Editing Family tree (section)

== Graph theory ==
While family trees are depicted as trees, family relations do not in general form a [[tree (graph theory)|tree]] in the strict sense used in [[graph theory]], since distant relatives can mate. Therefore, a person can have a common ancestor on both their mother's and father's side. However, because a parent must be born before their child, an individual cannot be their own ancestor, and thus there are no loops. In this regard, ancestry forms a [[directed acyclic graph]]. Nevertheless, graphs depicting [[matrilineal]] descent (mother-daughter relationships) and [[patrilineal]] descent (father-son relationships) do form trees. Assuming no common ancestor, an ancestry chart is a [[perfect binary tree]], as each person has exactly one mother and one father; these thus have a regular structure. A Descendant chart, on the other hand, does not, in general, have a regular structure, as a person can have any number of children or none at all.