A pedigree diagram is a family tree that shows which individuals in a family express a trait and which do not, allowing you to deduce the alleles each member likely carries. By working through a GCSE pedigree systematically, you can determine whether a condition is dominant or recessive, and whether it is autosomal or sex-linked.
What is a pedigree diagram?
A pedigree diagram (also called a pedigree chart or family tree) is a standardised way of recording and displaying the inheritance of a genetic trait across multiple generations of a family. Unlike a Punnett square (which predicts probabilities for one cross), a pedigree shows what actually happened in a real family.
GCSE biology uses pedigrees to examine inheritance of conditions such as cystic fibrosis (autosomal recessive), Huntington's disease (autosomal dominant), and red–green colour blindness (X-linked recessive).
What are the symbols in a pedigree diagram?
| Symbol | Meaning |
|---|---|
| Square (□) | Male individual |
| Circle (○) | Female individual |
| Filled shape (■ / ●) | Affected by the condition |
| Half-filled shape | Carrier (not affected but carries one copy of a recessive allele) |
| Unfilled shape (□ / ○) | Unaffected and not known to be a carrier |
| Horizontal line between ■/□ and ●/○ | Mating pair |
| Vertical line downward | Line of descent to offspring |
| Horizontal line connecting siblings | Siblings from the same parents |
| Roman numerals (I, II, III…) | Generation number (I = oldest) |
| Arabic numerals (1, 2, 3…) | Individual within a generation (left to right) |
In pedigree questions, examiners usually tell you which convention they are using for half-filled shapes (carriers).
How do you analyse a pedigree step by step?
Work through this method for any GCSE pedigree question:
Step 1 — Decide: dominant or recessive?
Look for a case where two unaffected parents have an affected child. If this occurs, the condition must be recessive — the parents each carry one copy of the allele but do not express it. Both must be carriers (Aa × Aa → some aa children).
If every affected person has at least one affected parent, the condition is likely dominant.
Step 2 — Assign alleles
Let the dominant allele be A and the recessive allele be a.
- Affected individual with a recessive condition: aa
- Unaffected parent of an affected child (for a recessive condition): Aa (must be a carrier)
- Unaffected individual with no affected children: may be AA or Aa (cannot always tell)
Step 3 — Check for sex-linkage
If the condition appears far more often in males than females, suspect X-linked recessive. Key patterns:
- Affected fathers cannot pass X-linked alleles to sons (they pass their Y chromosome to sons)
- Carrier mothers pass the allele to 50% of sons (who are affected) and 50% of daughters (who become carriers)
- Affected daughters must have an affected father AND a carrier or affected mother
If affected individuals are equally distributed between males and females, the condition is likely autosomal (on a non-sex chromosome).
Worked example: autosomal recessive pedigree
A family pedigree shows two unaffected parents (I-1 and I-2) with three children. Child II-2 is affected; siblings II-1 and II-3 are unaffected. II-2 is male.
Analysis:
- Two unaffected parents have an affected child → the condition is recessive
- Let A = unaffected allele, a = affected allele
- II-2 is affected → genotype aa
- Both parents must each have contributed one a allele → both parents are Aa (carriers)
- Unaffected siblings (II-1, II-3): each could be AA (probability 1/3 among unaffected offspring from Aa × Aa) or Aa (probability 2/3)
- The condition appears in a male, but both male and female offspring can be affected → consistent with autosomal inheritance (not X-linked, since the father I-1 is unaffected and would need to be affected to pass an X-linked allele to a son)
Probability question: What is the probability that unaffected sibling II-1 is a carrier?
From Aa × Aa: offspring are ¼ AA, ½ Aa, ¼ aa. Among unaffected offspring (¾ of total): ¼ AA and ½ Aa, so ²/₃ of unaffected individuals are carriers.
Answer: probability = 2/3
How do you identify X-linked recessive inheritance in a pedigree?
Key indicators:
- More males than females are affected
- Affected males have carrier (unaffected) mothers, not affected mothers
- An affected male never passes the condition to his sons (his sons get his Y chromosome)
- All daughters of an affected male are at least carriers
Example — red–green colour blindness (X-linked recessive):
Let X^B = normal vision allele, X^b = colour-blind allele.
- Affected male: X^b Y
- Carrier female: X^B X^b (normal vision but carries the allele)
- Affected female: X^b X^b (must have inherited one X^b from her father and one from her carrier/affected mother)
If a pedigree shows an unaffected woman (I-2) whose father (generation 0) was colour-blind, she must be a carrier (X^B X^b), because she received X^b from her affected father.
What are common GCSE conditions and their inheritance patterns?
| Condition | Inheritance | Affected genotype | Notes |
|---|---|---|---|
| Cystic fibrosis | Autosomal recessive | aa | 1 in 25 people in the UK are carriers |
| Huntington's disease | Autosomal dominant | Aa or AA | Just one copy causes the condition; late onset |
| Red–green colour blindness | X-linked recessive | X^b Y (male), X^b X^b (female) | Affects ~8% of males, ~0.4% of females |
| Haemophilia A | X-linked recessive | X^h Y (male) | Queen Victoria was a carrier; spread through European royal families |
Frequently asked questions
How can I tell if two unaffected parents will have affected children?
If both parents are carriers of a recessive allele (genotype Aa × Aa), on average 1 in 4 of their children will be affected (aa). You can identify carriers in a pedigree if they are the unaffected parents of an affected child — both must be Aa. Once you know both parents are Aa, the probability that any future child will be affected is ¼ (25%), a carrier is ½ (50%), and completely unaffected (AA) is ¼ (25%).
Can a pedigree ever prove that a condition is dominant?
A pedigree can provide strong evidence for dominant inheritance if every affected individual has at least one affected parent, and if unaffected individuals never have affected children. However, it is difficult to prove dominance conclusively from a pedigree alone if the family is small — chance could mean some dominant heterozygotes (Aa) produce only unaffected offspring in a small family. Proving dominance with certainty requires large-scale genetic testing.
Why might an X-linked condition be harder to spot in a female than in a male?
A male with one X chromosome is affected if he has even one copy of the recessive X-linked allele (X^b Y). A female has two X chromosomes, so if she inherits one X^b, she is likely to also have a normal X^B from the other parent — making her an unaffected carrier. A female is only affected if she receives X^b from both parents (X^b X^b), which requires an affected father and at least a carrier mother. This is why X-linked recessive conditions affect males much more often than females.
What is the difference between a carrier and an affected individual?
A carrier is a heterozygous individual (Aa) who carries one copy of a recessive allele but does not show the condition because the dominant allele (A) masks the recessive one. The carrier has normal phenotype but can pass the recessive allele to their children. An affected individual is homozygous recessive (aa) and shows the condition. Only individuals with two copies of the recessive allele are affected (for autosomal recessive conditions).
For Socratic GCSE biology with Professor Darwin — working from family patterns back to the alleles that generate them — visit aitutors.me.