Pedigree Collapse

I've encountered this concept before, but have not been aware of its name.

Taken to its very extreme, we have pedigree collapse where a single individual occupies all male spots: https://en.wikipedia.org/wiki/Y-chromosomal_Adam, and a single female occupes all female spots, https://en.wikipedia.org/wiki/Mitochondrial_Eve 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.

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.