Question
How do you identify autosomal dominant, autosomal recessive, and X-linked recessive inheritance patterns from a pedigree chart? Give the key clues for each.
(NEET + CBSE Class 12 — high-value question type)
Solution — Step by Step
Key clues:
- Affected individuals appear in every generation (no skipping)
- An affected person has at least one affected parent
- Both males and females are affected equally
- Unaffected parents produce only unaffected children
Examples: Huntington’s disease, achondroplasia (dwarfism), polydactyly
Key clues:
- Trait can skip generations (carrier parents, affected children)
- Two unaffected parents can have affected children (both must be carriers)
- Affected individuals are often born to consanguineous (related) parents
- Males and females affected equally
Examples: Sickle cell anaemia, cystic fibrosis, albinism, phenylketonuria
Key clues:
- Males are predominantly affected (they need only one copy of the mutant allele)
- Carrier females (heterozygous) are unaffected but pass the trait to sons
- An affected father cannot pass the trait to sons (he gives Y to sons, X to daughters)
- All daughters of an affected father are at least carriers
Examples: Haemophilia, colour blindness, Duchenne muscular dystrophy
Pedigree Pattern Identification Decision Tree
flowchart TD
A["Look at the pedigree"] --> B{"Does the trait skip generations?"}
B -->|"No — every generation"| C["Likely Autosomal Dominant"]
B -->|"Yes — skips"| D{"Are mostly males affected?"}
D -->|"No — equal in both sexes"| E["Likely Autosomal Recessive"]
D -->|"Yes — mostly males"| F{"Does affected father pass to sons?"}
F -->|"No"| G["X-linked Recessive"]
F -->|"Yes"| E
C --> H["Confirm: every affected has 1 affected parent"]
E --> I["Confirm: unaffected parents have affected children"]
G --> J["Confirm: carrier mothers, affected sons"]Why This Works
Pedigree analysis is pattern recognition. Each inheritance mode creates a predictable pattern in the family tree. Autosomal dominant traits cannot hide — they show up every generation. Autosomal recessive traits can hide in carriers. X-linked recessive traits disproportionately affect males because males have only one X chromosome (hemizygous).
The decision tree approach works because you can systematically eliminate possibilities: first check if it skips generations, then check sex ratio, then check father-to-son transmission.
Common Mistake
The trickiest distinction: autosomal recessive vs X-linked recessive. Both can skip generations. The key differentiator is: in X-linked recessive, affected fathers NEVER pass the trait to sons (because fathers give Y, not X, to sons). If you see an affected father with an affected son, it CANNOT be X-linked — it must be autosomal. NEET uses this logic to design trap options.
For NEET: always check the male-to-female ratio of affected individuals first. If mainly males are affected and the trait comes through the mother’s side, think X-linked. If the ratio is roughly equal, think autosomal.