2.4.4Topic 2 · Genes and Health

Pedigree Diagrams — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

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2.4.4 Pedigree Diagrams — Edexcel A Level Biology A (SNAB) 9BI0 notes: Interpret pedigree diagrams to deduce whether an allele is dominant or recessive, identify genotypes and calculate probabilities.A pedigree diagram (family tree) shows how a characteristic is inherited through…

Exam-style question 1
Exam-style question 2
Exam-style question 3
Sex-linked
Genetic counselling
Probability

Section 1 of 9Reading a pedigree

A pedigree diagram (family tree) shows how a characteristic is inherited through generations.

Squares represent males and circles represent females.

Shaded symbols show affected individuals; unshaded show unaffected.

A horizontal line joins parents, and a vertical line leads down to their children.

Generations are labelled with Roman numerals (I, II, III), and individuals within a generation with Arabic numbers (1, 2, 3),

so you can refer to 'individual II-3'.

A half-shaded symbol or a dot sometimes marks a carrier.

Section 2 of 9Deducing recessive inheritance

The key clue for a recessive condition is two unaffected parents who have an affected child. This is only possible if both parents are heterozygous carriers, so the allele must be recessive and be hidden in the parents. Features of recessive inheritance:

The trait can skip generations.

Affected individuals are homozygous recessive (aa).

Unaffected parents of an affected child are both carriers (Aa).

Affected and unaffected people appear in both sexes about equally (autosomal).

Section 3 of 9Deducing dominant inheritance

The key clue for a dominant condition is two affected parents who have an unaffected child.

Deducing dominant inheritance

This is possible only if both are heterozygous and the child is homozygous recessive.

Features: affected individuals have at least one affected parent, the trait appears in every generation,

and unaffected individuals are homozygous recessive (aa) and do not pass on the condition.

An affected parent with an unaffected spouse may have both affected and unaffected children if the affected parent is heterozygous.

Section 4 of 9Sex-linked inheritance

For a recessive allele carried on the X chromosome (for example haemophilia), more males than females are affected, because males have only one X chromosome.

An affected father cannot pass the allele to his sons (he gives them his Y), but passes it to all of his daughters, who become carriers.

A carrier mother has a 50% chance of passing the allele to each child, so half her sons are expected to be affected.

Pedigrees showing affected males in alternate generations, passing via unaffected carrier females, suggest an X-linked recessive allele.

An affected daughter requires an affected father and a carrier or affected mother.

Section 5 of 9Calculating probabilities

Once the genotypes are deduced you can find the probability of an outcome using a Punnett square.

Calculating probabilities

If two carriers (Aa × Aa) have a child, the probability that the child is affected is 1/4 and that an unaffected child is a carrier is 2/3 (two of the three unaffected genotypes).

Probabilities for independent events are multiplied: the probability that two children are both affected is 1/4 × 1/4 = 1/16.

The probability does not change with the previous children, because each fertilisation is independent.

Cross	Offspring	Probability affected
Aa × Aa	1 AA : 2 Aa : 1 aa	25%
Aa × aa	1 Aa : 1 aa	50%
AA × aa	All Aa	0% (all carriers)

Section 6 of 9A method for exam answers

To explain how you reached a conclusion, identify a family in which the clue appears, state the relevant individual numbers, and write out the genotypes of parents and offspring.

A method for exam answers

Then argue why the alternative (dominant or recessive) is impossible: for example, if the condition were dominant, the unaffected parent would have to be homozygous recessive and unable to have an affected child.

Pedigrees are also used in genetic counselling, to estimate the risk to future children and to decide whether to use testing such as amniocentesis or preimplantation genetic diagnosis.

Practise by choosing a pedigree and writing the genotype on every person, using a letter such as A/a.

Start with people who must be homozygous recessive (affected, in a recessive condition) because their genotype is certain,

then deduce parents and children.

Where genotype is uncertain, write A- to show that the second allele is unknown.

If a question asks whether a pedigree could show X-linked inheritance, check whether any affected woman has an unaffected father or unaffected son: this would rule it out.

State your reasoning with the individual numbers, as examiners expect a clear chain of logic.

Consider also complications such as incomplete penetrance, where some people with the genotype do not show the phenotype, and consanguineous marriages, shown by a double line, which raise the chance that two carriers of a rare recessive allele have children together.

Section 7 of 9Exam-style questions

In a pedigree, individuals I-1 and I-2 are unaffected and have an affected daughter II-2. Deduce whether the allele is dominant or recessive and whether it is sex-linked. [4]4 marks
Two carriers of cystic fibrosis have two children. Calculate the probability that both have cystic fibrosis. [2]2 marks
Explain why X-linked recessive conditions are more common in males. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Pedigree	A family tree showing inheritance of a trait over generations.
Carrier	An unaffected individual with one copy of a recessive allele.
Autosomal	Located on a non-sex chromosome.
Sex-linked	Located on the X or Y chromosome.
Affected	Shows the phenotype in question.
Genetic counselling	Advice on the risk of inherited conditions to families.
Probability	The likelihood of an event, expressed as a fraction, ratio or percentage.

Section 9 of 9Exam technique

Using the number of affected individuals alone to decide dominant or recessive; look for the parents-and-child clue.

Forgetting that unaffected individuals in a recessive pedigree may be carriers.

Treating probabilities of consecutive children as dependent.

Confusing squares and circles, or writing that females are squares.

Last updated: 13 October 2026

Section 1 of 9Reading a pedigree

Section 2 of 9Deducing recessive inheritance

Section 3 of 9Deducing dominant inheritance

Section 4 of 9Sex-linked inheritance

Section 5 of 9Calculating probabilities

Section 6 of 9A method for exam answers

Section 7 of 9Exam-style questions

In a pedigree, individuals I-1 and I-2 are unaffected and have an affected daughter II-2. Deduce whether the allele is dominant or recessive and whether it is sex-linked. [4]4 marks

Two carriers of cystic fibrosis have two children. Calculate the probability that both have cystic fibrosis. [2]2 marks

Explain why X-linked recessive conditions are more common in males. [3]3 marks

Section 8 of 9Key terms

Section 9 of 9Exam technique

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