Pedigree Collapse Calculator

Pedigree Collapse

In theory, each person has 2^N ancestors at generation N (2 parents, 4 grandparents, 8 great-grandparents, etc.). At generation 30 — about 900 years ago — that gives over 1 billion ancestors, which is more people than were alive at the time.

Pedigree collapse is the resolution to this paradox: many of those theoretical ancestor slots are filled by the same people. Distant cousins marry, lineages cross, and the realistic count of distinct people is much smaller than 2^N.

The collapse formula (simplified): Realistic ancestors ≈ Population × (1 - e^(-2^N / Population))

This caps at the size of the contributing population, since you cannot have more distinct ancestors than there were people alive at that time.

Effective population size matters:

• A fully isolated village (1,000 people, e.g., a remote island): collapse begins by generation 10 (~300 years ago)
• A regional population (100,000 people, typical pre-industrial European area): collapse meaningful by generation 17 (~500 years ago)
• A national population (10 million, e.g., medieval France): collapse meaningful by generation 24 (~700 years ago)
• A continental population (50-100 million): collapse hits by generation 28-30 (~900 years ago)

Common findings:

• By 1200 AD, every European has roughly the same set of ancestors as every other European (Genetic Genealogy MRCA studies)
• Charlemagne (born 742 AD) is an ancestor of essentially every person of European descent
• For globally mixed populations, the universal MRCA is around 5,000 years ago

Practical implications:

• Most ancestors at deep generations are duplicates — your tree fills out fast on paper but slowly in distinct people
• If you find a “shared ancestor” with someone in DNA results 8+ generations back, it may be one of dozens of shared ancestors, not the unique link
• This is why DNA segments shrink so quickly — most distant cousin matches share many tiny segments from many shared ancestors, not one big segment from one