Pedigree Collapse Calculator

Pedigree Collapse

In theory, each person has 2^N ancestors at generation N: 2 parents, 4 grandparents, 8 great-grandparents, and so on. Run that to generation 30, about 900 years ago, and it gives over a billion ancestors. There were nowhere near a billion people alive in 1100.

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. The generation given is the one where 2^N first exceeds the population, so it is just the base-2 logarithm of the population rounded up. Years assume 30 to a generation, the same figure our ancestor count calculator uses.

• A fully isolated village (1,000 people, a remote island say): your tree outgrows it at generation 10, about 300 years back
• A regional population (100,000, a typical pre-industrial European area): generation 17, about 500 years back
• A national population (10 million, medieval France): generation 24, about 720 years back
• A continental population (100 million): generation 27, about 810 years back
• A global pre-industrial population (500 million): generation 29, about 870 years back

Common findings:

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

Practical implications:

• Most ancestors at deep generations are duplicates. Your tree fills out fast on paper and 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 inherited from many shared ancestors, rather than one big segment from a single one