Incomplete Dominance: Snapdragon Flower Colour Example

Incomplete dominance is a pattern of inheritance in which neither allele is fully dominant, so the heterozygous offspring shows a phenotype in between the two homozygous forms. The classic example is flower colour in snapdragon (Antirrhinum), where red and white parents produce pink offspring.

This topic matters because it shows that Mendel's rule of complete dominance is not universal. Recognising incomplete dominance explains many intermediate traits in plants and animals and changes the ratios expected in crosses.

Key Concepts

• Incomplete dominance: the heterozygote phenotype is intermediate between the two homozygotes.
• Alleles in snapdragon: C^R C^R gives red flowers (full pigment), C^W C^W gives white (no pigment), and C^R C^W gives pink (half the pigment).
• Pigment dose effect: one functional allele makes about half the red pigment of two copies, so the colour appears diluted as pink.
• Genotype-phenotype correspondence: because each genotype looks different, the genotypic and phenotypic ratios are identical.
• F2 ratio: crossing two pink heterozygotes gives 1 red : 2 pink : 1 white both genotypically and phenotypically.

Formulas

Cross between two heterozygotes: C^R C^W x C^R C^W. Gametes from each parent: 1/2 C^R and 1/2 C^W. Offspring probabilities: C^R C^R = 1/2 x 1/2 = 1/4 red; C^R C^W = 2 x (1/2 x 1/2) = 1/2 pink; C^W C^W = 1/4 white. So the ratio is 1:2:1 for both genotype and phenotype, unlike the 3:1 phenotypic ratio of complete dominance. Cross of pink with red: C^R C^W x C^R C^R gives 1/2 C^R C^R (red) and 1/2 C^R C^W (pink), a 1:1 ratio with no white offspring possible.

Worked Example

A gardener crosses two pink snapdragons and grows 240 offspring. Expected numbers: red = 1/4 x 240 = 60 plants; pink = 1/2 x 240 = 120 plants; white = 1/4 x 240 = 60 plants. Now the gardener crosses a pink plant with a white plant (C^R C^W x C^W C^W): gametes C^R or C^W from pink, only C^W from white. Offspring: 1/2 C^R C^W (pink) and 1/2 C^W C^W (white), so out of 240 seedlings expect about 120 pink and 120 white, and never red.

Common Misconceptions

• "Pink flowers prove blending inheritance, where genes mix permanently." Wrong. The alleles stay separate: crossing two pink plants produces pure red and pure white offspring again in F2. Only the phenotype looks blended.
• "The F2 ratio in incomplete dominance is 3:1 like Mendel's peas." It is 1:2:1, because the heterozygote has its own visible phenotype.
• "Incomplete dominance means the alleles are damaged." Both alleles are normal; the intermediate colour simply reflects the amount of pigment one copy can make.

Real-World Applications

Flower breeders deliberately cross red and white varieties to create pink cultivars for the market. Incomplete dominance also appears in animals, such as palomino coat colour in horses (between chestnut and cremello). In plant breeding, recognising the 1:2:1 ratio helps identify heterozygotes by sight without test crosses.

In incomplete dominance, the heterozygote shows an intermediate phenotype — pink between red and white in snapdragon — giving a 1:2:1 ratio in F2, while the alleles themselves remain unblended.