Pedigrees and Family Inheritance

HSC Biology | Study Notes

Pedigrees and family inheritance are a key part of NSW Biology Stage 6, Module 5, Heredity. This topic matters because Module 5 specifically requires students to model genotype combinations and inheritance patterns using pedigrees and Punnett squares, including autosomal and sex-linked inheritance. HSC questions and marking guidelines regularly assess pedigree interpretation, identification of inheritance type, and justification of genetic risk.  

In this lesson

• the main pedigree symbols

• how pedigrees show inheritance patterns

• how to identify risk in families

• how to approach pedigree questions in exams

What is a pedigree?

A pedigree is a family diagram used to track the inheritance of a trait across generations.

It helps students:

• identify who is affected or unaffected

• follow a trait through a family

• work out likely genotypes

• predict the chance of future offspring showing the trait

Pedigree symbols

Pedigrees use standard symbols.

Basic symbols

• square = male

• circle = female

• shaded symbol = affected individual

• unshaded symbol = unaffected individual

These standard symbols appear repeatedly in HSC pedigree questions. 

Family lines

• a horizontal line joining a male and female shows a mating pair

• a vertical line leading down shows offspring

• siblings are connected by a horizontal branching line

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Generations

Pedigrees are usually arranged by generations:

• oldest generation at the top

• youngest generation lower down

Inheritance patterns

Pedigrees can help identify the type of inheritance.

The main patterns students usually consider are:

• autosomal dominant

• autosomal recessive

• sex-linked dominant

• sex-linked recessive

HSC questions directly test whether students can identify these patterns from pedigree diagrams.  

Autosomal dominant

In autosomal dominant inheritance:

• only one dominant allele is needed for the phenotype to appear

• affected individuals usually have an affected parent

• the trait often appears in every generation

Clue in a pedigree

If the trait does not skip generations very often, autosomal dominant inheritance may be likely.

Autosomal recessive

In autosomal recessive inheritance:

• two recessive alleles are needed for the phenotype to appear

• unaffected parents can have an affected child if both are carriers

• the trait may skip generations

Clue in a pedigree

A 2025 HSC marking guideline explains autosomal recessive inheritance by noting that unaffected parents can produce an affected child, showing both parents must carry the recessive allele. 

Sex-linked inheritance

In sex-linked inheritance, the gene is located on a sex chromosome, usually the X chromosome.

Why this matters

Because males have only one X chromosome:

• recessive X-linked traits may appear more often in males

• inheritance patterns can differ between sons and daughters

Clue in a pedigree

When deciding whether a trait is sex-linked, look carefully at:

• whether affected males are common

• whether fathers pass traits to sons

• whether daughters are affected in a way that fits X-linkage

HSC questions often ask students to distinguish between sex-linked and autosomal inheritance from pedigrees. 

Identifying risk

Pedigrees are useful for estimating the risk of an offspring inheriting a condition.

How this is done

To identify risk:

1. Work out the most likely inheritance pattern.

2. Assign likely genotypes to the parents.

3. Use a Punnett square if needed.

4. Calculate the probability of affected or unaffected offspring.

Example idea

If two carrier parents for an autosomal recessive condition are crossed:

• there is a 25% chance of an affected child

• a 50% chance of a carrier child

• a 25% chance of an unaffected non-carrier child

How pedigrees and Punnett squares work together

A pedigree helps you infer likely genotypes from family evidence.

A Punnett square then helps you calculate:

• genotype probabilities

• phenotype probabilities

• risk to future offspring

This combination is a very common HSC exam approach. HSC marking schemes often reward correct pedigree interpretation plus appropriate Punnett square working. 

Exam approach

Pedigree questions are often less about memorising facts and more about careful reasoning.

A NSW exam-skills document advises students to:

• read the question carefully

• identify exactly what is being asked

• jot down an outline for longer responses

• include all required steps in a process

• answer with only the relevant amount of detail 

Good pedigree method

Step 1: read the key

Make sure you know:

• who is male or female

• who is affected or unaffected

• 
  

Step 2: look for the pattern

Ask:

• does the trait skip generations?

• do unaffected parents have affected children?

• are mostly males affected?

• does an affected parent appear in each generation?

Step 3: assign likely genotypes

Use letters carefully and logically.

Step 4: justify with evidence from the pedigree

In exam answers, refer to specific individuals or relationships if possible.

Step 5: calculate risk if needed

Use a Punnett square after you have decided the likely parental genotypes.

Worked example

Exam-style question

How can a pedigree suggest that a trait is autosomal recessive?

Worked answer

A pedigree may suggest autosomal recessive inheritance if two unaffected parents have an affected child. This means both parents are likely to be carriers and the affected child inherited a recessive allele from each parent.

Why this works

This answer:

• identifies the pattern

• explains the genetic reasoning

• links the pedigree evidence to genotype

Common mistakes

• Mixing up males and females by reading the symbols incorrectly.

• Forgetting that unaffected parents can have an affected child in recessive inheritance.

• Assuming every affected male means the trait is sex-linked.

• Writing a Punnett square before working out the likely parental genotypes.

• Giving an inheritance type without justifying it from the pedigree.

Quick quiz

1. What does a square represent in a pedigree?

2. What does a shaded symbol represent?

3. What inheritance pattern is suggested if unaffected parents have an affected child?

4. Why are pedigrees useful for identifying risk?

5. What should you do after deciding the likely parental genotypes?

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