Genetic inheritance is the basic process by which traits are passed from parents to their offspring. It ensures the continuity of species by transmitting characteristics across generations. Genes, the basic units of heredity, carry the instructions for these traits, and they exist in different versions called alleles. The specific combination of alleles an individual inherits determines their genetic makeup, or genotype, which in turn influences their observable characteristics, known as their phenotype. While some traits follow simple dominant-recessive patterns, where one allele completely masks another, many others exhibit more intricate inheritance patterns.
Incomplete Dominance
Incomplete dominance describes a genetic scenario where the heterozygous phenotype is an intermediate blend of the two homozygous phenotypes. This means that neither allele completely dominates the other when both are present. Instead, the resulting trait appears as a mixture or dilution of the parental traits.
A classic example of incomplete dominance is seen in the flower color of snapdragons (Antirrhinum majus). When a true-breeding red-flowered snapdragon (homozygous for red alleles, RR) is crossed with a true-breeding white-flowered snapdragon (homozygous for white alleles, rr), their offspring in the first filial (F1) generation all have pink flowers. The pink color arises because the single red allele in the heterozygous (Rr) plant is not sufficient to produce enough red pigment for a fully red flower. The red allele does not completely mask the effect of the white allele, leading to a blended appearance.
If these F1 pink-flowered plants are then self-pollinated, the subsequent F2 generation will display a phenotypic ratio of 1 red: 2 pink: 1 white. This 1:2:1 phenotypic ratio directly corresponds to the genotypic ratio (1 RR: 2 Rr: 1 rr).
Codominance
Codominance is an inheritance pattern where both alleles in a heterozygote are fully and distinctly expressed. Unlike incomplete dominance where traits blend, codominance results in a phenotype that displays characteristics of both homozygous traits simultaneously, without any mixing.
A prominent example of codominance is the ABO blood group system in humans. The A and B alleles are codominant, while the O allele is recessive. An individual inheriting both an A allele and a B allele (genotype AB) will have type AB blood, meaning their red blood cells express both A and B antigens. Both antigens are present and fully functional, demonstrating simultaneous expression.
Another illustrative example is the roan coat color in cattle. When a red-coated cow is bred with a white-coated bull, the offspring often have a roan coat, which is a mixture of individual red and white hairs. This is not a blend, but distinct red and white hairs appearing together, showcasing the full expression of both alleles.
Distinguishing the Patterns
The fundamental distinction between incomplete dominance and codominance lies in the phenotype of the heterozygous individual.
In incomplete dominance, the heterozygous phenotype is an intermediate blend. For instance, a red flower and a white flower producing a pink flower exemplifies this blending. The effect of one allele is not completely masked by the other, leading to a new, intermediate phenotype.
Conversely, codominance involves the full and distinct expression of both alleles in the heterozygote. For example, in roan cattle, both red and white hairs are present, rather than a uniform “pinkish” coat. Similarly, human AB blood type clearly exhibits both A and B antigens, not an intermediate form. The key difference is that incomplete dominance results in a novel, blended phenotype, while codominance results in a phenotype where both original traits are expressed side-by-side.