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 thirdphenotype in which the expressed physical trait is a combination of the phenotypes of both alleles. Unlike in complete dominanceinheritance, one allele does not dominate or mask the other allele. Incomplete dominance occurs in the polygenic inheritanceof 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 ABblood type inheritance. 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 genotypesare: 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.