In genetics, incomplete dominance describes a pattern of inheritance where the heterozygous offspring display a phenotype that is intermediate between the phenotypes of the two homozygous parents. This means that neither allele completely masks the effect of the other, leading to a blended appearance of the trait. It represents a deviation from simpler Mendelian genetics, where one allele typically fully dominates another.
How Incomplete Dominance Works
Incomplete dominance occurs because the expression of one allele does not fully suppress the expression of the other allele. This blending often happens at a molecular level, where the gene product from one allele may not be sufficient to produce the full effect seen in an individual with two copies of that allele. For instance, if a gene produces a pigment, a single copy of the allele might only produce half the amount of pigment compared to two copies, leading to a diluted or intermediate color.
Incomplete Dominance Compared to Other Patterns
Incomplete dominance is distinct from other common patterns of inheritance, such as complete dominance and codominance. In complete dominance, one allele completely masks the presence of another, meaning a heterozygous individual will exhibit the same phenotype as a homozygous dominant individual. For example, if a red allele is completely dominant over a white allele, a heterozygous organism would still appear red.
In contrast, codominance involves the full and simultaneous expression of both alleles in the heterozygote, without any blending. An individual with codominant alleles will show both traits distinctly, such as a flower with patches of red and white, or human blood type AB where both A and B antigens are present. Incomplete dominance, however, results in a blended phenotype, where the heterozygous trait is an intermediate form, like a pink flower resulting from red and white parents.
Real-World Examples
Incomplete dominance is observed in various organisms. Flower color in snapdragons (Antirrhinum majus) is a classic example. When a true-breeding red-flowered snapdragon is crossed with a true-breeding white-flowered snapdragon, their offspring in the first generation (F1) all produce pink flowers. If these pink F1 generation snapdragons are then crossed, the next generation (F2) will show a phenotypic ratio of 1 red: 2 pink: 1 white, demonstrating the intermediate nature of the pink phenotype.
Human hair texture is another instance. Individuals with two alleles for curly hair have very curly hair, while those with two alleles for straight hair have straight hair. However, heterozygous individuals exhibit wavy hair, an intermediate texture. Andalusian chickens also display incomplete dominance in feather color. A cross between a black-feathered chicken and a white-feathered chicken results in offspring with blue feathers, a blend of the parental colors.
Predicting Outcomes with Punnett Squares
Punnett squares are useful tools for predicting the genotypes and phenotypes of offspring in crosses involving incomplete dominance, similar to how they are used for other inheritance patterns. When setting up a Punnett square for incomplete dominance, both alleles are typically represented by capital letters, sometimes with superscripts, to indicate that neither is fully dominant or recessive. For instance, in snapdragons, red flowers might be denoted as CᴿCᴿ and white flowers as CᵂCᵂ, with pink flowers being CᴿCᵂ.
To predict outcomes, the alleles from each parent are placed along the top and side of the square, and the possible combinations for the offspring are filled into the inner boxes. For a cross between two pink snapdragons (CᴿCᵂ x CᴿCᵂ), the Punnett square would show a genotypic ratio of 1 CᴿCᴿ: 2 CᴿCᵂ: 1 CᵂCᵂ. Correspondingly, the phenotypic ratio would be 1 red: 2 pink: 1 white, directly reflecting the genotypic ratio because each genotype produces a distinct phenotype in incomplete dominance. This method allows for a clear visualization of the probabilities of inheriting each blended trait.