A pedigree tree is a diagram depicting members of a family, their inter-relations, and their disease or phenotypic status. It provides an overview of the inheritance pattern and frequency of a specific trait.

Each family member is represented by a symbol: circles for females and squares for males. Filled symbols represent affected individuals, and empty ones represent normal individuals. Matings are illustrated by connecting horizontal lines, while their offspring is depicted beneath. Each generation is assigned a Roman number (e.g. I, II, III) while individuals of a generation are assigned Arabic numbers (e.g. 1, 2, 3).

A pedigree tree showing 3 generations of a family and their status for a genetic condition. Squares represent males, circles represent females, with a blank shape indicating an unaffected individual and a filled shape representing an affected individual with the trait. Generation one is labelled with a Roman numeral one and contains two individuals. Individual one is an affected male and individual two is an unaffected female. A horizontal line connects both of them. A vertical line descends from this horizontal line, and branches connect to their offspring in generation 2, which is labelled with roman number 2. Individuals 2 and five on this branch are females and are both descended from generation 1, with a horizontal line connecting them to a male, 1 and 6, respectively. Individual 3 is also a direct descendent from generation 1 and is connected by a horizontal line to a female labelled as individual 4. None of the six individuals in the second generation have the affected trait.  Each generation pair has 2 offspring indicated by a vertical line downwards to their children, so there are a total of 6 individuals in generation 3, labelled with Roman number 3. Individual 1 is an unaffected female, and individual 2 is an affected male. They are the offspring of individuals 1 and 2 in generation 2. None of the other offspring have the affected trait. Individuals 3 and 4 from generation 2 have one female and one male offspring, labelled 3 and 4, respectively. Individuals 5 and 6 from generation 2 have one female and one male offspring, labelled 5 and 6 respectively.

Figure 1: Pedigree illustrating a three generation family and the inheritance of an X-linked recessive trait.