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My 1st cousin 1xR and my Aunt and I have all taken Ancestry DNA tests. For clarity sake I will refer to the parties as "Z", "N" and "R"
"R" is related (by document) to "N" as nephew and Aunt "Z" is related (by document) To "N" as grand-nephew and grandaunt
"R" and "N" share 2067 centimorgans over 65 segments.
"Z" and "N" share 1097 centimorgans over 44 segments.
But Ancestry DNA reports that there is no shared DNA between "R" and "Z".
Is there any way that makes sense?
Whoa, that chart popped up in the middle of my answering this. I was getting nowhere fast because the numbers weren't adding up. Now I see it's because your explanations of relationship were not clear enough. Z isn't anybody's aunt, she's a younger generation. That was throwing me.
Okay, I'll start over.
So R and N are uncle/nephew and should share 1349-2175 cM. They share 2067, which is in that range.
(If R and N were half uncle/nephew, they would share 500-1446 cM, so we can rule that one out.)
Z and N are granduncle/niece and should share 251-2108 cM. They share 1097, and we're good.
(If Z and N are half granduncle/niece, they would share 125-765 cM, so we can rule that one out.)
R and Z are (on paper) first cousins once removed. They should share 141-851 cM. They actually share zero.
The most obvious explanation is there is a half-sibling relationship going on here. A NPE (non parental event or “not parent expected”) from the sound of it, or you wouldn’t be asking.
But it still doesn’t fit. The half uncle/nephew and half granduncle/niece possibilities don’t come out even close with the numbers.
The only thing I can come up with is that GC has one parent different from N and RH (who are full siblings).
AND that Z is GC’s child, not grandchild.
That would make N and Z half uncle/niece with an expected range of 500-1446 cM, perfect for the actual amount of 1097.
Z and R would be half first cousins and that doesn’t work because the range is 137-856.
I’m at a loss.
Perhaps there was an error when you compared R and Z. I’d put everyone in Gedmatch and check it again.
Note: My cM ranges come from Blaine Bettinger. http://thegeneticgenealogist.com/wp-content/uploads/2017/08/Relationship_Chart_FINAL_August_2017.jpg
The range for a full aunt is 1349 to 2175 cM. A half aunt is 500 to 1446 cM. So 2064 definitely indicates a full aunt.
Your 1C1R would be a great-niece/nephew to your aunt. The range for a great-niece/nephew is 251 to 2108 cM. Your value of 1097 cM fits in there. If that's the situation, then something is amiss. You should share DNA with your 1C1R as well as any and all close relatives (<= 2nd cousins) to your aunt. It wouldn't matter whether they are half relatives to your aunt or not because both your aunt's parents are your grandparents.
I'd definitely recommend looking at the In-Common-With (ICW) list of people between your aunt and your suspected 1C1R. You have already said that you will not be listed as one of the in common people. Compare that to your ICW list between you and your aunt.
If the two lists have few people in common between them, then maybe you need to form a new hypothesis about how this person is related to your aunt.
If the two lists have a lot of people in common, but you are not in it, then something else is going on. It is rare that a DNA company would make this sort of error for matches that are this close. But if in fact your full aunt's great niece/nephew does not match you, then you should contact Ancestry DNA to see what's going on.
