The study of heredity shows that traits are passed from parents to offspring through genes, which exist in different forms called alleles. Early concepts of genetics, established by Gregor Mendel, focused on complete dominance, where one allele entirely masks the effect of another. While this simple dominant-recessive relationship explains many traits, the expression of some genes is more complex, demonstrating variations in how alleles interact.
Defining Incomplete Dominance
Incomplete dominance is an inheritance pattern where the heterozygous offspring displays a phenotype that is a mixture or blend of the traits of the two homozygous parents. Neither allele is entirely dominant over the other, resulting in a third, intermediate physical trait. For example, if a plant with a red flower allele is crossed with a white flower allele, the resulting offspring would exhibit pink flowers.
Incomplete dominance is distinct from complete dominance, where a single dominant allele fully expresses the trait and masks the recessive allele. It also differs from codominance, where both parental alleles are fully and separately expressed at the same time, such as a flower with both red and white patches.
The Molecular Reason for Intermediate Traits
The intermediate phenotype seen in incomplete dominance is often explained by the dosage effect. Genes carry instructions for building proteins, frequently enzymes responsible for producing a pigment or other physical characteristic. A fully expressed trait, such as a deep red color, results from having two functional copies of the allele, which produce a maximum quantity of the necessary enzyme.
In a heterozygous individual, only one functional allele is present. This single functional allele produces about half the amount of the required enzyme compared to the homozygous dominant organism. This reduced quantity of the enzyme is insufficient to synthesize the full concentration of pigment needed for the trait to be fully realized. Consequently, the trait appears diluted or intermediate.
Applying the Concept: The Snapdragons Example
The classic illustration of incomplete dominance is observed in the flower color of the snapdragon, Antirrhinum majus. When a true-breeding red-flowered plant (genotype RR) is crossed with a true-breeding white-flowered plant (genotype WW), the first generation (F1) all have pink flowers. These pink flowers possess the heterozygous genotype RW, demonstrating the intermediate phenotype.
If two heterozygous pink flowers (RW x RW) are crossed, the resulting F2 generation exhibits a distinct pattern of inheritance. The alleles segregate and recombine to produce three genotypes: RR, RW, and WW. This cross results in a genotypic ratio of 1:2:1 (one red, two pink, and one white).
Because the heterozygote (RW) has its own unique phenotype (pink), the phenotypic ratio in the F2 generation is also 1:2:1. This result is a distinction from simple Mendelian complete dominance, where the phenotypic ratio of a heterozygous cross is typically 3:1.