Incomplete dominance and codominanceSpec D3.2.10
In short
In incomplete dominance, heterozygotes have an intermediate phenotype, such as pink four o'clock flowers from red and white parents. In codominance, heterozygotes have a dual phenotype, with both alleles fully expressed, such as blood group AB from Iᴬ and Iᴮ. Both differ from complete dominance, where heterozygotes look like the homozygous dominant.
| Pattern | Heterozygote phenotype | Example |
|---|---|---|
| Complete dominance | Same as homozygous dominant | Pea plants: Tt is tall, like TT |
| Incomplete dominance | Intermediate between the two homozygotes | Four o'clock flower (Mirabilis jalapa): red × white gives pink |
| Codominance | Dual phenotype: both alleles expressed | Blood group AB (IᴬIᴮ): both A and B antigens present |
Incomplete dominance in the four o'clock flower
A pure-breeding red Mirabilis jalapa (CᴿCᴿ) is crossed with a pure-breeding white plant (CᵂCᵂ). The F1 are self-pollinated. Deduce the F1 and F2 phenotypes and ratio.
- P gametes: Cᴿ and Cᵂ, so all F1 are CᴿCᵂ: pink.
- F1 × F1: each produces Cᴿ and Cᵂ gametes in equal numbers.
- Punnett grid: CᴿCᴿ, CᴿCᵂ, CᵂCᴿ, CᵂCᵂ.
Answer: F1 all pink. F2 1 red : 2 pink : 1 white; the genotype ratio and phenotype ratio are the same.
In the pink heterozygote, the single Cᴿ allele produces only enough red pigment to make the petals pink. In blood group AB, each allele makes its own enzyme, so both antigens appear side by side.
A 1:2:1 phenotype ratio in an F2 is a sign of incomplete dominance or codominance, because the heterozygotes can be told apart. Either the common or the scientific name of the organism is acceptable.
Written and checked against the IB Biology SL specification · Updated October 2026