4.8.5Additional lessons

Pedigree Charts — IB Biology HL Revision Notes & Topical Questions

5 min read 5 parts Lesson 72 of 101

📚 IB Biology HL Hub / Topical Revision

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Part 1 of 5Reading a Pedigree

Symbols

Squares stand for males and circles for females, and shaded symbols show people who have the characteristic.

Lines

A horizontal line joins a couple, and a vertical line leads down to their children, who are usually drawn left to right in birth order.

Generations

Generations are labelled with Roman numerals and individuals with numbers, so 'II-3' is the third person in the second generation.

Purpose

Pedigrees allow genotypes to be deduced when direct genetic testing is not available.

Deducing the Pattern of Inheritance

Recessive clue

Two unaffected parents with an affected child show that the condition is recessive and both parents are heterozygous.

Dominant clue

Two affected parents with an unaffected child show that the condition is dominant and both parents are heterozygous.

Autosomal clue

An affected daughter whose father is unaffected cannot be X-linked recessive,
- because such a daughter must receive an affected X from her father.

X-linked clue

Many more affected males than females, with the allele passing through unaffected carrier mothers, suggests X-linked recessive inheritance.

Observation	Conclusion
Unaffected parents, affected child	Recessive
Affected parents, unaffected child	Dominant
Affected mother, all sons affected	Likely X-linked recessive

Decide whether the allele is dominant or recessive by looking for a couple who have a child with a different phenotype from both of them.

Assign letters to the alleles and write down the genotype of every person who can be certain, starting with affected recessive individuals (aa).

Use parents and children to work out the unknown genotypes, because an aa child must have received an a allele from each parent.

Calculate probabilities from the genotypes of the parents using a Punnett grid.

Give the final answer with a reason referring to the specific individuals shown on the chart.

Question

Two unaffected parents have an affected daughter with a recessive autosomal condition.
Their next child is unaffected.
What is the probability that this child is a carrier?

The affected daughter is aa, so both parents passed on an a allele and each parent is Aa.
The cross Aa × Aa gives 1 AA : 2 Aa : 1 aa.
The child is unaffected, so aa is excluded and only AA and Aa remain: 3 out of 4 outcomes.
Of those three, two are Aa, so the probability is 2/3.

Answer The unaffected child has a 2/3 (about 67%) probability of being a carrier.

Pedigree A diagram showing the inheritance of a characteristic through several generations of a family.
Autosomal Carried on a chromosome that is not a sex chromosome.
Affected Showing the phenotype under study, drawn as a shaded symbol.
Carrier An unaffected individual heterozygous for a recessive allele.

Giving the answer 1/4 for an unaffected child, forgetting that the aa possibility has been excluded.
Calling a trait dominant just because it appears in every generation, without checking the parents of unaffected children.
Mixing up the symbols for males and females.
Ignoring the question's wording about which individual is being asked about.

Last updated: 28 October 2026