The transmission of traits follows predictable patterns, first explored by Gregor Mendel in the mid-19th century. Mendel established that heritable factors, now known as genes, determine an organism’s characteristics, with different versions called alleles. While Mendelian genetics provides a foundational framework, the expression of these alleles is not always simple. Many traits exhibit complex inheritance patterns that deviate from standard Mendelian rules, leading to variations like incomplete dominance and codominance.
The Foundation of Dominance
Mendel’s experiments laid out the concept of complete dominance, where one allele completely masks the presence of another allele for the same trait. An individual inherits two alleles for every gene, one from each parent. If the individual is heterozygous (one dominant and one recessive allele), the resulting trait, or phenotype, will exclusively be that of the dominant allele.
For example, in pea plants, the allele for purple flowers is dominant over white. A plant that is homozygous dominant or heterozygous will both display the purple flower phenotype. The recessive white allele is fully hidden in the heterozygote, resulting in only two possible visible traits (purple or white), despite three possible allele combinations.
How Incomplete Dominance Results in Blending
Incomplete dominance is a non-Mendelian pattern where the heterozygous genotype produces a phenotype that is an intermediate physical mixture of the two parental traits. This occurs because neither allele fully asserts its dominance, resulting in a partial expression of both.
A classic illustration is flower color in snapdragons (Antirrhinum majus). When a true-breeding red plant is crossed with a true-breeding white plant, the offspring are all pink. The pink color is an intermediate phenotype, demonstrating that the single red allele is insufficient to produce the full pigment intensity seen in the red parent. When two pink (heterozygous) plants are crossed, the next generation exhibits a 1:2:1 ratio of red, pink, and white phenotypes, which mirrors the 1:2:1 genotypic ratio.
Codominance Shows Both Traits Simultaneously
Codominance is an inheritance pattern where both alleles in a heterozygous individual are fully and equally expressed, resulting in both traits being visible simultaneously. Unlike incomplete dominance, codominance involves no mixing or intermediate trait; the traits appear side-by-side.
A prominent human example is the ABO blood group system, where the \(I^A\) and \(I^B\) alleles are codominant. A person who inherits both alleles will have type AB blood, meaning their red blood cells display both A and B antigens on their surface. Another visual example is the roan coat color in cattle, where an animal inheriting alleles for red and white hair will have a coat made up of individual red hairs and individual white hairs, creating a speckled appearance.
Comparing the Two Inheritance Patterns
The distinction between incomplete dominance and codominance lies in the final appearance of the heterozygous phenotype. In incomplete dominance, the alleles result in an intermediate, blended trait, much like mixing red and white paint to get pink. This blending results from the partial or reduced activity of the gene product from one allele, which leads to a dose-dependent effect on the resulting trait.
Conversely, codominance results in the full and independent expression of both parental phenotypes, analogous to painting red and white stripes next to each other. In this pattern, the products of both alleles are fully functional and do not interfere with each other, so both traits are clearly visible in the organism. Therefore, incomplete dominance dilutes the traits into a new one, while codominance displays both original traits without alteration.