Explain incomplete dominance with the snapdragon flower example

Incomplete dominance is when neither allele is fully dominant over the other. Instead of one allele “winning,” the heterozygote shows an intermediate phenotype — a blend of the two parental phenotypes.

This was a challenge to Mendel’s law of dominance (which said one allele is dominant and fully expressed). Incomplete dominance shows that dominance is not always complete.

In snapdragons (Antirrhinum majus):

• Pure breeding red-flowered plant: genotype $R^1R^1$ (phenotype: red)
• Pure breeding white-flowered plant: genotype $R^2R^2$ (phenotype: white)

Note: We use $R^1$ and $R^2$ (or $R$ and $r$ or $C^R$ and $C^W$) because neither allele is dominant. We cannot use capital/lowercase notation (which implies dominance-recessiveness).

When crossed (F₁ generation):

$R^1R^1 \times R^2R^2$

All F₁ offspring: $R^1R^2$ → Pink flowers (intermediate between red and white)

Now cross two pink-flowered plants: $R^1R^2 \times R^1R^2$

F₂ genotypes: $R^1R^1 : R^1R^2 : R^2R^2 = 1:2:1$

F₂ phenotypes: Red : Pink : White = 1 : 2 : 1

Key difference from Mendel’s results: In standard dominance, F₂ shows 3:1 phenotypic ratio. In incomplete dominance, F₂ shows 1:2:1 phenotypic ratio — because the genotypic and phenotypic ratios are the same (each genotype gives a distinct phenotype).

In red plants, $R^1$ codes for red pigment. Two copies of $R^1$ produce abundant red pigment.

The $R^2$ allele is non-functional (doesn’t produce red pigment).

Heterozygotes ($R^1R^2$) have only one working copy of $R^1$ — producing half the normal amount of red pigment. This diluted pigment concentration produces the intermediate pink colour.

This is called gene dosage effect — the phenotype depends on how many functional copies of the gene are present.