D3.2.13—Pedigree charts
Pedigree charts use family relationships and phenotypes to infer genotypes, carriers and likely inheritance patterns in inheritance problems in inheritance problems.
- Syllabus
- First assessment 2025
- Objective
- D3.2.13
- Level
- SL
Exam analysis
Chance of appearing12%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1
Typical marks1
Common command terms
- Identify
- Deduce
- Determine
- Draw
- Calculate
- State
- Explain
Recent exam appearances
November 2025Paper1A ["SL"] · TZ120[ 1 ]D3.2.13—Pedigree charts
May 2025Paper2 ["SL"] · TZ34(b)(ii)[ 2 ]D3.2.13—Pedigree charts
May 2025Paper2 ["SL"] · TZ34(b)(i)[ 1 ]D3.2.13—Pedigree charts
May 2025Paper1A ["SL"] · TZ113[ 1 ]D3.2.13—Pedigree charts
November 2024Paper2 ["SL"] · TZ13(b)[ 2 ]D3.2.13—Pedigree charts
Practice this objective
Coverage 2010–2025 · Updated 16 Jul 2026
Use Pedigrees to Test Inheritance Hypotheses
A pedigree records phenotype and family relationships across generations so inheritance patterns and possible genotypes can be deduced.
| Step | Reasoning |
|---|---|
| Read symbols and relationships | Identify affected/unaffected individuals, sex, partners and offspring |
| Look for a pattern | Recessive traits may skip generations; sex linkage and dominance give different parent-offspring constraints |
| Assign only forced genotypes | Use each mating and offspring to test the hypothesis; leave uncertain alleles unknown |
Inductive reasoning proposes a pattern from the observed family data; deductive reasoning predicts who could be affected or carry an allele if that pattern is correct.
Two unaffected parents with an affected child support a recessive hypothesis; if the trait is autosomal recessive, both parents must carry the allele.
Consanguineous partners are more likely to share a rare ancestral recessive allele, but relatedness does not guarantee an affected child. Small pedigrees may fit more than one model.
Pedigree charts
Assessment in practice
1 marksHow it is assessed
This objective is assessed through structured response, commonly using Identify / Deduce / Determine.
Command terms
Identify / Deduce / Determine / Draw / Calculate / State / Explain
What earns marks
Build the answer around this relationship: Pedigrees use affected and unaffected relatives to infer hidden genotypes.
Representative question
Question 1
Explain how the pedigree chart shows that the dominant allele causing PKD is not on the X chromosome.
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
- Pedigrees use affected and unaffected relatives to infer hidden genotypes.
- Unaffected parents with an affected child can indicate recessive inheritance.
- Sex-linked traits must be traced through X chromosome transmission.