Incomplete Dominance

What makes incomplete dominance different

In normal dominance, a heterozygote (Tt) looks identical to a homozygous dominant (TT). In incomplete dominance, neither allele fully masks the other, so the heterozygote shows an intermediate phenotype that is a blend of both alleles' effects.

Because of this, genotype and phenotype can be read directly from each other, there are three phenotypes for three genotypes.

Allele notation

Since neither allele is dominant or recessive, capital and lower-case letters are not used. Instead, both alleles are written as capital letters with different superscripts, e.g. C^R for red and C^W for white flower colour.

Worked example: red x white four o'clock flowers

A true-breeding red-flowered plant (C^R C^R) is crossed with a true-breeding white-flowered plant (C^W C^W).

Parental genotypes: C^R C^R x C^W C^W. Gametes: C^R only, and C^W only.

All F1 offspring are C^R C^W, which are pink, the intermediate phenotype.

Two F1 pink plants are then crossed: C^R C^W x C^R C^W.

Reading the result

The genotype ratio 1:2:1 is identical to the phenotype ratio, because each genotype produces its own distinct phenotype. This differs from a normal monohybrid cross, where a 1:2:1 genotype ratio collapses into a 3:1 phenotype ratio.

Worked problem 1 (easy): red x pink

Question. In four o'clock flowers, flower colour shows incomplete dominance: C^R C^R is red, C^W C^W is white, and C^R C^W is pink. A red-flowered plant is crossed with a pink-flowered plant.

Determine the genotype and phenotype ratio of the offspring.

Key. Parents: red (C^R C^R) x pink (C^R C^W). Gametes: C^R only from the red plant; C^R and C^W from the pink plant.

Worked problem 2 (medium): pink x white

Question. A pink four o'clock plant (C^R C^W) is crossed with a white one (C^W C^W). Determine the phenotype ratio of the offspring and state why no red plants appear.

Key. Parents: pink (C^R C^W) x white (C^W C^W). Gametes: C^R and C^W from the pink plant; C^W only from the white plant.

Worked problem 3 (SPM-level): blue Andalusian fowl

Question. In Andalusian fowl, feather colour shows incomplete dominance: C^B C^B is black, C^W C^W is white (splashed), and the heterozygote C^B C^W is blue-grey. Two blue-grey birds are crossed.

Determine the genotype and phenotype ratio of the offspring, and explain why blue-grey birds never breed true.

Key. Parents: blue-grey (C^B C^W) x blue-grey (C^B C^W). Gametes: C^B and C^W from each parent.

Practice problems

1. In four o'clock flowers (C^R red, C^W white, C^R C^W pink), a red plant is crossed with a white plant. State the phenotype of all the F1 offspring.
2. Two pink four o'clock plants are crossed. State the genotype ratio and the phenotype ratio of the offspring.
3. A cross between two four o'clock plants gives offspring in the ratio 1 red : 1 pink. Deduce the genotypes of both parents.

Answers to the practice problems

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions