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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

Dominant and Recessive Describe an Allele Relationship

For a complete-dominance locus, the dominant allele determines the heterozygous phenotype; the recessive phenotype appears only when no dominant allele is present.

One dominant allele may produce enough functional product for the dominant phenotype, so AA and Aa look alike in this model, whereas aa lacks that contribution.

Genotype Phenotype in a complete-dominance model
AA Dominant
Aa Dominant
aa Recessive

In Aa × Aa, the expected genotypes are 1 AA : 2 Aa : 1 aa but the expected phenotypes are 3 dominant : 1 recessive.

Dominant does not mean common, beneficial or stronger. Dominance describes the phenotype of a heterozygote.

Dominant and recessive alleles

Assessment in practice

1–2 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / Explain / Deduce.

Command terms

Identify / Explain / Deduce

What earns marks

Build the answer around this relationship: Dominant alleles are expressed in heterozygotes.

Representative question

Question 1

[Maximum number: 4]

Many genetic diseases are due to recessive alleles of autosomal genes that code for an enzyme. Using a Punnett grid, explain how parents who do not show signs of such a disease can produce a child with the disease.

Retrieve the Core Inheritance Route

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.

  • haploid gametes carry one allele and fertilization restores a diploid genotype
  • dominance, codominance, incomplete dominance, environment, and plasticity affect the observed trait
  • PKU, ABO, sex determination, and haemophilia use different inheritance rules and notation
  • pedigrees infer inheritance patterns and box plots summarize continuous variation

Solve Core Inheritance Questions

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.

  • Use allele and genotype notation correctly for monohybrid, ABO, PKU, haemophilia, and sex-determination contexts.
  • Connect genotype, dominance pattern, environment, or plasticity to phenotype.
  • Use pedigree or box-plot evidence to justify an inheritance or variation conclusion.

Concept essentials

  • Dominant alleles are expressed in heterozygotes.
  • Recessive phenotypes usually require two recessive alleles.
  • Dominance describes expression, not allele frequency or value.