Interpret pedigrees to determine the mode of inheritance and predict genotypes

A focused answer to the WACE Year 12 Biology dot point on pedigree analysis. Covers reading pedigree symbols, distinguishing dominant from recessive and autosomal from sex-linked inheritance, and assigning genotypes from a family tree.

Generated by Claude Opus 4.86 min answerUpdated 2026-06-02

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

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What this dot point is asking

SCSA wants you to read standard pedigree symbols, apply a logical set of rules to work out the mode of inheritance, and deduce genotypes including carriers. A strong answer states the reasoning, not just the conclusion.

Reading a pedigree

Standard symbols are used: squares are males, circles are females, shaded shapes are affected individuals, and unshaded shapes are unaffected. Horizontal lines join mating pairs, and vertical lines connect parents to offspring. Generations are usually numbered with Roman numerals.

Deciding dominant or recessive

Two questions usually settle this.

If two affected parents have an unaffected child, the trait is dominant (the child must be homozygous recessive, so the unaffected phenotype is recessive).
If two unaffected parents have an affected child, the trait is recessive (the parents must both be carriers).

Deciding autosomal or sex-linked

Once you know dominant or recessive, check the pattern across the sexes.

For a recessive trait, if it is far more common in males and appears to pass from carrier mothers to sons, it is likely X-linked recessive.
If affected and unaffected individuals appear in roughly equal numbers across both sexes, the trait is more likely autosomal.
An affected father who never passes an X-linked recessive trait to his sons (because he gives them his Y) is a useful check.

Assigning genotypes

Work systematically:

Assign genotypes to affected individuals first, since their phenotype fixes their genotype (for example homozygous recessive for a recessive trait).
Use the affected children to deduce that unaffected parents must be carriers.
Fill in the remaining individuals, leaving a question mark where the genotype cannot be fully determined.

Spotting sex linkage in a pedigree

Once a trait is shown to be recessive, a few extra patterns reveal whether it sits on the X chromosome. An X-linked recessive trait is usually far more common in males, because a male ( $XY$ ) shows the trait with just one copy of the recessive allele, whereas a female ( $XX$ ) needs two. A second strong clue is the father-to-child pattern: an affected father passes his single X to all of his daughters, so every daughter of an affected father is at least a carrier, but he passes only his Y to his sons, so he never transmits an X-linked recessive trait directly to a son. If you see an affected condition apparently jumping from an affected grandfather, through unaffected carrier daughters, to affected grandsons, that criss-cross pattern is a hallmark of X-linked recessive inheritance. By contrast, an autosomal recessive trait affects both sexes about equally and can pass from father to son.

A logical sequence

A reliable approach is: first decide dominant or recessive using the two-parent rule, then decide autosomal or sex-linked using the pattern across sexes, and only then assign genotypes. Doing the steps in this order avoids guessing and produces an answer you can justify, which is what marks reward.

Why pedigrees matter

Pedigree analysis is the practical tool of genetic counselling. By working out the mode of inheritance, a counsellor can estimate the chance that a couple's child will inherit a condition such as cystic fibrosis or haemophilia. It connects the abstract rules of inheritance to real decisions made by Australian families and the genetic services that support them.

Exam-style practice questions

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

WACE 20226 marksA pedigree shows a condition that appears in every generation, affects males and females in roughly equal numbers, and is passed from an affected parent to roughly half the children. Two affected parents have one unaffected child. Determine the most likely mode of inheritance, justify your answer, and assign the genotypes of that unaffected child and its parents.

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A 6 mark answer needs the mode, the justification and genotypes.

Mode of inheritance: Autosomal dominant. Let $A$ be the dominant (affected) allele and $a$ the recessive (unaffected) allele.
Justification: Two affected parents producing an unaffected child shows the trait is dominant (the unaffected child must be homozygous recessive, so unaffected is the recessive phenotype). Roughly equal numbers of affected males and females, and the trait appearing in every generation, point to autosomal (not sex-linked) dominant inheritance.
Genotypes: The unaffected child is $aa$ . Because each parent passed an $a$ allele to that child, and both parents are affected, both parents must be heterozygous $Aa$ .

Markers reward identifying autosomal dominant, the two-affected-parents-to-unaffected-child justification, the equal-sex evidence, and correct genotypes ( $aa$ child, $Aa$ parents).

WACE 20245 marksIn a pedigree, a condition is much more common in males than females, and an affected male never passes the condition to his sons but his daughters can be carriers. Explain why these observations indicate X-linked recessive inheritance.

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A 5 mark answer must link each observation to X-linked recessive inheritance.

More common in males: Males are $XY$ with only one X chromosome, so a single recessive allele on the X ( $X^a Y$ ) is expressed. Females ( $XX$ ) need two copies ( $X^a X^a$ ) to be affected, which is rarer, so the condition appears more often in males.
Affected father not passing to sons: A father gives his Y chromosome to his sons, not his X, so an X-linked recessive allele cannot pass from father to son.
Daughters as carriers: A father gives his single X (carrying the recessive allele) to all his daughters, so each daughter receives $X^a$ and becomes at least a carrier ( $X^A X^a$ ).

Markers reward the single-X reasoning for males, the father-to-son Y inheritance and the father-to-daughter X inheritance making daughters carriers.