D3.2.5—Dominant and recessive alleles
Dominant alleles affect phenotype in heterozygotes, while recessive alleles are expressed only when no dominant allele is present in inheritance problems.
- Syllabus
- First assessment 2025
- Objective
- D3.2.5
- Level
- SL
Exam analysis
Chance of appearing12%of analysed past papers
Latest appearanceMay 2025
Most common paperPaper1
Typical marks1–2
Common command terms
- Identify
- Explain
- Deduce
Recent exam appearances
May 2025Paper2 ["SL"] · TZ15(c)[ 3 ]D3.2.5—Dominant and recessive alleles
November 2023Paper1 ["SL"] · TZ216[ 1 ]D3.2.5—Dominant and recessive alleles
May 2023Paper2 ["SL"] · TZ15(b)[ 2 ]D3.2.5—Dominant and recessive alleles
May 2023Paper2 ["SL"] · TZ15(a)[ 2 ]D3.2.5—Dominant and recessive alleles
May 2021Paper2 ["SL"] · TZ25(c)[ 3 ]D3.2.5—Dominant and recessive alleles
Practice this objective
Coverage 2011–2025 · Updated 16 Jul 2026
Concept essentials
- Dominant alleles are expressed in heterozygotes.
- Recessive phenotypes usually require two recessive alleles.
- Dominance describes expression, not allele frequency or value.