How can two different generations of one family be a half sibling to an outsider?


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?


Posted 2020-12-07T19:15:52.103

Reputation: 31

Did your father's mother have any known sisters, or your father's father any known brothers? might start to be relevant.

– shoover – 2020-12-07T21:28:01.623

@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. – MOMMAMIA9 – 2020-12-22T18:44:13.910



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.


Posted 2020-12-07T19:15:52.103

Reputation: 2 427


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.)


Posted 2020-12-07T19:15:52.103

Reputation: 458

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. – MOMMAMIA9 – 2020-12-22T19:01:14.687

@MOMMAMIA9 this is not a theory!!! Remember: "I'm not commenting on the likelihood ..., but just how it can happen." – RonJohn – 2020-12-22T19:15:38.010


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.

Jamie Cox

Posted 2020-12-07T19:15:52.103

Reputation: 861

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. – MOMMAMIA9 – 2020-12-22T18:51:53.330

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 – 2020-12-22T19:05:27.523