Incomplete Dominance — Definition & Examples

What's Incomplete Dominance?

Incomplete dominance occurs when neither of two alleles is fully dominant nor recessive towards each other. The alleles are both expressed and the phenotype, or physical trait, is a mixture of the two alleles. In less technical terms, this means that the two possible traits are blended together.

What Is an Example?

In the Punnett square above, two homozygous flowers are crossed (RR x rr), producing all heterozygous (Rr) offspring. It is easy to tell that this is an example of incomplete dominance when looking at the phenotypes of the offspring. As you can see in the picture, all of the offspring are drawn as pink flowers. Since neither of the parents (the white flower and the red flower) has pink petals, we can conclude that the alleles for petal color in this specific flower show incomplete dominance over each other. This means that the three different genotypes result in three unique phenotypes:

• RR: Red petals
• Rr: Pink petals
• rr: White petals

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Allele}}$ : Genes of the same type that contain different kinds of information. Alleles can be dominant or recessive

$\color{#8925AE}{\text{Dominant}}$ : The stronger of two expressed genes

$\color{#8925AE}{\text{Recessive}}$ : The weaker of two expressed genes. Only shown if a gene is homozygous recessive

$\color{#8925AE}{\text{Heterozygous}}$ : When an organism has one dominant allele and one recessive allele for a trait

$\color{#8925AE}{\text{Homozygous}}$ : Two of the same type of allele, both dominant or both recessive

$\color{#8925AE}{\text{Codominance}}$ : Occurs when two heterozygous alleles are fully expressed. Neither allele can dominate so they both show up, but they do not blend

$\color{#8925AE}{\text{Incomplete Dominance}}$ : Occurs when two alleles are neither dominant nor recessive. The alleles are both expressed and show up as a mixture of the two alleles

$\color{#8925AE}{\text{Sex-Linked Traits}}$ : Traits caused by alleles located on a sex-chromosome

$\color{#8925AE}{\text{Polygenic}}$ : Traits that are controlled by two or more genes

(Video) Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis

by Amoeba Sisters

This video by the Amoeba Sisters explains incomplete dominance by showing snapdragon phenotypes. Snapdragon flowers can come in red, white, or pink, but why does this happen? Well, dominant alleles can sometimes be expressed together, producing an intermediate phenotype. This video explains how this happens by showing a Punnett Square for both Mendelian and non-Mendelian genetics. (Codominance, polygenic traits, and epistasis are also covered in the video.)