Pedigree analysis - TCE Biology (Tasmania)
“Use pedigree charts to describe and analyse the inheritance of genetic disorders and disease: identify and justify the likely mode of inheritance (dominant or recessive, autosomal or X-linked) and the likely genotypes of individuals”
A pedigree is a family tree showing who has a trait. To find the mode of inheritance, look for evidence that only one mode can explain. Two unaffected parents with an affected child proves the trait is recessive; two affected parents with an unaffected child proves it is dominant. An X-linked recessive trait cannot pass from father to son, and an affected mother has only affected sons; an X-linked dominant trait passes from an affected father to all of his daughters and none of his sons. Always justify by naming the individuals in the pedigree that show the pattern.
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What this dot point is asking
Pedigree analysis is part of criterion 8 (Section E of the TASC exam). The external assessment specifications ask you to identify and justify the likely pattern of inheritance (dominant or recessive, X-linked or autosomal) and the likely genotypes of individuals.
Reading a pedigree
- Squares are males; circles are females.
- Shaded symbols are affected individuals; unshaded are unaffected. Some pedigrees show carriers with a dot or half shading.
- A horizontal line joining a male and female shows a mating; a vertical line leads to their children.
- Generations are numbered with Roman numerals (I, II, III); individuals in each generation with numbers from left to right (II-3 is the third person in generation II).
Dominant or recessive?
| Evidence in the pedigree | Conclusion |
|---|---|
| Two unaffected parents have an affected child | Recessive (both parents are carriers) |
| Two affected parents have an unaffected child | Dominant (both parents are heterozygous) |
| Every affected person has at least one affected parent, in every generation | Suggests dominant, but does not prove it |
| The trait appears in a child when neither parent shows it | Recessive |
Autosomal or X-linked?
X-linked recessive is ruled out if:
- an affected female has an unaffected son (she would pass her only allele type, an affected X, to every son); or
- an affected female has an unaffected father (he would have to give her an X carrying the allele, so he would be affected).
X-linked dominant is ruled out if:
- an affected male has an unaffected daughter (he passes his X to every daughter); or
- an affected male has an affected son whose mother is unaffected (sons get their X from their mother).
If X-linked inheritance is ruled out, the trait is autosomal. A trait appearing mostly in males suggests X-linked recessive, but in a small family this is weak evidence on its own.
"Skips a generation", "mostly males" and "uncommon in the family" are suggestive patterns, not proof. TASC markers reward evidence that only one mode of inheritance can explain, tied to named individuals.
Assigning genotypes
- Decide the mode of inheritance.
- Give every affected individual the genotype that fits (for recessive, aa or / ).
- Work out unaffected individuals from their parents and children: an unaffected parent of an affected child (recessive trait) must be a carrier, Aa.
- If a genotype could be either, write both (AA or Aa) and say why.
A family with an eye condition
Generation I: an unaffected man (I-1) and an unaffected woman (I-2) have three children: an affected son (II-1), an unaffected daughter (II-2) and an unaffected son (II-3). II-2 has children with an unaffected man (II-4): an affected son (III-1) and an unaffected daughter (III-2).
- Recessive
- I-1 and I-2 are unaffected but have an affected son (II-1), so the allele is recessive.
- X-linked or autosomal
- Only males are affected, and each affected son has an unaffected mother who could be a carrier. Nothing rules out X-linked recessive, so it is the most likely mode, but with so few individuals autosomal recessive cannot be excluded.
- Genotypes if X-linked recessive
- I-1 , I-2 (carrier), II-1 , II-2 (carrier, since her son III-1 is affected and his father is unaffected), III-1 .
Marker's note: name the individuals that provide each piece of evidence, and say when the evidence only suggests rather than proves a mode.
- Justifying with "skips a generation" alone
- Point to the specific unaffected parents and affected child.
- Forgetting the X and Y in sex-linked genotypes
- Write , not aa, for an affected male.
- Describing how the trait could be inherited instead of giving evidence
- The question asks you to justify from the pedigree, not to retell it.
- Ignoring the sex of individuals
- Always check father-to-son and mother-to-son transmission before deciding between autosomal and X-linked.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Original5 marksIn a pedigree, two unaffected parents (II-3 and II-4) have an affected daughter (III-2). Identify the mode of inheritance and justify your answer, and give the genotypes of II-3, II-4 and III-2 using A and a.Show worked answer →
- Mode (1 mark)
- Autosomal recessive.
- Justification (2 marks)
- The trait is recessive because two unaffected parents have an affected child, so both parents must carry a hidden recessive allele. It is autosomal, not X-linked, because an X-linked recessive trait in a daughter would require her father to be affected (he would pass his only X, carrying the allele, to her), but her father II-3 is unaffected.
- Genotypes (2 marks)
- II-3: Aa. II-4: Aa. III-2: aa.
Original4 marksExplain why the statement 'the trait skips a generation, so it must be recessive' is not a full justification of a mode of inheritance.Show worked answer →
Explanation (4 marks). Skipping a generation suggests a recessive trait but does not prove it, and it does not refer to specific individuals. A full justification points to evidence in the pedigree that only one mode can explain, such as two unaffected parents (named individuals) who have an affected child, which proves the allele is recessive. It must also address whether the trait is autosomal or X-linked, for example by showing an affected daughter with an unaffected father (ruling out X-linked recessive). TASC's 2025 assessment report noted that "skips a generation" without specific detail earned only half a mark.