels: the heterozygote has an intermediate level of enzyme activity or structural protein function.

• Thresholds not reached: one allele’s product alone may be insufficient to reach the “full” phenotype seen in the homozygote.

The important conceptual link is that the phenotype is often proportional to gene product quantity or activity, so heterozygotes fall between homozygotes.

How to describe incomplete dominance precisely

Use careful language

To stay accurate:

• Say the heterozygote shows an intermediate phenotype.

• Avoid implying alleles “mix” permanently; alleles segregate into gametes and can reappear unchanged in future generations.

• Emphasize that neither allele masks the other at the level of phenotype expression.

Distinguish from look-alikes (without changing the topic)

Some traits can resemble incomplete dominance but are conceptually different:

• True incomplete dominance: one gene, two alleles, three phenotypes, intermediate heterozygote.

• Traits influenced by measurement error or broad categories may falsely appear “intermediate”; careful phenotyping matters.

Common pitfalls students should avoid

• Confusing incomplete dominance with codominance: in incomplete dominance, the heterozygote is intermediate; it does not show both parental phenotypes side-by-side.

• Assuming “blending inheritance” means alleles lose identity; they do not.

• Treating the intermediate phenotype as environmental; incomplete dominance is a genetic pattern (though environment can still influence many traits in general).