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
- HL
Exam analysis
Chance of appearing4%of analysed past papers
Latest appearanceMay 2022
Most common paperPaper1
Typical marks1
Common command terms
- Identify
- Explain
- Deduce
Recent exam appearances
May 2022Paper1 ["HL"] · TZ113[ 1 ]D3.2.5—Dominant and recessive alleles
May 2018Paper1 ["HL"] · TZ112[ 1 ]D3.2.5—Dominant and recessive alleles
November 2017Paper1 ["HL"] · TZ012[ 1 ]D3.2.5—Dominant and recessive alleles
May 2016Paper2 ["HL"] · TZ05(c)[ 4 ]D3.2.5—Dominant and recessive alleles
May 2016Paper1 ["HL"] · TZ020[ 1 ]D3.2.5—Dominant and recessive alleles
Practice this objective
Coverage 2016–2022 · 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.