Family History Day

What the day is

Family History Day is observed on June 14, and the first useful thing to know is that your family tree cannot possibly be a tree.

Origin and background

The arithmetic forces it. You have two parents, four grandparents, eight great-grandparents, and the number doubles with every generation back. Forty generations, roughly a thousand years, would require more than a trillion ancestors, which is vastly more people than have ever existed. The only resolution is that the same individuals occupy many positions at once, because your ancestors were related to one another. Genealogists call this pedigree collapse, and it is not a curiosity or a sign of anything unusual. It is mandatory, for everyone.

The consequence is that populations are far more interrelated than people expect. Mathematical work on this suggests the most recent common ancestor of everybody alive is startlingly recent in historical terms, and that beyond a certain point every person then living is either an ancestor of all of us or of none of us.

DNA testing carries limits that are rarely spelled out. You inherit half your DNA from each parent, but past a handful of generations the shuffling means you carry no measurable DNA at all from most of your documented ancestors. They are genuinely your ancestors and they are genetically invisible, so an absent result is not evidence of an absent relative.

Ethnicity estimates are the most misunderstood part of all. They work by comparing your DNA against reference panels of living people from particular regions, so they estimate similarity to present-day populations rather than reading out the past. When a company updates its panels, customers' percentages shift, which is less a correction of an error than a reminder of what the number always was.

Paper records remain the backbone, and they contain errors, omissions and deliberate untruths.

How to take part

• Interview the oldest relative you have, and record it. That source expires.
• Write down where each claim came from, not only the claim itself.
• Treat ethnicity percentages as similarity scores rather than heritage.
• Expect pedigree collapse, because everybody has it.
• Check original documents instead of copying other people's trees.