What Is Incomplete Dominance and How Does It Work?
Incomplete dominance describes a genetic inheritance pattern where the heterozygous genotype results in an intermediate phenotype, rather than one allele completely masking the other. The resulting physical appearance in an organism appears as a blend of the two parental traits. This type of inheritance shows how traits can exhibit intermediate forms in offspring, moving beyond simple dominant and recessive expressions.
The Mechanism of Incomplete Dominance
Incomplete dominance occurs at a genetic level when neither of the two alleles is completely dominant over the other. This results in a phenotype that is a combination of both alleles. For example, if an organism inherits two different alleles for a trait, such as one for red color and one for white, the heterozygous combination expresses a phenotype distinct from both homozygous parents.
A plant with a homozygous dominant genotype might produce a large amount of pigment, resulting in a dark color. A homozygous recessive plant might produce no pigment, appearing white. In a heterozygous individual, only one copy of the pigment-producing allele is present. This single allele may not produce enough pigment to achieve the full, dark color seen in the homozygous dominant individual. Consequently, the phenotype is diluted or intermediate, appearing as a lighter shade or a blend of the two original colors. This partial expression means that the effect of one allele is not entirely hidden by the other, leading to a unique appearance in the heterozygote.
Common Examples in Nature
Classic examples of incomplete dominance are observed in flower color, such as snapdragons. When a true-breeding red snapdragon is crossed with a true-breeding white snapdragon, the offspring in the first generation (F1) are uniformly pink. This pink color is an intermediate blend of the red and white parental phenotypes. If these pink snapdragons are then crossed with each other, the next generation (F2) typically shows a phenotypic ratio of one red, two pink, and one white flower.
Another instance of incomplete dominance is seen in the feather color of Andalusian chickens. A cross between a black-feathered chicken and a white-feathered chicken results in offspring with blue-tinged feathers. The blue color is a diluted or blended version of the black and white parental traits.
How Incomplete Dominance Differs from Other Patterns
Incomplete dominance stands apart from other common inheritance patterns like complete dominance and codominance. In complete dominance, one allele fully masks the other, so the heterozygous individual displays the same phenotype as the homozygous dominant individual. For instance, in Gregor Mendel’s pea plants, a tall allele completely masks a short allele, so a heterozygous plant (one tall, one short allele) is still tall. There is no blending; only the dominant trait is visible.
Codominance, by contrast, involves the simultaneous and full expression of both alleles in the heterozygote. Instead of blending, both traits appear distinctly. An example of codominance is the ABO blood group system in humans, where an individual with both A and B alleles expresses both A and B antigens on their red blood cells, resulting in AB blood type. Similarly, roan cattle exhibit both red and white hairs, not a blended pink, because both red and white coat color alleles are fully expressed. Incomplete dominance results in a new, intermediate phenotype, while codominance shows both parental phenotypes at once.