CourseCourseQuestionQuestionDocumentDocument

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
HL

Exam analysis

Chance of appearing11%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1

Common command terms

  • Identify
  • Deduce
  • Determine
  • Draw
  • Calculate
  • State
  • Explain

Recent exam appearances

November 2024Paper1 ["HL"] · TZ012[ 1 ]D3.2.13—Pedigree charts
May 2023Paper1 ["HL"] · TZ213[ 1 ]D3.2.13—Pedigree charts
May 2022Paper1 ["HL"] · TZ212[ 1 ]D3.2.13—Pedigree charts
November 2020Paper1 ["HL"] · TZ010[ 1 ]D3.2.13—Pedigree charts
May 2019Paper1 ["HL"] · TZ218[ 1 ]D3.2.13—Pedigree charts

Coverage 2013–2024 · Updated 16 Jul 2026

Read Pedigree Grammar Before Inferring an Inheritance Pattern

Pedigree mark Meaning
square / circle male / female in the standard convention
filled symbol individual shows the tracked phenotype
horizontal partner line mating or reproductive partnership
vertical descent line offspring connection
shared sibship line siblings from the same parents
I, II, III … generations
1, 2, 3 … individuals within a generation
  1. identify the phenotype being tracked and whether filled means affected
  2. trace parents, siblings and generations accurately
  3. mark genotypes that are forced by phenotype or parent–child transmission
  4. keep uncertain alleles as alternatives until more evidence removes them

A pedigree records observed family evidence, not every conception or every relative. Small families and missing records can make more than one inheritance model possible.

Pedigree Inference Eliminates Models That Cannot Produce the Family

Observation Strong inference
two unaffected parents have an affected child supports recessive inheritance; contradicts simple complete-dominant inheritance
affected person has an affected parent in every generation supports dominance, but does not prove it alone
affected father has an affected son contradicts X-linked transmission from father to son
affected sons arise from unaffected mothers consistent with X-linked recessive carrier mothers
males and females affected similarly supports autosomal inheritance

Start with forced genotypes. For an autosomal recessive trait, affected people are aa; each unaffected parent of an affected child must carry a. For an X-linked recessive trait, an affected son XʰY proves that his mother supplied Xʰ.

Treat each inheritance mode as a hypothesis. Predict transmissions that must or cannot occur, compare them with the pedigree, and reject a model only when a relationship contradicts it.

Skipping a generation suggests recessiveness but is not a proof. Chance, incomplete records, late onset and variable expression can obscure the apparent pattern.

Human Inheritance Evidence Runs from Chromosomes to Families to Populations

X and Y contributions establish the usual chromosomal starting point → unequal X-linked gene dosage changes inheritance risk → pedigrees reveal transmission across families → polygenes and environment create continuous population variation → box plots summarize centre and spread

Evidence source Reliable question
X-linked cross which parent supplies the allele to sons or daughters?
pedigree which inheritance hypotheses are contradicted by family relationships?
continuous measurements where is the median and how wide is the middle 50%?
apparent outlier biological extreme, data error or different process?

Match the claim to the evidence scale. One Punnett grid predicts a fertilization probability, a pedigree constrains family genotypes, and a box plot describes a sample distribution; none alone proves a universal biological rule.

Pedigree charts

[ 2 ]

Explain how the pedigree chart shows that the dominant allele causing PKD is not on the X chromosome.

Drawing canvas

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.