I was surprised to find someone show up in FTDNA as a 2nd-3rd cousin to me (168 shared cM, 34 segments with a minimum length of 5 cM) but not among the 17,667 matches of my father or the 18,178 matches of my father.

Fortunately, the person's DNA was also on GEDmatch, where I could compare his DNA to mine and my parents'. The chromosome browser showed me as inheriting most of the segments that my parents had in common with that person; however, on chromosome 14, I had a 3.1 cM match to the person in a region in which neither of my parents matches him. Something similar occurs on chromosome 20, where my match with the unknown person (4.47 cM) is longer than my mother's match to him (3.13 cM) and my father has no match.

Is the likely explanation that my parents have matches too small to show up unless they're combined in me? For example, they could each have a 2 cM stretch:

Mom ------XX-------
Dad --------XX-----
me  ------XXXX-----

Or is there a different explanation? In general, what is a reasonable minimum cM threshold?

I am an Ashkenazi Jew. My parents are not closely related to each other.

What I see often said is that any matching segments under 5 cM, if not under 7 cM, are more likely to be identical by chance rather than by descent, and should usually be disregarded as noise.

  • 2
    Your answer could be improved by citations. I added a few and can add more as I find them. Not sure if the minimum cM threshold should be changed when dealing with an endogamous population, but if so, my guess would be it should be adjusted upwards, not down. – Jan Murphy Aug 8 at 3:58
  • 2
    @JanMurphy Thanks for adding the citations; I had been mostly thinking of what I've seen said in genetic genealogy groups on FB, so didn't have any citations to add myself. – Leah Worster Aug 8 at 18:10

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