The inheritance of traits is governed by genes, with variations known as alleles. While introductory genetics often focuses on simple dominant and recessive relationships, many traits follow more complex patterns known as non-Mendelian inheritance. These patterns reveal how genes interact to determine an organism’s physical characteristics, or phenotype. Two important variations are incomplete dominance and codominance, which describe how the heterozygous genotype—having two different alleles—is expressed.
The Foundation: Simple Dominance
The simplest model of inheritance, described by Gregor Mendel, involves two alleles for a trait where one is completely dominant over the other. This model, often called simple dominance, dictates the resulting phenotype in a straightforward manner. If an organism inherits at least one copy of the dominant allele, that trait will be the one expressed.
The dominant allele essentially masks the presence of the recessive allele in a heterozygous individual. For example, in Mendel’s pea plants, the allele for purple flowers is dominant over the allele for white flowers. A plant that is heterozygous (one purple and one white allele) displays the purple flower phenotype, looking the same as a homozygous dominant plant. Only an individual inheriting two copies of the recessive allele will express the white flower phenotype.
Defining Incomplete Dominance
Incomplete dominance is a pattern where the heterozygous genotype produces a phenotype that is an intermediate blend of the two homozygous traits. This occurs because neither allele is fully dominant over the other. The resulting physical characteristic appears to be a mix or dilution of the two parental traits.
A common illustration is found in the flower color of snapdragons. When a plant with red flowers is crossed with a plant with white flowers, the offspring are all pink. The pink color is a distinct, third phenotype that is physically intermediate between the red and white parent colors. This blending often happens because the single copy of the pigment-producing allele in the heterozygote produces only half the amount needed for the fully colored trait.
Defining Codominance
Codominance describes an inheritance pattern where both alleles in a heterozygous individual are fully and separately expressed simultaneously. Unlike incomplete dominance, there is no blending or mixing of the traits to form an intermediate phenotype. Instead, the phenotypes associated with both alleles are distinctly observable at the same time.
A classic example in humans is the AB blood type. The \(I^A\) and \(I^B\) alleles are codominant with respect to each other. An individual inheriting both alleles will have type AB blood, meaning their red blood cells display both A-type antigens and B-type antigens on their surface.
Another common example is the roan coat color in cattle or horses. A cross between a homozygous red animal and a homozygous white animal results in a roan offspring. The coat contains a mixture of both red hairs and white hairs, where both phenotypes are fully expressed side-by-side, resulting in a distinctly mixed appearance.
Key Differences in Phenotype Expression
The fundamental distinction between incomplete dominance and codominance lies in the resulting phenotype of the heterozygous individual. Incomplete dominance results in a novel, intermediate phenotype that appears to be a blend of the two parental traits, such as a red and white flower producing a uniformly pink flower.
Codominance, conversely, results in the full and distinct expression of both parental phenotypes simultaneously. Both traits are visible, such as red and white hairs appearing side-by-side on a roan coat, or both A and B antigens being present on an AB blood cell. The heterozygous phenotype in codominance is a combination of both traits, not an intermediate mixture.