Odd Fact: you aren’t related to most of your ancestors.

September 15, 2011  |  Evolution

John Hawks, on the mathematics of family trees and recombinant DNA:

In practice, even though we have billions of nucleotides, our DNA cannot follow billions of genealogical lines. Recombination over 30 — 40 generations does not divide chromosomes down to individual nucleotides. In the medium term, most human DNA is separated by recombination hotspots into lengths of around 50 kilobases. Across very short spans of 30 generations, DNA is for the most part inherited in chunks of hundreds of kilobases or longer. So dividing six billion nucleotides by 50 kilobases yields a number of around 120,000 ancestral lines at most from which any individual inherits his or her DNA. Recombination will increase this number somewhat further and further back in time, but not nearly so fast as the doubling of possible ancestral lines in every generation. This means that the vast majority of your ancestral lines more than around 17 generations ago have left no DNA to you whatsoever.

Granted, this is relative to the massive redundancy in our family trees– humankind is one huge, partially-inbred extended family. I.e.– if you go back 40 generations, you have over a trillion great-great-great-(etc) grandparents. There weren’t a trillion people alive in 1000AD, so a lot of those slots were filled by the same people.



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