In genetics, incomplete dominance occurs when neither of two different alleles completely masks the effect of the other. As a result, an organism with two different alleles—called a heterozygote—shows a trait that is intermediate between the traits associated with the two homozygous genotypes.
A classic example comes from flower color. Suppose a plant has one allele associated with red flowers and another associated with white flowers. If the red allele were completely dominant, a plant with one red allele and one white allele would have red flowers. Under incomplete dominance, however, the heterozygous plant may have pink flowers, producing an intermediate phenotype.
The important point is that incomplete dominance does not mean that the two alleles blend together permanently. The alleles remain distinct genetic variants and can be passed separately to offspring.
How incomplete dominance works
An allele is a version of a gene. For a particular gene, an individual typically inherits one allele from each biological parent.
Consider a simplified flower-color example:
- RR → red flowers
- WW → white flowers
- RW → pink flowers
The two homozygous genotypes, RR and WW, produce different phenotypes. The heterozygous genotype, RW, produces a phenotype between them.
This is why incomplete dominance is sometimes described as partial dominance. Neither allele is sufficiently dominant to determine the entire phenotype by itself in the heterozygote.
The intermediate appearance results from how the gene’s products influence the organism’s biology. It is not because the DNA sequences of the two alleles physically merge.
Incomplete dominance vs. complete dominance
The distinction becomes clearer when the same genetic situation is compared with complete dominance.
With complete dominance, one allele determines the phenotype of a heterozygote. If R is completely dominant over W:
| Genotype | Complete dominance | Incomplete dominance |
|---|---|---|
| RR | Red | Red |
| RW | Red | Pink |
| WW | White | White |
The key difference is the heterozygote.
In complete dominance, RR and RW have the same observable phenotype even though their genotypes differ. In incomplete dominance, RR, RW, and WW can all have distinguishable phenotypes.
This makes incomplete dominance especially useful when explaining why genotype and phenotype are not interchangeable terms. A genotype describes the alleles an organism carries; a phenotype describes the observable characteristics produced by the interaction of those alleles with biological processes and, in many cases, the environment.
Why the heterozygote can have an intermediate phenotype
Incomplete dominance often arises because a heterozygote produces an amount or activity of a gene product that falls between the levels produced by the two homozygotes.
For example, imagine that a particular flower-color pigment is produced when a functional allele contributes to pigment production. A plant with two copies of the allele might produce abundant pigment and appear red. A plant with two nonfunctional copies might produce little or no pigment and appear white. A heterozygote with one copy of each could produce an intermediate amount of pigment and appear pink.
This is a simplified model, but it illustrates an important principle: dominance is a relationship between alleles as expressed in a particular phenotype; it is not a statement that one allele is inherently stronger than another.
The molecular mechanisms behind real traits can be considerably more complicated. Many traits involve multiple genes, regulatory processes, environmental influences, or nonlinear biological pathways.
Incomplete dominance is not genetic blending
The pink-flower example can create a misleading impression that red and white alleles have blended into a new “pink allele.” They have not.
If two pink heterozygous plants in the simplified example reproduce, their offspring can inherit the alleles independently:
RW × RW
The possible offspring genotypes are:
- RR
- RW
- RW
- WW
That gives an expected genotype ratio of 1 RR : 2 RW : 1 WW.
Because the three genotypes have different phenotypes in this example, the expected phenotype ratio is also 1 red : 2 pink : 1 white.
This is one of the clearest ways to distinguish incomplete dominance from the old idea of hereditary blending. The intermediate phenotype in the parent generation does not prevent the original allele variants from reappearing in later generations.
How incomplete dominance differs from codominance
Incomplete dominance is often confused with codominance, but the two patterns are different.
With incomplete dominance, the heterozygote has a phenotype that is intermediate between the two homozygotes.
With codominance, both alleles are expressed distinctly in the heterozygote.
Human ABO blood types provide a familiar example of codominance. A person with an A allele and a B allele has type AB blood because the A and B antigens are both expressed. The phenotype is not an intermediate “A-B” version of the two.
The distinction can be summarized this way:
| Inheritance pattern | Heterozygote |
|---|---|
| Complete dominance | Resembles the dominant homozygote |
| Incomplete dominance | Shows an intermediate phenotype |
| Codominance | Shows contributions of both alleles distinctly |
These are patterns of how alleles are expressed, not categories that apply universally to every trait controlled by a particular gene.
A classic plant example
The four-o’clock plant, Mirabilis jalapa, is commonly used in genetics education to illustrate incomplete dominance. In a simplified cross, plants with red flowers and plants with white flowers can produce heterozygous offspring with pink flowers.
Crossing two pink-flowered plants can then produce red, pink, and white offspring in the characteristic 1:2:1 ratio, assuming the simplified single-gene model and sufficiently large offspring numbers.
The example is valuable because the phenotype of the heterozygote makes the underlying genotype pattern relatively easy to see.
Does incomplete dominance apply to humans?
Incomplete dominance can occur in humans, but it should not be used as a general explanation for most human traits.
Human characteristics are frequently influenced by multiple genes and environmental factors. Even when a single gene has an allele relationship that resembles incomplete dominance, the resulting human phenotype may not fit a simple textbook three-genotype, three-phenotype pattern.
A useful rule is to avoid assuming that every trait with an intermediate appearance demonstrates incomplete dominance. An intermediate phenotype can arise for many biological reasons, including interactions among multiple genes and environmental effects.
Why the concept matters
Incomplete dominance helps illustrate several fundamental ideas in genetics.
First, dominant does not mean stronger, better, or more common. Dominance describes how different alleles affect a phenotype when they occur together in a heterozygote.
Second, heterozygotes can have their own recognizable phenotype. They are not necessarily phenotypically identical to one of the two homozygotes.
Third, alleles remain discrete units of inheritance even when their effects produce an intermediate appearance. The pink flowers in the classic example do not carry a permanently blended red-and-white allele.
Finally, incomplete dominance shows why genetic inheritance is best understood by considering both genotypes and the biological mechanisms that connect genotype to phenotype. A simple Punnett square can predict possible allele combinations, but the observable trait depends on how those genetic differences are actually expressed.
