 Family trees may be seen as directed acyclic graphs , with a vertex for each family member and an edge for each parent @-@ child relationship . Despite the name , these graphs are not necessarily trees because of the possibility of marriages between relatives ( so a child has a common ancestor on both the mother 's and father 's side ) causing pedigree collapse . The graphs of <unk> descent ( " mother " relationships between women ) and <unk> descent ( " father " relationships between men ) are trees within this graph . Because no one can become their own ancestor , family trees are acyclic . 
