The big idea: A pedigree chart is a family tree that tracks one inherited condition through several generations.
Every person is a shape: a square is male, a circle is female.
If the shape is filled in, that person is affected (shows the condition). If it is clear, they are unaffected.
By following who is affected and who is not, you can work out how the condition is inherited — and that is exactly what the exam asks.
Reading a pedigree: squares are males, circles are females, and a filled symbol means the person is affected (shows the condition).
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- Pedigree chart
- A diagram showing how a single inherited trait or disease passes through the generations of a family.
- Affected individual
- A person who shows the condition — drawn as a filled (shaded) symbol.
- Carrier
- An unaffected person who carries one copy of a recessive allele (genotype, e.g., Dd) and can pass it on.
- Generation
- Each horizontal row of the chart, labelled with a Roman numeral (I, II, III), oldest at the top.
- Autosomal allele
- An allele on one of the 22 ordinary chromosome pairs (not the sex chromosomes).
- X-linked allele
- An allele carried on the X chromosome, so its inheritance is tied to a person's sex.
| Symbol | What it means |
|---|---|
| Square ▢ | A male |
| Circle ◯ | A female |
| Unfilled (clear) symbol | Unaffected — does NOT show the condition |
| Filled (shaded) symbol | Affected — shows the condition |
| Horizontal line joining two symbols | A mating pair (parents) |
| Vertical line dropping from a couple | Leads down to their children |
| Horizontal sibship line | Joins brothers and sisters (one set of children) |
| Roman numerals (I, II, III) | Label each generation, oldest at the top |
Square = male, circle = female: An easy way to remember: a square has corners like a man's broad shoulders; a circle is round.
And filled = affected — the condition has 'filled the person in'. Get these two facts right and the rest of the chart reads itself.
Once you can read the symbols, every pedigree question is really two questions: is the allele dominant or recessive, and is it autosomal or X-linked?
You answer each one by hunting for a specific clue in the chart.
Step 1 — dominant or recessive?
- Look for two UNAFFECTED parents with an AFFECTED child. If you find this, the allele must be recessive — the parents are hidden carriers (Dd × Dd).
- If instead the trait appears in every generation and affected children almost always have an affected parent, it is most likely dominant.
- Quick test: 'skips a generation' → recessive; 'shows up every generation' → dominant.
The give-away for recessive: Two unaffected parents producing an affected child is the single most powerful clue.
It can only happen if the condition is recessive and both parents are carriers (each Dd), so a quarter of their children can be dd (affected).
A dominant condition cannot 'hide' in an unaffected parent — if you have the dominant allele, you show the trait.
Step 2 — autosomal or X-linked?
- Count affected males vs females. Roughly equal numbers → likely autosomal.
- Far more affected males, often inherited from carrier mothers → likely X-linked recessive.
- Key X-linked test: an affected father passes his single X to EVERY daughter but never to a son (a son gets his father's Y). So an affected father with an unaffected daughter rules X-linked dominant OUT.
| Clue in the pedigree | Most likely pattern |
|---|---|
| The trait appears in EVERY generation (no skipping); affected children usually have an affected parent | DOMINANT |
| The trait SKIPS a generation; two unaffected parents have an affected child | RECESSIVE |
| Affected ≈ equal in males and females; an affected father can have affected daughters AND unaffected daughters | AUTOSOMAL (allele on a normal chromosome) |
| Many more affected males than females; an affected father passes it to ALL his daughters (X-linked dominant) or never to his sons (X-linked) | X-LINKED (allele on the X chromosome) |
Why a father's pattern reveals X-linkage: A father passes his Y chromosome to every son and his X chromosome to every daughter.
So if a disease allele were on the X, an affected father could never give it to a son (sons get his Y), but would give it to all his daughters.
If a pedigree shows an affected father with an affected son, or with an unaffected daughter, the allele is not X-linked — it must be autosomal.
Allele location: affected sons and an affected daughter both appear, and males and females are affected fairly evenly — consistent with an autosomal (not X-linked) allele.
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Then read the genotypes: Once you know the pattern, you can write each person's possible genotypes using the letters given in the question.
For an autosomal dominant allele D: an affected person is DD or Dd; an unaffected person can only be dd.
For an autosomal recessive allele: an affected person is dd; an unaffected person is DD or Dd (and 'Dd' makes them a carrier).
| If the condition is... | Affected person's genotype | Unaffected person's genotype |
|---|---|---|
| Autosomal DOMINANT (allele D) | DD or Dd (at least one D) | dd only |
| Autosomal RECESSIVE (allele d) | dd only | DD or Dd (a carrier) |
| X-linked RECESSIVE (allele on X) | XʳXʳ (female) or XʳY (male) | XᴿX_ (female) or XᴿY (male) |
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How this is tested: On Paper 1 a 1-mark item gives a pedigree and asks you to conclude the inheritance pattern (dominant/recessive) or deduce where the allele is located (autosomal/X-linked).
On Paper 2 a structured question can ask you to state the possible genotypes of a named person using given symbols, calculate the probability that a couple's child is affected, or explain how the pedigree shows an allele is (for example) not X-linked.
IB-style question — pattern, location and genotype
In one family, a condition called Tarian syndrome is studied. Two unaffected parents, Mara and Joss, have three children: an affected daughter, an unaffected son and an unaffected daughter. Using the symbols T (allele for the syndrome) and t:
(a) Deduce, with a reason, whether Tarian syndrome is dominant or recessive. [2]
(b) State whether the allele is most likely autosomal or X-linked, and give a reason. [2]
(c) State the genotype of Mara (the unaffected mother). [1]
How to score all five marks
- (a) Dominant or recessive? Two unaffected parents have an affected child. A dominant allele cannot hide in an unaffected parent, so the syndrome must be recessive — both parents are carriers. (1 mark: recessive. 1 mark: because two unaffected parents have an affected child / both parents carry the allele.)
- (b) Autosomal or X-linked? The affected child is a daughter whose father is unaffected. If the recessive allele were on the X, an affected daughter would need an affected father (she gets one X from him). The father is unaffected, so the allele is most likely autosomal. (1 mark: autosomal. 1 mark: an X-linked recessive affected daughter would need an affected father, but the father is unaffected.)
- (c) Mara's genotype. The syndrome is autosomal recessive (allele t). Mara is unaffected but has an affected (tt) child, so she must carry one t. Her genotype is Tt (a carrier). (1 mark: Tt.)
Final answer
(a) Recessive — two unaffected parents have an affected child, which only a recessive allele allows. (b) Autosomal — an X-linked recessive affected daughter would need an affected father, but her father is unaffected. (c) Mara is Tt (a carrier).
✓ Why this scores full marks: Each part states the answer AND gives the reason from the pedigree — the reason is where the marks usually are.
Part (c) uses the rule that an unaffected parent of an affected (recessive) child must be a carrier (Tt), not the homozygous TT — that single deduction is the mark.
Spotting the pattern: the trait appears in an affected parent and in their children every generation, with no 'skipping' — that points to a dominant allele.
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