Each individual inherits two copies, known as alleles, for most genes, receiving one from each parent. These alleles reside at specific locations on chromosomes and can have different versions, influencing how a particular trait is expressed. While some allele interactions follow simple dominant-recessive patterns, others exhibit more complex inheritance.
What is Incomplete Dominance?
Incomplete dominance describes a genetic situation where neither allele completely masks the effect of the other in a heterozygous individual. The resulting phenotype is an intermediate blend of the two homozygous phenotypes. This means that the dominant allele does not fully assert itself over the recessive one, leading to a new, combined appearance. For instance, if a red-flowered plant is crossed with a white-flowered plant, the offspring might display pink flowers.
A classic example of incomplete dominance is seen in snapdragon flowers (Antirrhinum majus). When a homozygous red-flowered snapdragon (RR) is crossed with a homozygous white-flowered snapdragon (genotype WW), their first-generation offspring (F1) will have pink flowers (genotype RW). This pink color arises because the pigment produced by the red allele is diluted in the presence of the white allele, creating an intermediate shade. If two pink-flowered snapdragons (RW x RW) are then crossed, their offspring will show a phenotypic ratio of 1 red: 2 pink: 1 white, corresponding to the genotypic ratio of 1 RR: 2 RW: 1 WW.
What is Codominance?
Codominance is an inheritance pattern where both alleles in a heterozygous individual are simultaneously and fully expressed, without blending or masking each other. This results in offspring that display traits associated with both alleles distinctly and equally. Unlike incomplete dominance, where a new, intermediate phenotype emerges, codominance allows both parental traits to be visible at the same time.
A prominent example of codominance in humans is the ABO blood group system. Individuals with type AB blood possess one allele for A antigens (Iᴬ) and one allele for B antigens (Iᴮ). Both Iᴬ and Iᴮ alleles are expressed equally, meaning both A and B antigens are present on the surface of their red blood cells. Roan cattle also illustrate codominance, having both red and white hairs in their coat, not a blended pink color. A roan cow (RW) results from a cross between a red cow (RR) and a white cow (WW), showcasing individual red and white hairs simultaneously.
How They Differ
The fundamental distinction between incomplete dominance and codominance lies in how the heterozygous phenotype is expressed. In incomplete dominance, the traits appear to blend, producing an intermediate phenotype different from either parent. For example, red and white snapdragons yield pink flowers. This blending occurs because the dominant allele’s effect is not complete, leading to a diluted expression.
Conversely, codominance involves the simultaneous and distinct expression of both alleles in the heterozygote. Neither allele is diluted or masked; instead, both contribute fully to the phenotype, appearing side-by-side. This is seen with AB blood type, where both A and B antigens are present, or in roan cattle, which exhibit both red and white hairs. Therefore, incomplete dominance creates a new, blended phenotype, while codominance results in the individual expression of both original parental phenotypes.