Human pedigree


A human pedigree is a diagrammatic representation of the pattern of inheritance of a particular trait, usually a disease, in a family tree. Pedigree analysis is a powerful tool in medical genetics for predicting whether a newborn child will or will not suffer from a genetic disease. In modern medical genetics, pedigree analysis is typically combined with tools such as polymorphism analysis via PCR and electrophoresis of the DNA of patients and their relatives in order to make an accurate diagnosis.

The symbols conventionally used in a human pedigree are listed below.

Symbols used in pedigree diagrams.
Symbols used in pedigree diagrams.


Important rules and conventions to remember when studying pedigrees are:
  • ´╗┐Males are always represented by squares, and females by circles
  • An empty square or circle indicates that the individual does not suffer from the disease
  • A shaded square or circle indicates that the individual does suffer from the disease
  • A square or circle with a line through it indicates a deceased individual
  • A horizontal line between a male and a female indicates a mating pair
  • A double-horizontal line between a male and a female indicates an incestuous mating pair (i.e. between two people who are second cousins or closer)
  • The male should always be drawn left of the female in a mating pair
  • A vertical line that leads from a horizontal 'mating' line represents the connection between parents and their children
  • Children are always represented in order of birth, the eldest being the farthest left
  • An arrow pointing to a square or circle (the proband) indicates the individual who is under consideration (i.e. usually the person who first seeks medical attention)
  • Individuals in a pedigree are denoted first by generation (in Roman numerals) and then in order of their appearance horizontally (in Arabic numerals), so for example, the second-from-left individual in the third generation would be denoted III-2, and the fifth-from-left individual in the fourth generation would be denoted IV-5.