Aunt nephew shared dna

[Yonnie3]

Shouldn't the nephew show a dna match relation to everyone the aunt is related to?  Is there a scenario where there can be matches for the aunt that are not shared with the nephew?

[sugarfizzle]

Several reasons, I think.

Aunt inherited different DNA from nephew's parent, one reason to get as many family members tested as possible. They will match a lot of the same people, but both will have matches unique to themselves.

Aunt generation before nephew, retains more DNA -
Aunt's 4th to 6th cousin matches (if we are talking about Ancestry, similar reasoning elsewhere perhaps) could be nephew's 5th to 8th matches, so will not show up, direct search should bring them up. Similarly, aunt's 5 to 8 might not show up as matches for nephew. Uploading elsewhere might pick up smaller segment matches.

If no matches at all are shared, then one of them is not who they appear to be, suggesting NPE.

Regards Margaret

[Yonnie3]

We share a lot of people except for one 3rd cousin to her but no relation to me and I find that puzzling.  The only thing I can think of is she may have a different father.  Is there another scenario that could account for that?

I'm not familiar with the acronym "NPE", could you provide a definition?

[sugarfizzle]

Sorry! NPE is non paternity event, i.e. illegitimacy.

I'll have to think about the 3rd cousin issue, but going out soon until this afternoon, haven't got time now.

Hopefully someone else will chip in before I get back.

Providing he/she has a paper trail, this should show the exact relationship between you all, without that you will probably never know.

Regards Margaret

[sugarfizzle]

My first reason in my first reply applies, I think.

https://thegeneticgenealogist.com/2017/08/26/august-2017-update-to-the-shared-cm-project/

3rd cousins expected to share 0 - 217 cM (your aunt).

You would be 4th cousins, expected to share 0 - 127 cM

Paper trail essential in this case.

Regards Margaret

[Gadget]

I'm not sure if I've got this correct but the nephew would probably have quite a number of relatives that the aunt would not be related to - the relatives of the parent who the aunt is not related to. 

Is this the case or are the matches all on the one side?


If same side, it's as Margaret says.


Gadget

[Gadget]

PS

Are you assuming that the aunt and her sibling  inherited the exact same set of DNA from their parents? The aunt's  sibling/nephew's parent might have inherited  slightly  different DNA  and this would show as a different relationship.  It's not the same 50:50 -> 25:25.

[Yonnie3]

Everybody who matters in this have been dead for a very long time.  My aunt's sister was my mother who died 5 or so years ago.  Their mother died in 1943 and their father died in 1978.  We only have dna samples for the aunt and my siblings. 
As I understand this, this dna science, I should have a 1 to 2 generation in addition to my aunts for everyone she matches.  But if she has a different father I would have no relation to him or his cousins that match to him and my aunt.  That should be the only scenario?

[Gadget]

I think you are assuming that your mother and her sister had the same dna - it's unlikely that they did unless they were identical twins.