Expert Opinion Please... on DNA Siblings

[JMCWSW]

We've just tested two of our cousins. (I paid for the kits, another cousin is managing them, as we collaborate a lot).

The first anomaly is: they don't appear as sisters, but as first cousins to each other.

Given there are even more anomalies in which other cousins neither of them DNA match to.... I'm fretting for them, a little, in having them discover that they've got this anomaly.

Sooooo....

ANYONE had any experience of genealogical siblings, not actually being genetic DNA matched siblings, simply by the quirk of genes not being evenly shared? i.e. Absolutely no risk/chance/possibility of NPE needing to be considered?

 

[peter]

The biggest danger in a situation like this is uninformed speculation, which is presumably why you've asked for advice. However there are some details that need to be clarified.

First, you say that another cousin is managing these tests, but do you have access to them as a Viewer or Collaborator, or are you relying on what you cousin has told you? People who are unfamiliar with Ancestry DNA often misinterpret what they see on the screen.

Secondly, on what basis are you saying that they appear as 1st cousins. Is it that the relationship suggested by Ancestry, or is it the relationship that you (or the cousin managing the test) have deduced by comparing the amount of shared DNA with the figures in the chart in my DNA Masterclass? With such a close relationship and such a large amount of shared DNA there may be only one answer, but usually there are other possibilities, For example, the range for 1st cousin overlaps with the range for half-aunt.

As you can see from the chart, there is no overlap between the range of DNA shared by full siblings and that shared by 1st cousins. If they really are 1st cousins it would mean that they do not share either parent, which suggests an adoption within the extended family. However it would be foolish to speculate further without knowing precisely how much DNA they share - which is why this information should ideally be provided when asking questions of this type.

If for some reason you don't want to share the precise cM look up the chart, then make an insignificant adjustment to the figure before posting.

 

[JMCWSW]

Am crunching some numbers from the information I do have... but my partner in crime isn't available to answer a couple of specific things I can't get to do just yet.

He tried to assign me as a manager also, (using the cousins' logins) but couldn't appoint a second one, so now he'll catch up with both of them later this week, to add me as collaborator; so at the moment (as I have access to his DNA) I can only tweak around from his perspective. But he can tweak around from theirs... which will give better answers as to cMs etc.

In the meantime I looked at the cMs of four siblings and their relationship to my Mum (as a comparison as to how much they might differ to a third person).
Genealogically they're half 3rd cousins, once removed, and I'd adjusted ancestry's relationship accordingly.
~ THE FIRST HAS A DIFFERENT FATHER FROM THE OTHER THREE (the mother is our relative)

M T and M T
21 cM across 2 segments

M T and V S
51 cM across 4 segments

M T and L B
32 cM across 3 segments (so quite a difference from her full sisters, almost closer to her half-sister)

M T and D B
50 cM across 3 segments

But seeing how the other two cM to each other is going to be the crunch... even if neither match to the rest of the family!!!

J