Blood Relations

A blood relations problem describes a chain of family relationships in words — often stated from one person's own point of view ("my father's only son," "the daughter of my mother's brother") — and asks how two people in that chain are related to each other.

As a template, a small three-generation family can be shown as an indented list:

• G (grandparent)
  • P1, P2 (G's two children — a sibling link between them)
    • C (P1's child — a downward parent-child link from P1 to C, making C a grandchild of G)

Each indentation level marks one generation down; a link is drawn only between a parent and a child, or between siblings on the same row.

The reliable method is to draw a small family tree as each clue is read, rather than trying to hold the whole chain in memory. Represent each person by their initial, draw a horizontal line between two people who are married or are siblings, and a line going downward from a parent to a child; work through the relationship chain one clue at a time, adding one new person (or one new connection) to the diagram at each step, until every person named in the problem has been placed. The relationship asked about is then read directly off the completed diagram by counting generations and branches.

A handful of standard family terms recur constantly in these problems and are worth fixing clearly: