Incomplete dominance blends heterozygote phenotype, while codominance expresses both alleles fully in the heterozygote in inheritance problems in inheritance problems.
Coverage 2015–2025 · Updated 16 Jul 2026
In incomplete dominance the heterozygote has an intermediate phenotype; in codominance both allele products are detectably expressed.
| Pattern | Heterozygote | IB example |
|---|---|---|
| Incomplete dominance | Intermediate phenotype | Red × white four-o'clock flower (Mirabilis jalapa) can produce pink F1 flowers |
| Codominance | Both products expressed | IᴬIᴮ produces both A and B antigens |
Self-crossing two pink Mirabilis F1 plants predicts a 1 red : 2 pink : 1 white phenotype ratio when the two alleles show incomplete dominance.
Codominance is not blending: both products remain present. Incomplete dominance does not make either allele 'partly dominant' in every genotype.
This objective is assessed through multiple choice, commonly using Identify / Describe.
Identify / Describe
Build the answer around this relationship: Incomplete dominance produces an intermediate heterozygote phenotype.
Representative question
A Mirabilis jalapa plant with red flowers was crossed with one with white flowers. All plants in the F1 generation had pink flowers. What phenotype ratio would be expected in the F2 generation?
100 % pink
50 % red and 50 % white
25 % white, 50 % pink and 25 % red
75 % red and 25 % white
Core D3.2 is secure when the student can move from allele rules into predictions and evidence: gametes form genotypes, genotypes can produce phenotypes, different dominance patterns need different notation, and pedigrees or plots require evidence-based interpretation.
Core inheritance exam questions reward disciplined reasoning. First identify the inheritance rule, then write the correct notation or evidence, then state the phenotype, ratio, or conclusion. This prevents the common mistake of writing definitions without solving the genetic problem.
