directed acyclic graphs
... that because of pedigree collapse, some family trees are better modeled mathematically as directed acyclic graphs than as trees?
... that because of <a href="./Pedigree_collapse" id="mwBwY" rel="mw:WikiLink" title="Pedigree collapse">pedigree collapse</a>, some <a href="./Family_tree" id="mwBwc" rel="mw:WikiLink" title="Family tree">family trees</a> are better modeled mathematically as <b id="mwBwg"><a href="./Directed_acyclic_graph" id="mwBwk" rel="mw:WikiLink" title="Directed acyclic graph">directed acyclic graphs</a></b> than as <a href="./Tree_(graph_theory)" id="mwBwo" rel="mw:WikiLink" title="Tree (graph theory)">trees</a>?