Incomplete Dominance - A Genetics Definition

Pink Snapdragons
Pink Snapdragons. Credit: Crezalyn Nerona Uratsuji/Moment/Getty Image

Definition: Incomplete dominance is a form of intermediate inheritance in which one allele for a specific trait is not completely expressed over its paired allele. This results in a third phenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. Unlike in complete dominance inheritance, one allele does not dominate or mask the other allele. Incomplete dominance occurs in the polygenic inheritance of traits such as eye color and skin color.

Incomplete Dominance vs Co-dominance

Incomplete genetic dominance is similar to, but different from co-dominance. In co-dominance, an additional phenotype is produced, however, both alleles are expressed completely. Co-dominance is exemplified in AB blood type inheritance. Blood type is determined by multiple alleles recognized as A, B, or O. In blood type AB, both phenotypes are fully expressed. For additional information see: Differences Between Incomplete Dominance and Co-dominance.

Incomplete Dominance in Snapdragons

Example: Incomplete dominance is seen in cross-pollination experiments between red and white snapdragon plants. In this monohybrid cross, the allele that produces the red color (R) is not completely expressed over the allele that produces the white color (r). The resulting offspring are all pink. The genotypes are: Red (RR) X White (rr) = Pink (Rr).

When the F1 (first filial) generation (consisting of all pink plants) is allowed to cross-pollinate, the resulting plants (F2 generation) consist of all three phenotypes [1/4 Red (RR): 1/2 Pink (Rr): 1/4 White (rr)].

The phenotypic ratio is 1:2:1.

When the F1 (first filial) generation is allowed to cross-pollinate with true breeding red plants, the resulting plants (F2 generation) consist of red and pink phenotypes [1/2 Red (RR): 1/2 Pink (Rr)]. The phenotypic ratio is 1:1.

When the F1 (first filial) generation is allowed to cross-pollinate with true breeding white plants, the resulting plants (F2 generation) consist of white and pink phenotypes [1/2 White (rr): 1/2 Pink (Rr)].

The phenotypic ratio is 1:1.

In incomplete dominance, the intermediate trait is the heterozygous genotype. In the case of snapdragon plants, the pink plants are heterozygous with the (Rr) genotype. The red and white plants are both homozygous for plant color with genotypes of (RR) red and (rr) white.

Polygenic Traits

Polygenic traits are determined by more than one gene and by interactions among several alleles. The genes contributing to these traits equally influence the phenotype and the alleles for these genes are found on different chromosomes. The alleles have an additive effect on the phenotype resulting in varying degrees of phenotypic expression. Individuals may express varying degrees of a dominant phenotype, recessive phenotype, or intermediate phenotype. Those that inherit more dominant alleles will have a greater expression of the dominant phenotype, while those that inherit more recessive alleles will have a greater expression of the recessive phenotype. Those that inherit various combinations of dominant and recessive alleles will express the intermediate phenotype to varying degrees. Height, weight, eye color, and skin color are examples of polygenic traits.