Genetics explains how traits are passed from parents to offspring through genes. An organism inherits two versions of each gene, called alleles, one from each parent. These alleles interact to determine the observable physical characteristics, or phenotype. While many traits follow straightforward rules where one allele completely overshadows the other, inheritance can be more complex. Incomplete dominance is a variation where the interaction between alleles does not result in the full expression of one trait over the other.
Comparing Complete and Incomplete Dominance
Complete dominance is the more commonly recognized form of inheritance, where a dominant allele fully masks a recessive allele in a heterozygous pairing. An individual with one dominant and one recessive allele displays the same phenotype as an individual with two dominant alleles, resulting in only two possible phenotypes for a given trait. In contrast, incomplete dominance occurs when neither allele is fully expressed over the other. The heterozygous genotype results in a third, physically distinct phenotype that is an intermediate blend of the two homozygous phenotypes.
The Molecular Mechanism of Intermediate Traits
The intermediate trait’s appearance is explained by the amount of functional protein the alleles produce. The allele associated with the fully expressed trait codes for a functional protein, such as an enzyme that synthesizes a pigment. In a homozygous dominant individual, both alleles are functional, resulting in a full “dose” of the protein and maximal trait expression. However, a heterozygous individual possesses only one copy of the functional allele, producing approximately half the amount of protein. This reduced quantity, or gene dosage, is insufficient to achieve the full effect, leading to the intermediate phenotype.
Observable Instances of Incomplete Dominance
A classic example is the flower color in snapdragons (Antirrhinum majus). When a plant with homozygous red flowers (RR) is crossed with homozygous white flowers (rr), the resulting heterozygous offspring (Rr) display a uniform pink color. In human genetics, hair texture is often cited as an instance of incomplete dominance. A child of parents homozygous for straight hair and curly hair will have wavy hair, which is an intermediate characteristic. Another example is the feather color of the Andalusian chicken, where a cross between black and white parents yields blue-tinged offspring.
Calculating Offspring Ratios (Genotype and Phenotype)
Predicting the outcome of incomplete dominance uses the same Punnett square method as standard Mendelian genetics. If two pink-flowered snapdragons (Rr) are crossed, the genotypes (RR, Rr, and rr) appear in a 1:2:1 ratio, which is standard for a cross between two heterozygous parents. The key difference is how the phenotype is expressed. Since the heterozygous genotype (Rr) results in a unique, intermediate phenotype (pink), the phenotypic ratio also becomes 1:2:1 (25% red, 50% pink, and 25% white). This direct correlation between each genotype and a distinct phenotype makes the genotypic and phenotypic ratios identical.