What is "pedigree collapse"

n. (context genealogy English) The phenomenon in which ancestral inbreeding causes the number of a descendant’s distinct ancestors to be smaller than that predicted by a binary tree ($sum\_\{i=1\}^n\; 2^i$, where $n$ represents ''number of generations'').

In genealogy, pedigree collapse describes how reproduction between two individuals who share an ancestor causes the number of distinct ancestors in the family tree of their offspring to be smaller than it could otherwise be. Robert C. Gunderson coined the term, which is also known by the German term Ahnenschwund (loosely translated: “loss of lineage”) or the term implex.