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D3.2.10—Incomplete dominance and codominance

Incomplete dominance blends heterozygote phenotype, while codominance expresses both alleles fully in the heterozygote in inheritance problems in inheritance problems.

Syllabus
First assessment 2025
Objective
D3.2.10
Level
SL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1

Common command terms

  • Identify
  • Describe

Recent exam appearances

May 2025Paper1A ["SL"] · TZ321[ 1 ]D3.2.10—Incomplete dominance and codominance
May 2025Paper1A ["SL"] · TZ220[ 1 ]D3.2.10—Incomplete dominance and codominance
May 2025Paper1A ["SL"] · TZ120[ 1 ]D3.2.10—Incomplete dominance and codominance
November 2024Paper1 ["SL"] · TZ114[ 1 ]D3.2.10—Incomplete dominance and codominance
November 2019Paper1 ["SL"] · TZ015[ 1 ]D3.2.10—Incomplete dominance and codominance
Practice this objective

Coverage 2015–2025 · Updated 16 Jul 2026

Concept essentials

  • Incomplete dominance produces an intermediate heterozygote phenotype.
  • Codominance expresses both alleles in the heterozygote.
  • Non-Mendelian allele relationships can make phenotype ratios match genotype ratios.
ConceptIB Biology SL