What Is Incomplete Dominance in Genetics?

The Blending of Polygenic Traits

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

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 complete dominance inheritance, one allele does not dominate or mask the other.

Incomplete dominance occurs in the polygenic inheritance of traits such as eye color and skin color.

It is a cornerstone in the study of non-Mendelian genetics.

Incomplete Dominance vs. Co-Dominance

Incomplete genetic dominance is similar to but different from co-dominance. Whereas incomplete dominance is a blending of traits, in co-dominance an additional phenotype is produced and both alleles are expressed completely. 

The best example of co-dominance is AB blood type inheritance. Blood type is determined by multiple alleles recognized as A, B, or O and in blood type AB, both phenotypes are fully expressed. 

The Discovery of Incomplete Dominance

Going back to ancient times, scientists have noted the blending of traits though none used the words "incomplete dominance." In fact, Genetics was not a scientific discipline until the 1800s when Gregor Mendel (1822–1884) began his studies.

Like many others, Mendel focused on plants and the pea plant in particular. He helped define genetic dominance when he noticed that the plants had either purple or white flowers.

They would not have a combination such as a lavender color as one might suspect.

Previous to this, scientists believed that physical traits would always be a blending of the parent plants. Mendel proved quite the opposite, that the offspring can inherit different forms separately. In his pea plants, traits were visible only if an allele was dominant or if both alleles were recessive.

Mendel described a genotype ratio of 1:2:1 and a phenotype ratio of 3:1. Both would be consequential in further research.

In the early 1900s, German botanist Carl Correns (1864–1933) would conduct similar research on four o'clock plants. While Mendel's work laid a foundation, it is Correns who is credited with the actual discovery incomplete dominance.

In his work, Correns observed a blend of colors in flower petals. This led him to the conclusion that the 1:2:1 genotype ratio prevailed and that each genotype had its own phenotype. In turn, this allowed the heterozygotes to display both alleles rather than a dominant one, as Mendel had found.

Incomplete Dominance in Snapdragons

As an 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 first filial (F1) 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 generation is allowed to cross-pollinate with true breeding red plants, the resulting F2 plants consist of red and pink phenotypes [1/2 Red (RR): 1/2 Pink (Rr)]. The phenotypic ratio is 1:1.
  • When the F1 generation is allowed to cross-pollinate with true breeding white plants, the resulting F2 plants 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, such as height, weight, eye color, and skin color, 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.
  • 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.