A Pedigree Is a Family Tree, Showing Cancer Events

A pedigree is simply a type of “family tree” which show’s cancer events that have occurred in a family.

In human pedigrees, squares represent males and circles represent females. A horizontal line between a square and a circle represents a union or marriage. A vertical line down from the horizontal line leads to the offspring of that union or marriage. A vertical line down from the horizontal line leads to the offspring of that union. If there are more than one offspring, they are joined by a horizontal line.

Each line of symbols is a generation line and numbered with a Roman numeral from top to bottom. Each individual within a generation is assigned an Arabic number starting from the left of the pedigree and going to the right.

This pedigree traces hereditary breast ovarian cancer (HBOC) through five generations. Note that the proband (indicated by the arrow) had breast cancer at age 38. We see that her mother (III-1) had breast cancer at age 30, her grandmother (II-1) had ovarian cancer at age 58, and her great-grandmother (I-2) had breast cancer at age 39. Thus, we see four generations affected with breast or ovarian cancer. Individual (II-4) a male who dies at age 70 without cancer had a daughter (III-6) with breast cancer at age 42 and a son (III-7) living at age 70 without cancer. This son in turn had two daughters, (IV-8) who had ovarian cancer, (IV-9) who had breast cancer. In this situation, we infer that individuals II-4 and III-7, both males, were actually obligate gene carriers in that they inherited the deleterious gene from their mother/grandmother (I-2) and transmitted it to their daughters who manifested the syndrome cancer. Individual IV-4 had breast cancer at 31, a second breast cancer at 48 and then ovarian cancer at 52.

Hopefully, this has shown you how the construction of a pedigree can be useful in tracking the disorder throughout the family and in identifying individuals who are at increased risk for this disease.

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