While DNA passes from parent to child, each child only gets half of each of their individual parent’s DNA. Consequently, as a lineage is worked back in time, there will be ancestors in your genealogical tree with whom you might not share any DNA. It doesn’t mean that the ancestor is not your ancestor. It simply means that their DNA did not makes it’s way all the way down to you. While DNA is microscopically small, there’s only so much your body needs.

Some suggest (for example, Blaine Bettinger in his  The Family Tree Guide to DNA Testing and Genetic Genealogy) that once a lineage is traced back to the 4th great-grandparents that there are paper genealogy tree ancestors with whom you do not share DNA.  That’s why you may share no DNA with another descendant of one of your 5th great-grandparents.

Of course there have to be ancestors in your tree with whom you do share DNA.



5 Responses

  1. What many do not realize is that siblings (one or more) each receive half their parents’ DNA, and portions may or may not overlap with yours. Therefore, testing siblings is a smart way to learn if they received DNA that you did not.

    • Very good point. After all, siblings are not identical and sometimes look significantly different and that’s just the DNA that impacts “looks.”

  2. This is a well-researched mathematical problem . The theories of probability and of combinations and permutations formed part of my maths degree course in the 1950s. The answer depends on how much of your DNA is taken into account We know that we share DNA even with Neandertal man. So when we’re talking about sharing DNA in genealogy, we just mean the selection of genes that the companies are looking at. The probability of not sharing any of those genes is fairly small, because they’ve been selected for the purpose of matching us in the short term. So it just .depends on how far back the tree goes, how many people are under consideration, and on the properties of the family genes.

  3. What I hadn’t realized is that if you don’t have the DNA of the an ancestor – for example a GGGGgrandfather, you will have the DNA of the GGGGgrandmother. If you don’t inherit the segment one of the pair, you get the segment from the partner. https://gcbias.org/2013/11/11/how-does-your-number-of-genetic-ancestors-grow-back-over-time/

    Eva makes a good point – if we were fully sequenced, you’d find segments from the missing ancestor somewhere. This explains why my genome shares as much DNA with Neanderthals as I do with a second cousin (4%).

  4. So true. I have four half-cousins (siblings of one another) who recently completed Ancestry DNA tests at my urging. The variation in the DNA they shared with me was astounding. One shared so little cMs that she did not appear as a “DNA match” as defined by Ancestry! we did an analysis together to explain this seeming ‘mis-match’. Of course , all the cM numbers fell within the statistical bounds. And when future unknown matches do appear, I will only use the cousin match with the most shared DNA. Hilariously, he of the one of the four totally disinterested in genealogy. His sisters coerced him into joining the DNA experience. ( I have not even met him, as he does not attend family events. )

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