I've encountered this concept before, but have not been aware of its name.<p>Taken to its very extreme, we have pedigree collapse where a single individual occupies all male spots: <a href="https://en.wikipedia.org/wiki/Y-chromosomal_Adam" rel="nofollow">https://en.wikipedia.org/wiki/Y-chromosomal_Adam</a>, and a single female occupes all female spots, <a href="https://en.wikipedia.org/wiki/Mitochondrial_Eve" rel="nofollow">https://en.wikipedia.org/wiki/Mitochondrial_Eve</a> Interestingly, these happened at different points in history.
Of particular interest to me was how pedigree collapse transforms our family trees from exponentially growing binary trees into directed acyclic graphs.<p>We are accustomed to thinking that, because we have four unique grandparents, we must have eight unique great-grandparents. But this is not always so. For example, if our grandparents are two brothers who married two sisters, then we have four great-grandparents, not eight.