My full paternal uncle and I enough cM with an outsider to both be listed as half siblings to an unknown male. My uncle shares more cM with that unknown male than I do (a female).

Both my uncle and I are listed as half siblings to this person. I don't understand how two different generations of known lineage can be half siblings with with the same person.

Here are the numbers:

Uncle and "the male outsider" 26% shared DNA | 1,809 cM across 41 segments Unweighted shared DNA: 1,809 cM Longest segment: 125 cM 100% 1/2 sibling

Me and the "male outsider" Shared DNA: 1,655 cM across 28 segments Unweighted shared DNA: 1,655 cM Longest segment: 165 cM 100% 1/2 sibling.

My Uncle and I Shared DNA: 1,599 cM across 41 segments Unweighted shared DNA: 1,599 cM Longest segment: 125 cM 99% uncle

Can you start to unravel this?

  • Did your father's mother have any known sisters, or your father's father any known brothers? genealogy.stackexchange.com/a/17712/10577 might start to be relevant.
    – shoover
    Dec 7, 2020 at 21:28
  • @shoover Yes to both. My paternal grandmother had 3 sisters. and my paternal grandfather had several brothers. Unfortunately, this person will not consider any other options other than my dad.
    Dec 22, 2020 at 18:44

3 Answers 3


Half-sibling is not the only likely relationship at that cM range, it also includes aunt/uncle and niece/nephew. So, depending on the age of the outside match, it could be either your uncle's half-brother, and therefore also your uncle; or your half-brother, and therefore your uncle's nephew.



One way for both you and your uncle to also be half siblings is that the same man (probably of your grandparents' generation) impregnated both your maternal grandmother and -- some decades later -- your mother.

(Note that I'm not commenting on the likelihood or the squick factor, but just how it can happen.)

  • Interesting theory. Problem is it took my parents took 9 years to conceive me. She was a Para 0 when I was born... Doctors can tell.
    Dec 22, 2020 at 19:01
  • @MOMMAMIA9 this is not a theory!!! Remember: "I'm not commenting on the likelihood ..., but just how it can happen."
    – RonJohn
    Dec 22, 2020 at 19:15

I ran this through DNA Painter's What Are The Odds tool. The only scenario that works is if the "outside male" is your half-sibling. In that case, your uncle is also his uncle, and the centimorgan results are consistent with those relationships. No incest or other weird relationships are necessary. I considered the possibility that the "outside male" is a half-sibling or full sibling to your uncle, but those didn't work. If “outside male” were your uncle’s half-brother, that would make him your half-uncle, but you share too much DNA for that relationship. Congratulations, you found a new half-brother! See the diagram I used here.

  • Sounds like a great tool, What I think I that my paternal grandfather impregnated the outsider's mom. And as we know DNA doesn't always go 50-50. Meaning my genetics are a bit more like my father's. So that would make the outsider my Uncle 1/2 sibling and my half-uncle. Also, the timing of conception and age difference between us leads me to that consideration Many thanks for your input.
    Dec 22, 2020 at 18:51
  • I believe that the outsider being your half uncle is ruled out. The world record maximum shared DNA for a half-aunt or uncle is 1315 cM. You share 1655, way outside that range. If it is biologically possible that your father was also outsider's father, I think you better believe that scenario. Unless, maybe, your family is Ashkenazi Jewish, or some other group that has a lot of endogamy (DNA shared among the group).
    – Jamie Cox
    Dec 22, 2020 at 19:05

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