What Pedigree Analysis Is
A pedigree chart is a family tree that shows the occurrence of a genetic trait across multiple generations. Geneticists use them to determine how a trait is inherited — autosomal or X-linked? Dominant or recessive?
If a doctor sees a child with cystic fibrosis, both parents are apparently healthy, and the maternal grandmother had it — pedigree analysis lets the doctor figure out the inheritance pattern and predict the risk for future children.
CBSE Class 12 Biology Chapter 5 (Principles of Inheritance) includes pedigree analysis. NEET tests it at a moderate level — typically asking students to identify inheritance patterns from a given pedigree.
Pedigree Symbols
Before reading a pedigree, learn the standard symbols:
| Symbol | Meaning |
|---|---|
| Square (□) | Male individual |
| Circle (○) | Female individual |
| Filled square/circle (■/●) | Affected individual (shows the trait) |
| Half-filled symbol | Carrier (heterozygous, does not show trait) |
| Horizontal line between □ and ○ | Mating/marriage |
| Vertical line from couple | Offspring |
| Horizontal line connecting siblings | Sibship line |
| Roman numerals (I, II, III…) | Generation number |
| Arabic numerals (1, 2, 3…) | Individual number within generation |
The Four Inheritance Patterns
Pattern 1 — Autosomal Dominant
Characteristics:
- Every affected person has at least one affected parent
- The trait appears in every generation (no skipping)
- Both males and females equally affected
- Affected individuals are usually heterozygous (Aa)
- Unaffected individuals are homozygous (aa)
Key clue: If an unaffected child has TWO affected parents, impossible for autosomal dominant (unless parents are both Aa and child got aa).
Examples of traits: Huntington’s disease, polydactyly (extra fingers), achondroplasia (dwarfism)
Test cross reasoning: Trait cannot skip generations. If Generation II is all unaffected but one parent in Generation I is affected, something is wrong — reconsider your pattern.
Pattern 2 — Autosomal Recessive
Characteristics:
- Affected individuals can have two unaffected parents (both carriers)
- The trait can skip generations (carrier parents → affected child)
- Both males and females equally affected
- Affected individuals are homozygous recessive (aa)
- Unaffected individuals can be AA or Aa
Key clue: Two unaffected parents producing an affected child = autosomal recessive (or X-linked recessive if the child is male).
Examples: Cystic fibrosis, sickle cell anaemia, PKU (phenylketonuria), albinism, alkaptonuria (first inherited condition described by Garrod)
Pattern 3 — X-linked Recessive
Characteristics:
- More males affected than females (females need two copies of the recessive allele; males only need one)
- No male-to-male transmission (an affected father passes his X to daughters, not sons)
- Affected males are sons of carrier mothers
- An affected female must have an affected father AND a carrier or affected mother
- Can skip generations (carrier females)
Key clue: If only males are affected, and none of the daughters are affected, consider X-linked recessive.
Examples: Haemophilia A and B, Colour blindness (red-green), Duchenne muscular dystrophy, G6PD deficiency
Pattern 4 — X-linked Dominant
Characteristics:
- Affected father transmits to ALL daughters (not to sons)
- Affected males produce carrier daughters (not carriers — they express it)
- Both males and females affected, but more females (since they have two X chromosomes)
- Males tend to be more severely affected
- Does NOT skip generations
Key clue: If an affected father has ALL affected daughters and NO affected sons — X-linked dominant.
Examples: Hypophosphataemia (Vitamin D-resistant rickets), Rett syndrome (in females)
How to Analyse a Pedigree — Step-by-Step
Follow this systematic approach for any pedigree question:
- Look at the parents of affected individuals. Are both parents affected or unaffected?
- Check sex ratio. Are males and females equally affected?
- Check generation skipping. Does the trait skip a generation?
- Check father-son transmission. If an affected father has an affected son, it cannot be X-linked (since the son gets the Y chromosome from father, not X).
- Assign genotypes to all individuals and verify consistency.
Worked Example 1
A pedigree shows: Generation I: unaffected male × unaffected female. Generation II: 2 unaffected females, 1 affected male, 2 unaffected males. No other generations.
Analysis:
- Parents are unaffected but produced an affected child → suggests recessive
- The affected individual is male → could be autosomal recessive OR X-linked recessive
- Other males in Generation II are unaffected → doesn’t conclusively rule out X-linked
- If X-linked recessive: the mother must be a carrier (X^A X^a). Probability of affected son = 1/2. This is consistent.
- If autosomal recessive: both parents are carriers (Aa × Aa). Probability of affected = 1/4. Also consistent.
- Conclusion: Cannot distinguish between autosomal recessive and X-linked recessive from this pedigree alone — need more data (affected females, or affected father). In exams, if there are no affected females and the question asks which is MORE likely, X-linked recessive is a better fit if the ratio of affected males is high.
Worked Example 2
A pedigree shows: Generation I: affected male × unaffected female → Generation II: 2 affected females, 1 unaffected female, 3 unaffected males.
Analysis:
- Affected father → ALL daughters affected, NO sons affected → X-linked dominant!
- Father (X^A Y) passes X^A to all daughters → all daughters get the dominant allele → all affected ✓
- Father passes Y to sons → sons unaffected ✓
- Pattern: X-linked dominant
Important Genetic Conditions and Their Patterns
| Condition | Inheritance | Key Feature |
|---|---|---|
| Haemophilia A | X-linked recessive | Blood clotting disorder |
| Haemophilia B | X-linked recessive | Factor IX deficiency |
| Colour blindness | X-linked recessive | Mainly affects males |
| Duchenne MD | X-linked recessive | Progressive muscle weakness |
| Huntington’s disease | Autosomal dominant | Late-onset neurodegeneration |
| Cystic fibrosis | Autosomal recessive | Lung and digestive problems |
| Sickle cell anaemia | Autosomal recessive | Abnormal haemoglobin |
| Phenylketonuria (PKU) | Autosomal recessive | Enzyme deficiency |
| Thalassaemia | Autosomal recessive | Haemoglobin disorder |
| Polydactyly | Autosomal dominant | Extra digits |
NEET frequently asks about haemophilia and colour blindness pedigrees. Royal family pedigree (Queen Victoria was a carrier of haemophilia A) is a standard textbook example. Know: carrier females transmit the disease; affected males get it from carrier mothers; no male-to-male transmission.
Calculating Probabilities from Pedigrees
Once you’ve identified the pattern and assigned genotypes, calculate probabilities using Mendelian ratios.
Example: A carrier woman (X^A X^a) × normal man (X^A Y). What is the probability of a haemophiliac daughter?
Probability of haemophiliac daughter = 0 (daughters either normal or carrier, not affected since they’d need two copies).
Probability of haemophiliac son = 1/2 among sons = 1/4 overall.
Autosomal vs X-linked — Quick Decision Rules
If affected father has affected son → NOT X-linked (son gets Y from father, not X)
If only males affected, no female-to-male transmission → Likely X-linked recessive
If all daughters of affected father are affected, no sons affected → X-linked dominant
If trait skips generation (unaffected parents have affected child) → Recessive
If every affected person has at least one affected parent, no generation skipping → Dominant
If males and females equally affected → Likely autosomal (not X-linked)
Common Mistakes to Avoid
Mistake 1: Concluding “autosomal dominant” just because both sexes are affected. X-linked dominant also affects both sexes. Check whether affected fathers have all-affected daughters but no affected sons (X-linked dominant clue).
Mistake 2: Forgetting that carrier females in X-linked recessive may occasionally be mildly affected due to lyonisation (random X-inactivation). In a CBSE pedigree, assume carriers are unaffected unless told otherwise.
Mistake 3: Not checking all generations for consistency. After proposing a pattern, go through every individual and verify that the assigned genotypes are consistent with the proposed pattern. A single inconsistency invalidates the hypothesis.
Mistake 4: Treating haemophilia as “only seen in males.” Females CAN get haemophilia if they are homozygous X^a X^a. This requires an affected father (X^a Y) and a carrier mother (X^A X^a). While rare, it’s possible and has appeared in CBSE questions.
Practice Questions
Q1. A couple with normal vision has a colour-blind son. The father is normal. Is the mother definitely colour-blind?
No. Colour blindness is X-linked recessive. The son (X^a Y) got his X^a from the mother. The mother must be at least a carrier (X^A X^a). She could be a carrier with normal vision, OR she could be colour-blind (X^a X^a). Without more information, we know she is a carrier or colour-blind — “definitely colour-blind” is too strong a statement.
Q2. In a pedigree, two unaffected parents have four children — one affected male, one affected female, one unaffected male, one unaffected female. What is the most likely inheritance pattern?
Both parents unaffected but have affected children of both sexes → autosomal recessive. If it were X-linked recessive, affected females would be unusual (they’d need to inherit the recessive allele from both parents — possible but less likely). With equal sex representation among affected individuals, autosomal recessive is the most parsimonious explanation.
Q3. A woman with haemophilia (X^a X^a) marries a normal man. What fraction of their sons will have haemophilia?
The mother is X^a X^a; she passes X^a to ALL children. The father is X^A Y; he passes X^A to daughters and Y to sons. Sons get Y from father and X^a from mother → X^a Y → ALL sons have haemophilia (100%). Daughters get X^A from father and X^a from mother → X^A X^a → all daughters are carriers but unaffected.
Q4. How does a pedigree help in genetic counselling?
Genetic counselling uses pedigree analysis to: (1) identify the inheritance pattern of a family’s disease, (2) calculate the probability that future children will be affected, (3) identify carriers in the family, (4) advise about genetic testing options (prenatal diagnosis). Without a pedigree, the counsellor cannot determine whether a disease is dominant/recessive or autosomal/X-linked, making risk calculation impossible.
FAQs
Can a pedigree definitively prove an inheritance pattern? In theory, yes — if the pedigree is large enough and all individuals are correctly diagnosed. In practice, small pedigrees are ambiguous. Genetic testing (sequencing the gene or protein) provides definitive confirmation.
What is mitochondrial inheritance? Why isn’t it covered in standard pedigrees? Mitochondrial DNA is inherited exclusively from the mother (mitochondria in the egg). Mitochondrial diseases affect all offspring of an affected mother (both sons and daughters), with no paternal transmission. Standard Mendelian pedigree rules don’t apply. Examples: Leber’s hereditary optic neuropathy (LHON), MELAS syndrome.
What’s the difference between a carrier and an affected individual? Carriers possess the disease allele but don’t show the trait (because they have a normal copy too, in recessive disorders). Affected individuals show the trait — they are homozygous for the recessive allele, or have a dominant allele. In X-linked recessive disorders, carrier females are unaffected but can pass the disease to sons.
Can the same disease show different inheritance patterns in different families? Yes. Genetic heterogeneity means the same clinical disease can result from mutations in different genes, each with its own inheritance pattern. For example, some forms of deafness are autosomal dominant, others autosomal recessive, others X-linked. Family-specific pedigree analysis is essential for accurate counselling.
What is consanguinity and why does it matter in pedigrees? Consanguinity means mating between close relatives. It increases the probability that both parents carry the same rare recessive allele (inherited from a common ancestor), increasing the chance of homozygous affected offspring. Pedigrees for rare autosomal recessive diseases often show consanguinity.