Genetics often begins with Gregor Mendel’s foundational principles, which describe how traits are passed from parents to offspring. Mendel introduced the concepts of dominant and recessive alleles, where one version of a gene completely masks the effect of the other. However, genetic inheritance is a far more complex system, and not all traits follow this simple pattern. Incomplete dominance represents a deviation from Mendelian inheritance, resulting in a unique, intermediate trait.
The Blended Phenotype
Incomplete dominance occurs when neither allele for a gene is completely dominant over the other, leading to a heterozygous phenotype that appears as a blend of the two homozygous phenotypes. This results in a third, unique physical appearance. The classic example is the flower color of snapdragons or four o’clock plants, where a cross between a true-breeding red-flowered plant and a true-breeding white-flowered plant yields offspring with uniformly pink flowers.
The mechanism behind this blending relates to the quantity of gene product, such as a protein or pigment, produced by the alleles. The allele coding for the red color produces a full dose of red pigment when two copies are present in a homozygous red flower. The allele for white color, however, may be non-functional and produce no pigment.
In the heterozygous pink flower, only one functional red-pigment allele is present along with one non-functional white allele. This single functional allele produces only about half the amount of pigment compared to the homozygous red flower. The reduced concentration of pigment results in the lighter, intermediate pink color, visually representing a blending of the two parental traits.
How It Differs From Complete Dominance
The difference between incomplete dominance and complete dominance lies in the phenotypic expression of the heterozygote. In the traditional Mendelian model, the presence of just one dominant allele is enough to produce the full dominant phenotype. For example, a heterozygous pea plant with one allele for purple flowers and one for white flowers will have purple flowers, identical in color to a plant with two purple alleles.
In complete dominance, the recessive allele is entirely masked, meaning the homozygous dominant and the heterozygous genotypes have the same observable trait. In contrast, incomplete dominance results in three distinct phenotypes corresponding to the three possible genotypes. The heterozygous individual expresses a trait that is visibly intermediate between the two homozygous forms.
This distinction alters how researchers can identify an organism’s genetic makeup simply by looking at it. Under complete dominance, one cannot distinguish a homozygous dominant individual from a heterozygote based on appearance alone. With incomplete dominance, however, the heterozygous genotype is immediately recognizable because it possesses the unique, blended phenotype.
Predicting Genotype and Phenotype Ratios
The inheritance pattern of incomplete dominance leads to genotypic and phenotypic ratios in offspring that differ from a standard Mendelian cross. When two heterozygous individuals, such as two pink snapdragons, are crossed, the alleles still segregate and combine according to Mendel’s rules of inheritance. Because the heterozygote results in a unique phenotype, the resulting physical appearances reflect the underlying genetic combinations directly.
A monohybrid cross between two pink flowers (one red allele and one white allele) will yield a genotypic ratio of 1:2:1. The offspring will be 25% homozygous for red, 50% heterozygous, and 25% homozygous for white. The defining feature of incomplete dominance is that the phenotypic ratio is identical to the genotypic ratio.
The resulting offspring will show a phenotypic ratio of 1:2:1: one red flower, two pink flowers, and one white flower. This 1:2:1 correspondence between genotype and phenotype is a defining characteristic of incomplete dominance, contrasting with the 3:1 phenotypic ratio typically observed in a complete dominance monohybrid cross. Genetic notation often uses unique capital letters or superscripts to denote the different alleles, emphasizing that neither one is truly dominant or recessive over the other.
The Distinction From Codominance
Incomplete dominance is often confused with codominance, another non-Mendelian inheritance pattern, but they are genetically distinct in how the heterozygous trait is expressed. In incomplete dominance, the result is a physical blending or intermediate appearance, such as the pink flower from red and white parents. The two parental traits appear to merge into a new, uniform color or form.
Codominance, by contrast, occurs when both alleles are simultaneously and fully expressed in the heterozygote without any blending. Instead of an intermediate, the offspring exhibits both parental traits distinctly, often in separate areas. A classic example is the roan coat color in cattle, where the animal has both red hairs and white hairs, resulting in a speckled appearance. The human ABO blood group system also demonstrates codominance, as an individual with both the A and B alleles expresses both A and B antigens fully on the surface of their red blood cells, resulting in the AB blood type.