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Quiz yourself on Inherited DisordersGet your flashcardsBuild my revision planTrack my progress🌳 Reading Family Trees (Pedigree Diagrams)
Examiners often give you a family tree and ask you to work out genotypes or identify carriers. Here's how to read one:
Symbols
- Square = male, Circle = female
- Filled/shaded = affected by the condition
- Unshaded = not affected (could still be a carrier)
- Horizontal line = couple, Vertical line = their children
Worked Example: Is the Condition Dominant or Recessive?
Step 1: Look at affected children. If two unaffected parents have an affected child, the condition MUST be recessive (both parents are carriers — Ff × Ff → ff child).
Step 2: If an affected parent has unaffected children, the condition could be dominant (Dd × dd → some dd children who are unaffected).
Identifying Carriers
If a condition is recessive (like cystic fibrosis) and two unaffected parents have an affected child (ff), BOTH parents must be carriers (Ff). You can then calculate the probability for other children using a Punnett square.
Key exam skill: Always start by deciding if the condition is dominant or recessive. Then assign genotypes to the affected individuals first, and work backwards to the parents.
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