Submitted by **vesuvisian** t3_zwg75b
in **askscience**

Each generation back, the number of individuals doubles (two parents, four grandparents, etc.), but eventually, the same individuals start to appear in multiple parts of your family tree, since otherwise you’d be exceeding the population of the world. So the number of unique individuals in each generation grows at first before eventually shrinking. How many unique individuals can we expect in the ‘widest’ generation?

Edit: I’ve found the topic of pedigree collapse, which is relevant to my question.

Edit 2: Here's an old blog post which provides one example of an answer. For a typical English child born in 1947, "the maximum number of “real” ancestors occurs around 1200 AD — 2 million, some 80 percent of the population of England." Here's another post that delves into the concept more. England is more isolated than mainland Europe or elsewhere in the world, so it'd be interesting if these calculations have been done for other places.

Dorocchet1_j1v2q61 wroteThere's too many variables to answer universally. It would vary wildly depending on the individual you started with.

Edit: See /u/Tidorith's comment below, the rest of what I'm saying here isn't necessarily relevant.

For what it's worth, the point where unique ancestors would outnumber the population is precisely 30 generations. Whereas if we limited it to just the UK, it would be a number in the low twenties. So the possible variance here isn't dozens of generations, but more like fives.

So probably around 15-20 generations back? But again, it's impossible to give a universal answer.