Heredity often begins with simple Mendelian genetics, where traits are governed by a clear dominant or recessive relationship between two alleles. Many biological characteristics, however, involve more complex allelic interactions that do not follow this straightforward pattern. These variations, known as non-Mendelian inheritance, show that trait expression is more nuanced than one allele simply masking another. Two common forms of these complex interactions are incomplete dominance and codominance, both resulting in a third, unique phenotype when two different alleles are present.
The Mechanism of Incomplete Dominance
Incomplete dominance occurs when a heterozygous individual expresses a phenotype intermediate between the two homozygous parents. Neither allele is entirely dominant, meaning the resulting trait appears as a physical blending of the two parental traits. For example, in snapdragons, crossing a true-breeding red-flowered plant with a true-breeding white-flowered plant yields offspring with pink flowers. When two pink heterozygous plants are crossed, the offspring exhibit a genotypic ratio of 1:2:1 (red:pink:white). Because the heterozygote expresses a unique, intermediate phenotype, the phenotypic ratio also becomes 1:2:1, distinguishing this pattern from simple dominance.
The Mechanism of Codominance
Codominance is a pattern of inheritance where both alleles are fully and separately expressed in the heterozygous individual. The resulting phenotype displays both traits simultaneously and distinctly, rather than blending. This occurs because both alleles are equally functional, and their gene products are synthesized and visible at the same time. The most recognized human example is the ABO blood group system, specifically the AB blood type. An individual with the A allele and the B allele expresses both A and B antigens on the surface of their red blood cells, resulting in the AB classification.
Comparing Phenotypic Outcomes
The fundamental difference between incomplete dominance and codominance lies in the observable phenotype of the heterozygote. In incomplete dominance, the interaction leads to a single, third phenotype that appears as a physical fusion of the parental traits. This is analogous to mixing red and white paint to get a uniform pink color, where the original colors are no longer discernible. In contrast, codominance produces a third phenotype where the traits associated with both alleles are visibly expressed simultaneously, maintaining their separate identities. This is like having a canvas with both red and white stripes, where the two colors are present side-by-side without blending.