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

Q: 2019 HSC HSC: Two unaffected parents have a son with haemophilia, an X-linked recessive condition. Neither set of grandparents has any history of haemophilia. Explain how this is possible.

Haemophilia ($X^h$) is recessive on the X chromosome. **Most likely explanation: carrier mother.** The mother is a heterozygous carrier ($X^H X^h$). She is unaffected because the dominant $X^H$ allele compensates. She passes the $X^h$ allele to her son, who inherits only one X chromosome from her and so is affected ($X^h Y$). **Why no grandparent history?** 1. The mother may have inherited the $X^h$ allele from an unaffected carrier grandmother who never had affected sons (by chance, all her sons received $X^H$). 2. Alternatively, a **new (de novo) mutation** in the mother's gamete or in early embryonic development of the mother produced the $X^h$ allele. Roughly 30 percent of haemophilia cases are de novo mutations. Markers reward (1) the carrier mother explanation, (2) the X-linked inheritance logic ($X^h$ from mother, Y from father), and (3) the de novo mutation as an alternative.

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 OpusReviewed by Better Tuition Academy7 min answerUpdated 2026-05-18

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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.

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 example

A pedigree:

I:    1 (M, unaffected) x 2 (F, unaffected)
       |
II:   1 (F, unaffected) x 2 (M, unaffected)
       |
III:  1 (M, affected)

A boy in generation III is affected. His parents and grandparents are not.

Reasoning.

Skipped two generations → recessive likely.
Only one male affected, no other clues to X-linked or autosomal.

Most likely. Autosomal recessive with both parents being carriers ( $Aa \times Aa$ , 25 percent chance of affected child), or X-linked recessive with the mother being a previously unknown carrier ( $X^H X^h$ ). A de novo mutation is also possible if the parents are not carriers on testing.

How to distinguish. Genetic testing of the parents resolves which scenario applies.

Common traps

Assuming "appears in every generation" always means dominant. A common recessive allele in a small population can appear in every generation through unrelated carrier matings. Look at multiple features.

Forgetting that an affected female with X-linked recessive needs an affected father AND a carrier mother. This is rare and a useful diagnostic clue.

Calling a sporadic case "no inheritance pattern." Many sporadic cases are de novo mutations or are explained by parental mosaicism.

Ignoring consanguinity. Marriages between relatives concentrate rare recessive alleles, and the pedigree will often show a double horizontal line for consanguineous mating.

In one sentence

Pedigree analysis identifies inheritance patterns by examining which generations are affected, the sex ratio of affected individuals, and how the trait is transmitted between parents and children, allowing geneticists to classify mutations as autosomal dominant, autosomal recessive, X-linked recessive, X-linked dominant or new (de novo) mutations.

Past exam questions, worked

Real questions from past NESA papers on this dot point, with our answer explainer.

2022 HSC5 marksA pedigree shows a condition appearing in every generation, in roughly half the offspring of every affected parent, and in both males and females equally. Identify the most likely inheritance pattern and justify your answer.

Show worked answer →

A 5-mark answer needs the pattern named, the four key observations explained, and one alternative ruled out.

Most likely pattern: autosomal dominant.

Justification.

Every generation affected. Dominant alleles are expressed in heterozygotes, so the trait does not skip generations. Recessive conditions can skip generations through unaffected carrier parents, which is not seen here.
Roughly half of offspring of an affected parent. An affected heterozygote ( $Aa$ ) crossed with an unaffected homozygote ( $aa$ ) produces 50 percent $Aa$ (affected) and 50 percent $aa$ (unaffected). The observed ratio matches.
Both sexes affected equally. Rules out X-linked patterns, which usually affect males more (X-linked recessive) or show characteristic father-to-daughter transmission (X-linked dominant).
Affected father to affected son. Visible transmission from an affected father to a son confirms the gene is on an autosome (the father passes Y, not X, to sons).

Alternative ruled out. Autosomal recessive is ruled out because every generation is affected and unaffected x affected matings produce 50 percent affected children (a recessive condition would require two carrier parents and yield 25 percent affected).

Worked example. Huntington disease shows exactly this pattern.

Markers reward (1) the named pattern, (2) at least three justifications tied to specific pedigree features, and (3) at least one alternative explicitly ruled out.

2019 HSC3 marksTwo unaffected parents have a son with haemophilia, an X-linked recessive condition. Neither set of grandparents has any history of haemophilia. Explain how this is possible.

Show worked answer →

Haemophilia ( $X^h$ ) is recessive on the X chromosome.

Most likely explanation: carrier mother. The mother is a heterozygous carrier ( $X^H X^h$ ). She is unaffected because the dominant $X^H$ allele compensates. She passes the $X^h$ allele to her son, who inherits only one X chromosome from her and so is affected ( $X^h Y$ ).

Why no grandparent history?

The mother may have inherited the $X^h$ allele from an unaffected carrier grandmother who never had affected sons (by chance, all her sons received $X^H$ ).
Alternatively, a new (de novo) mutation in the mother's gamete or in early embryonic development of the mother produced the $X^h$ allele. Roughly 30 percent of haemophilia cases are de novo mutations.

Markers reward (1) the carrier mother explanation, (2) the X-linked inheritance logic ( $X^h$ from mother, Y from father), and (3) the de novo mutation as an alternative.