Incomplete dominance is a genetic phenomenon where neither allele for a trait is completely dominant over the other, leading to a blended or intermediate phenotype in heterozygotes.
Understanding Incomplete Dominance
This form of gene interaction, also known as partial dominance, occurs when both alleles of a gene at a specific locus are partially expressed. Instead of one allele completely masking the other, their combined expression results in a distinct, often intermediate, phenotype that is different from either homozygous parent.
For example, if you cross a plant with red flowers (homozygous for red) with a plant with white flowers (homozygous for white), and the trait exhibits incomplete dominance, their offspring (heterozygotes) will have pink flowers. Neither red nor white is fully expressed; instead, they blend to create a new, intermediate color.
Key Characteristics
Traits controlled by incomplete dominance exhibit several defining features:
- Intermediate Phenotype: Heterozygous individuals display a phenotype that is a mix or blend of the phenotypes of the two homozygous parents.
- Partial Expression: Neither allele completely masks the other; both contribute to the final phenotype in a partial manner.
- Distinct Genotypes and Phenotypes: Unlike complete dominance where the heterozygous phenotype is identical to the dominant homozygote, in incomplete dominance, the genotype directly correlates with a unique phenotype. This means:
- Homozygous dominant (e.g., RR) expresses one extreme phenotype.
- Homozygous recessive (e.g., WW) expresses the other extreme phenotype.
- Heterozygous (e.g., RW) expresses the intermediate phenotype.
- Identical Phenotypic and Genotypic Ratios: In a monohybrid cross between two heterozygotes (e.g., pink x pink), both the genotypic and phenotypic ratios in the F2 generation will be 1:2:1 (e.g., 1 red : 2 pink : 1 white). This differs from complete dominance, where the phenotypic ratio is typically 3:1.
Examples of Incomplete Dominance
While the classic example of flower color in snapdragons is often cited, incomplete dominance can be observed in various organisms:
- Snapdragons (Antirrhinum majus): Crossing red-flowered (RR) snapdragons with white-flowered (WW) ones produces F1 offspring with pink flowers (RW).
- Andalusian Fowl: When a black-feathered chicken (BB) is crossed with a white-feathered chicken (WW), the heterozygous offspring (BW) have blue feathers, a shade intermediate between black and white.
- Human Hair Texture: The trait for curly hair (CC) and straight hair (SS) can exhibit incomplete dominance. Individuals with heterozygous genotype (CS) often have wavy hair.
Understanding incomplete dominance is crucial for predicting inheritance patterns and understanding the diversity of traits observed in nature. It highlights that genetic interactions are not always straightforward "dominant or recessive," but can involve more nuanced expressions.
