What is assign genotypes as you go?

Label each relevant person (Aa, aa, X^A X^a) - it forces the logic to be consistent with descent and gives the marker the working they reward.

What is match depth to the verb?

"Identify" wants the pattern named; "justify"/"explain" wants the diagnostic features tied to individuals; "predict" (e.g. with CAG-repeat or gel data) wants you to read the data and link it to onset or outcome.

§-Syllabus dot point

NSWBiologySyllabus dot point

Inquiry Question 1: How does mutation introduce new alleles into a population?

Investigate the causes of genetic variation relating to the changes and conservation of the DNA sequence including: the use of pedigree analysis to identify patterns of inheritance and mutation

A focused answer to the HSC Biology Module 6 dot point on pedigree analysis. How to identify autosomal recessive, autosomal dominant, X-linked recessive and X-linked dominant inheritance patterns from pedigree charts, with a worked haemophilia example and rules for spotting new mutations.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-21

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
The answer
Examples in context
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What this dot point is asking

NESA wants you to read a pedigree chart, identify the inheritance pattern (autosomal recessive, autosomal dominant, X-linked recessive, X-linked dominant) and link the pattern to where the mutation occurred. New mutations and de novo events are a common twist.

The answer

Pedigree symbols

Symbol	Meaning
Square	Male
Circle	Female
Filled	Affected
Half-filled	Carrier (sometimes shown)
Horizontal line between two shapes	Mating
Vertical line	Offspring
Diamond	Sex unknown

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

Inheritance patterns

Autosomal dominant.

Trait appears in every generation (no skipping).
About 50 percent of offspring of an affected parent are affected.
Both sexes affected equally.
Affected father can pass to son (rules out X-linked).
Example: Huntington disease, achondroplasia.

Autosomal recessive.

Trait often skips generations (carriers are unaffected).
Two unaffected carrier parents have 25 percent affected children.
Both sexes affected equally.
Often appears in offspring of consanguineous (related) parents.
Example: cystic fibrosis, phenylketonuria, sickle cell anaemia.

X-linked recessive.

Affects males more than females (males are hemizygous; one allele is enough).
Affected father cannot pass the condition to sons (he passes Y), but all his daughters are carriers.
Affected sons usually have a carrier mother.
Can skip generations through unaffected carrier mothers.
Example: haemophilia A and B, Duchenne muscular dystrophy, red-green colour blindness.

X-linked dominant.

Affects both sexes but more females (they have two X chromosomes).
Affected father transmits the trait to all daughters and no sons.
No generational skipping.
Example: fragile X syndrome (partly), incontinentia pigmenti.

Y-linked.

Passed strictly father to son. Females never affected.
Rare in syllabus questions but worth recognising.

Reading a pedigree: the decision tree

Is the trait in every generation?
- Yes → likely dominant.
- No → likely recessive.
Are males and females affected equally?
- Yes → likely autosomal.
- More males affected → likely X-linked recessive.
- All daughters of affected father affected → X-linked dominant.
Does an affected father pass to a son?
- Yes → autosomal (rules out X-linked).
- No, with the rule "all daughters but no sons" → X-linked dominant.
Consanguinity present? Increases the chance of autosomal recessive.

The figure below walks the same decision through three contrasting pedigrees, with the genotypes that descent forces on each individual.

Worked example: haemophilia in the British royal family

Queen Victoria was a carrier ( $X^H X^h$ ). Her son Leopold was affected ( $X^h Y$ ). Her daughters Alice and Beatrice were carriers and married into European royal houses, introducing haemophilia into the Spanish, Russian and Prussian royal families. The Russian Tsarevich Alexei was famously affected, contributing to the political instability that preceded the 1917 revolution.

The pedigree shows the classic X-linked recessive pattern: affected males, carrier females, no female-to-female transmission, and skipping through unaffected carriers.

Mutations on pedigrees

A trait may appear in a pedigree with no prior family history because of a de novo (new) mutation in a parental gamete or early in the embryo. Clues that suggest a de novo mutation:

A single affected individual with no other family history and no consanguinity.
A high-penetrance dominant condition that should be visible in the parents (e.g. achondroplasia, where roughly 80 percent of cases are de novo).
An X-linked recessive condition (e.g. haemophilia) in a boy whose mother is not a known carrier.

Mosaicism is another explanation: a parent carries the mutation in only some of their gametes, so the mutation appears in only some of their children.

Worked exam answers

Worked example 1 - work out the mode of inheritance from a pedigree (3 marks)

Question. A pedigree shows the inheritance of a rare disorder. Two unaffected parents (individuals II-3 and II-4) have an affected son (III-1). No relatives are affected elsewhere, and the parents are not related. Identify the most likely mode of inheritance and justify your answer with reference to the individuals.

Model answer. The most likely mode of inheritance is autosomal recessive. Individuals II-3 and II-4 are both unaffected yet have an affected child (III-1), so the allele must be recessive (an affected child from two unaffected parents) and inherited from both parents, making each an unaffected carrier ( $Aa$ ). The affected son III-1 is therefore $aa$ . The trait is taken as autosomal rather than X-linked because there is no specific male bias across the family and no rule (such as all-daughters-affected) pointing to X-linkage; X-linked recessive cannot be fully excluded for a single affected male, so genetic testing of the parents would confirm it.

Marker's note: 1 mark for naming autosomal recessive; 1 mark for justifying recessive from two unaffected parents with an affected child; 1 mark for justifying autosomal and/or assigning the carrier genotypes ( $Aa \times Aa \rightarrow aa$ ). Writing only "recessive" without "autosomal", or giving no individual-referenced justification, caps at 2.

Worked example 2 - justify a genotype on a dominant pedigree (3 marks)

Question. A disorder is autosomal dominant. Individual H is affected and has both affected children and unaffected children. Justify the genotype of individual H.

Model answer. Individual H must be heterozygous, genotype $Aa$ (with $A$ = the dominant disease allele). Because the condition is dominant and H is affected, H must carry at least one $A$ allele. But H also has unaffected children, who must be $aa$ ; each unaffected child received an $a$ allele from H, so H must also carry a normal $a$ allele. H is therefore $Aa$ , not $AA$ .

Marker's note: 1 mark for stating $Aa$ (heterozygous); 2 marks for the two-sided justification - affected status forces at least one $A$ , and unaffected offspring force an $a$ . A Punnett square ( $Aa \times aa \rightarrow \tfrac{1}{2}$ affected) is accepted as part of the justification. Not explaining why H cannot be $AA$ caps at 2.

Worked example 3 - distinguish the scenarios for a single affected boy (4 marks)

Question. A boy in generation III is affected; his parents and grandparents are not, and the parents are unrelated. Identify the most likely modes of inheritance and explain how you would distinguish between them.

Model answer. The trait skips generations, so it is recessive. Two scenarios fit. (1) Autosomal recessive: both parents are unaffected carriers ( $Aa \times Aa$ ), giving a $\tfrac{1}{4}$ chance of an affected $aa$ son. (2) X-linked recessive: the mother is a previously unknown carrier ( $X^A X^a$ ) who passed $X^a$ to her son ( $X^a Y$ ); the father ( $X^A Y$ ) is unaffected and could not pass it to a son. A de novo mutation is a third possibility if neither parent carries the allele. To distinguish, sequence the affected gene in the boy and both parents: an autosomal recessive case shows the variant in both unaffected parents; an X-linked case shows it in the mother only; a de novo case shows it in the boy only.

Marker's note: 1 mark for deducing recessive from the skip; 1 mark each for the autosomal recessive and X-linked recessive scenarios with correct genotypes; 1 mark for a valid distinguishing test (trio sequencing) with correct interpretation.

Exam technique

Name the full pattern: Write "autosomal recessive", not just "recessive". NESA marking guidelines repeatedly dock answers that give only half the name; the autosomal-versus-X-linked half is worth its own mark.
Justify with named individuals: When a question says "refer to the letters on the pedigree", quote them: "individuals 8 and 9 are unaffected but have an affected child 17". A justification with no specific individuals rarely reaches full marks.
Assign genotypes as you go: Label each relevant person ( $Aa$ , $aa$ , $X^A X^a$ ) - it forces the logic to be consistent with descent and gives the marker the working they reward.
Match depth to the verb: "Identify" wants the pattern named; "justify"/"explain" wants the diagnostic features tied to individuals; "predict" (e.g. with CAG-repeat or gel data) wants you to read the data and link it to onset or outcome.

Examples in context

Example 1. Huntington's disease in a NSW family. Huntington's disease is an autosomal dominant disorder caused by an expansion of CAG repeats in the HTT gene on chromosome 4. A pedigree from a typical Hunter Valley family at the Royal North Shore neurogenetics clinic shows the disease appearing in every generation, affecting roughly half of offspring of each affected parent, and males and females equally. This pattern - vertical transmission, no skipping, no sex bias - is the classic signature of autosomal dominant inheritance. Genetic counsellors use the pedigree to estimate a 50 percent risk for each child of an affected parent and offer predictive genetic testing once the child is of legal age and capable of informed consent.

Example 2. Cystic fibrosis and the carrier-parent pedigree. Cystic fibrosis is autosomal recessive. In a typical pedigree presented at the Sydney Children's Hospital genetics clinic, two unaffected parents have an affected child, then have unaffected and affected siblings in roughly the 3:1 Mendelian ratio. The trait skips generations because carriers (heterozygotes) are phenotypically normal. Both parents must contribute the recessive allele, so both are obligate Aa carriers. The pedigree pattern - parents unaffected, child affected, no sex bias, often appearing in collateral relatives - confirms autosomal recessive inheritance, and counselling can offer prenatal testing for future pregnancies.

Try this

Q1. A pedigree shows a trait that affects only males, appears in every generation, and is passed from affected grandfathers to half of their grandsons through carrier daughters. Identify the inheritance pattern and justify your answer. [3 marks]

Cue. X-linked recessive: only males affected, no male-to-male transmission, daughters are unaffected carriers.

Q2. In a pedigree of an autosomal recessive disorder, both parents are heterozygous carriers. They have four children. Calculate the probability that (a) exactly two children are affected, and (b) at least one child is affected. [2+2 marks]

Cue. (a) Binomial: C(4,2) by (1/4) squared by (3/4) squared = 27/128. (b) 1 minus (3/4) to the fourth = 175/256.

Q3. A pedigree shows an unaffected mother and an unaffected father with an affected daughter. No relatives on either side have the disorder. (a) State why this is consistent with a new (de novo) autosomal dominant mutation. (b) Suggest two other inheritance patterns to consider before concluding it is de novo. (c) Describe a molecular test that could confirm a new mutation. [2+2+2 marks]

Cue. (a) Trait appears with no carrier family history; both parents unaffected but child affected with dominant phenotype. (b) Autosomal recessive with both parents carriers; non-paternity. (c) Sequence the daughter and both parents - mutation present only in daughter confirms de novo.

Exam-style practice questions

Practice questions written in the style of NESA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

2025 HSC3 marksA pedigree shows the inheritance of CAMT, a rare inherited disorder, across a family. What type of inheritance is shown in the pedigree? Justify your answer.

Show worked answer →

Name the precise inheritance pattern and justify with specific individuals/genotypes. Sample answer: Type of inheritance: autosomal recessive. Using B (unaffected) and b (affected): BB and Bb are unaffected, bb is affected. Individuals 8 and 9 are unaffected but have an affected child (17), so the allele must be recessive and inherited from both parents, making it autosomal. (If it were sex-linked, individual 15 would be expected to be affected.) For affected individual 10, both parents (3 heterozygous Bb, 4 homozygous recessive) must carry the recessive allele. Marks: 3 = identifies the type AND provides an appropriate justification; 2 = some understanding of the type; 1 = relevant information. Common error: writing 'recessive' rather than the full 'autosomal recessive', and using incorrect alleles/Punnett squares.

Source: NESA 2025 HSC Biology examination and marking guidelines.

2023 HSC3 marksHuntington's disease is an autosomal dominant genetic disease. Using the pedigree, justify the genotype of individual 'H'. In your answer, refer to the letters on the pedigree to identify individuals.

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Deduce the genotype from the affected/unaffected status of H's children. Sample answer: Individual H must be heterozygous for the Huntington's gene (genotype Hh). This is because she has children who do not have Huntington's disease (J and L) and children who do (I and K). Since Huntington's is autosomal dominant, an affected person showing the trait must carry at least one H allele, but producing unaffected children means she must also carry the normal allele h - therefore Hh. Marks: 3 = justifies the correct genotype with reference to H's offspring; 2 = identifies possible genotypes for autosomal dominant inheritance; 1 = relevant information. Punnett squares were accepted as part of the justification. Common error: not identifying all possible genotypes and confusing dominant/recessive patterns.

Source: NESA 2023 HSC Biology examination and marking guidelines.

2023 HSC3 marksDiagram 1 shows a pedigree of a family affected by Huntington's disease; Diagram 2 shows gel electrophoresis of chromosome-4 DNA fragments with the number of CAG repeats for each individual (P-V). Predict whether individuals S and U will be affected by Huntington's disease, and if so, at what age. Use data from the diagrams to justify your answer.

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Match each individual's CAG-repeat band to an affected/unaffected relative to predict outcome and onset. Sample answer: Individual S is not predicted to be affected. The gel shows S has a band at approximately 15 repeats, the same as his father P, who does not have the disease (normal range). Individual U will most likely develop Huntington's at around age 45. U has the same number of repeats (~38) as her mother Q, who has an age of onset of 45. (More CAG repeats - earlier onset; ~15 repeats is in the normal 10-26 range, while ~38 is in the disease 37-80 range.) Marks: 3 = predicts the outcome for BOTH S and U including age of onset, with justification from the data; 2 = predicts one individual with age of onset OR interprets the data; 1 = relevant information. Common error: not linking individuals' shared CAG repeats to predict age of onset.

Source: NESA 2023 HSC Biology examination and marking guidelines.

Practice questions

Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.

foundation2 marksState the four pedigree symbols for: an unaffected male, an affected female, a carrier, and a mating between two individuals.

Show worked solution →

1 mark - the two shapes and the fill. A square is a male and a circle is a female; an unfilled shape is unaffected, while a fully filled (shaded) shape is affected. So an unaffected male is an empty square and an affected female is a filled circle.

1 mark - carrier and mating. A half-filled shape is a carrier (heterozygous, unaffected but carrying the allele); a horizontal line joining two shapes is a mating, and a vertical line dropping from it leads to their offspring.

Award the marks only if both shape conventions AND the fill conventions are correct. Mixing up square/circle, or calling a half-fill "affected", loses the mark.

foundation3 marksList three features of a pedigree that point to autosomal recessive inheritance, and three that point to autosomal dominant inheritance.

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Autosomal recessive (up to 1.5 marks). (i) The trait often skips generations because carriers are unaffected; (ii) two unaffected parents can have an affected child (both are carriers); (iii) males and females are affected roughly equally, and the trait is more common where parents are related (consanguineous).

Autosomal dominant (up to 1.5 marks). (i) The trait appears in every generation (vertical transmission, no skipping); (ii) every affected child has at least one affected parent; (iii) males and females are affected equally, and an affected father can pass it to a son.

Full marks need three valid, distinct features for each pattern. "Both sexes affected equally" can only be credited once unless paired with a distinguishing feature, because it is true of both autosomal patterns.

foundation2 marksExplain why an autosomal recessive trait can skip a generation but a fully penetrant autosomal dominant trait usually cannot.

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1 mark - recessive skips via carriers. A recessive allele is masked in heterozygous carriers, who are unaffected but still pass the allele on. A generation of carriers can therefore show the trait nowhere, then it reappears when two carriers have an affected (homozygous recessive) child - so the trait skips.

1 mark - dominant cannot skip. A dominant allele is expressed whenever it is present, so an affected person must have inherited the allele from an affected parent. With full penetrance there are no "hidden" carriers, so an unaffected person does not carry the allele and the trait appears in every generation.

The discriminator is the idea of a masked/hidden allele (recessive) versus an always-expressed allele (dominant). Mentioning penetrance is a bonus, not required for the 2 marks.

core4 marksA couple are both unaffected. Their first son is affected by a condition; no other relatives on either side are affected. Using the pedigree information, work out the most likely mode of inheritance and justify it, then state one other possibility you cannot yet rule out.

Show worked solution →

1 mark - eliminate dominant: Both parents are unaffected yet have an affected child. For a fully penetrant dominant condition an affected child needs an affected parent, so the trait is almost certainly recessive, not dominant.
1 mark - identify the most likely pattern: The most likely mode is autosomal recessive: both parents are unaffected carriers ( $Aa \times Aa$ ), giving a $\tfrac{1}{4}$ chance of an affected ( $aa$ ) child each pregnancy.
1 mark - justify with genotypes: The affected son must be $aa$ , so he received one recessive allele from each parent; both parents must therefore be $Aa$ . This is consistent with an unaffected mother and unaffected father producing an affected child.
1 mark - the alternative you cannot exclude: Because only one male is affected, X-linked recessive is still possible - the mother could be an unaffected carrier ( $X^A X^a$ ) who passed $X^a$ to her son ( $X^a Y$ ). A de novo mutation is a third possibility. Genetic testing of the parents would distinguish these.

Top answers commit to autosomal recessive as most likely while explicitly keeping X-linked recessive open, because a single affected male fits both. Stating only one pattern with no alternative caps at 3 marks.

core5 marksA pedigree shows a disorder that affects males far more than females. Affected males never pass it to their sons, but all of an affected man's daughters are unaffected carriers. Identify the mode of inheritance, justify it using the rules of transmission, and give the genotype of an affected male and of a carrier female using X and Y notation.

Show worked solution →

1 mark - name the pattern: The mode of inheritance is X-linked recessive.
2 marks - justify from transmission rules: (i) The trait affects males far more than females because males are hemizygous - a male has only one X, so a single recessive allele ( $X^a Y$ ) is enough to be affected, whereas a female needs two ( $X^a X^a$ ). (ii) No male-to-male transmission: a father gives his son a Y, not his X, so an affected father cannot pass the condition to a son. (iii) An affected father gives his X (carrying the allele) to every daughter, so all his daughters are obligate carriers - exactly what the pedigree shows.
2 marks - genotypes: An affected male is $X^a Y$ (the single X carries the recessive allele). A carrier female is $X^A X^a$ - she has one normal and one mutant allele, so she is unaffected but can pass $X^a$ to half her children.

The three transmission facts (hemizygous males, no father-to-son, all daughters carriers) are the marking spine. Correct $X^a Y$ and $X^A X^a$ notation secures the genotype marks; lower-case/upper-case superscripts must match recessive/normal correctly.

core4 marksIn an autosomal recessive pedigree, two carrier parents (

Aa \times Aa

) have three children. Calculate the probability that (a) all three children are unaffected, and (b) exactly one of the three is affected.

Show worked solution →

1 mark - per-child probabilities: From $Aa \times Aa$ , each child independently has a $\tfrac{3}{4}$ chance of being unaffected and a $\tfrac{1}{4}$ chance of being affected.
1 mark - (a) all three unaffected: Multiply independent events: $\left(\tfrac{3}{4}\right)^3 = \tfrac{27}{64}$ (about 0.42).
2 marks - (b) exactly one affected: Use the binomial term for one affected and two unaffected: $\binom{3}{1}\left(\tfrac{1}{4}\right)^1\left(\tfrac{3}{4}\right)^2 = 3 \times \tfrac{1}{4} \times \tfrac{9}{16} = \tfrac{27}{64}$ (about 0.42).

Award 1 mark for setting up the binomial coefficient $\binom{3}{1}=3$ and 1 mark for the correct arithmetic. A common slip is forgetting the factor of 3 (the number of ways to choose which child is affected) and writing $\tfrac{9}{64}$ .

exam6 marksA rare disorder appears in a single child of two unaffected, unrelated parents. The child's only sibling is unaffected, and no relative in three generations on either side is affected. Distinguish between the possible inheritance explanations for this pedigree, and describe how a molecular test could determine whether the disorder arose from a de novo mutation. (Refer to genotypes in your answer.)

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Treat this as a "distinguish + describe a test" extended response. A Band 6 answer weighs at least three explanations against the evidence, then designs a decisive test.

Explanation 1 - autosomal recessive (about 2 marks): Two unaffected, unrelated carriers ( $Aa \times Aa$ ) can produce an affected $aa$ child with probability $\tfrac{1}{4}$ per pregnancy, while the other sibling is unaffected ( $AA$ or $Aa$ ). The lack of wider family history fits, because a rare recessive allele is usually carried silently. This is a strong candidate even without consanguinity.
Explanation 2 - X-linked recessive (about 1 mark): If the affected child is male, the mother could be an unaffected carrier ( $X^A X^a$ ) who passed $X^a$ , giving an affected son ( $X^a Y$ ); the absence of affected maternal uncles makes this less likely but not impossible. (If the affected child is female this is much less likely, needing an affected/carrier father.)
Explanation 3 - de novo (new) mutation (about 1 mark): A new mutation in a parental gamete (or early embryo) can produce an affected child from genotypically normal parents, especially for a dominant condition where neither parent carries the allele. Parental mosaicism is a related possibility.
The molecular test (about 2 marks): Sequence the affected gene (DNA sequencing) in the child and in both parents. If the causative variant is present in the child but absent from both parents' DNA, the mutation is de novo (it arose new in the child). If the same variant is found in one or both unaffected parents, the disorder is inherited (e.g. both parents heterozygous carriers for a recessive allele), not de novo. Testing additional tissues can detect parental mosaicism.

Full marks: at least three explanations tied to genotypes, plus a clearly described trio (child + both parents) sequencing test with the correct interpretation (variant in child only = de novo). Listing explanations without a decisive, well-interpreted test caps at 4.

exam7 marksA pedigree shows a neurological disorder appearing in every generation, affecting males and females in roughly equal numbers, with every affected individual having an affected parent. One affected man (individual III-2) has an unaffected partner; they have four children, two affected and two unaffected. Justify the mode of inheritance, determine the genotype of individual III-2 and the probability that any further child is affected, and evaluate one limitation of using this pedigree alone to advise the family.

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"Justify... determine... evaluate" is a layered command set; a Band 6 response does all three and reaches a judgement on the limitation.

Mode of inheritance (2-3 marks): The pattern is autosomal dominant. Justify with the pedigree features: (i) the trait appears in every generation with no skipping (vertical transmission); (ii) every affected person has an affected parent; (iii) males and females are affected roughly equally, ruling out X-linked dominant (which would over-affect females and show all-daughters / no-sons transmission from affected fathers). Autosomal, dominant, fully penetrant.
Genotype of III-2 and risk (2 marks): Because the disorder is dominant and III-2 has unaffected children, he must carry a normal allele as well as the disease allele, so he is heterozygous ( $Aa$ , with $A$ = disease allele). Crossed with his unaffected ( $aa$ ) partner: $Aa \times aa$ gives offspring $\tfrac{1}{2}\,Aa$ (affected) and $\tfrac{1}{2}\,aa$ (unaffected), so each further child has a $\tfrac{1}{2}$ ( $50\%$ ) chance of being affected. This matches the observed two-affected, two-unaffected split.
Evaluate a limitation (2 marks): A reasoned limitation: the pedigree gives phenotype and probability, not certainty or onset. For a late-onset condition (e.g. Huntington's), an apparently "unaffected" young relative may simply be presymptomatic, so the pedigree can misclassify them; and the $\tfrac{1}{2}$ figure is a per-child probability, not a guarantee of the family's actual outcomes. Direct DNA testing (e.g. counting CAG repeats) gives a more reliable, individual prediction including likely age of onset. A complete evaluation states that the pedigree is a powerful first-pass tool but should be confirmed with molecular testing and genetic counselling.

Marking spine: 3 justified pedigree features, correct $Aa \times aa$ cross giving $\tfrac{1}{2}$ , and a genuine evaluated limitation (presymptomatic/penetrance/onset) with a supported judgement. An answer that states "dominant" without the three features, or gives the risk without the $Aa \times aa$ working, cannot reach 7.