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Genetics

Pedigree Analysis: Reading Family Inheritance Diagrams

Learn to interpret pedigree diagrams and determine inheritance patterns — autosomal dominant, autosomal recessive, X-linked, and more.

V
Vectora Team
STEM Education
7 min read
2025-10-04

What is a Pedigree?

A pedigree is a family tree diagram showing the inheritance of a trait across generations. Squares = males, circles = females, filled = affected.

Learning Goals:

  1. Read standard pedigree symbols.
  2. Determine if a trait is dominant or recessive.
  3. Determine if a trait is autosomal or X-linked.
  4. Assign genotypes to individuals in a pedigree.

Quick Decision Rules

ObservationInheritance Pattern
Two unaffected parents → affected childAutosomal recessive
Affected parent → all children affectedLikely autosomal dominant
More males affected, never father-to-sonX-linked recessive
Affected in every generationLikely dominant
Skips generationsLikely recessive

Worked Examples

Example 1: Autosomal Recessive

Two unaffected parents have an affected child → both parents must be carriers (Aa). Probability of next child being affected = 1/4 (25%).

Example 2: X-linked Recessive

A carrier mother (XAXaX^AX^a) × normal father (XAYX^AY):

  • Daughters: XAXAX^AX^A (normal) or XAXaX^AX^a (carrier) — none affected
  • Sons: XAYX^AY (normal) or XaYX^aY (affected) — 50% affected

Common Mistakes

  1. Assuming affected = dominant — If two unaffected parents produce an affected child, the trait must be recessive.
  2. Forgetting carrier status — Unaffected individuals can be carriers (Aa) for recessive traits.
  3. Missing X-linkage clues — If affected individuals are mostly male and there's no father-to-son transmission, think X-linked.

Exam Tips

  • Start by determining dominant vs recessive, then autosomal vs X-linked.
  • Write genotypes for all individuals you can determine, then deduce the rest.
  • Always check if the pattern is consistent across all generations shown.