A family pedigree is a diagram that traces how a particular characteristic (often a genetic disorder) passes through several generations of a family. Pedigree diagrams are useful for working out:
- Whether the characteristic is dominant or recessive
- The probability that a particular family member is affected, or could pass the condition on to their children
Pedigree symbols
Pedigree diagrams use standard symbols:
- Squares represent males; circles represent females
- Filled-in (shaded) shapes represent affected individuals (those showing the characteristic)
- Empty shapes represent unaffected individuals
- A horizontal line between a male and a female means they are a couple who have produced children
- A vertical line going down from a couple connects them to their children, who are shown side by side
Reading a pedigree
To work out whether a trait is dominant or recessive:
- If two unaffected parents have affected children, the trait must be recessive (because both parents are carriers but don't show the trait).
- If every affected child has at least one affected parent, the trait is likely to be dominant.
- If the trait appears mainly in males and skips generations, it might be X-linked recessive (covered in the next section).