3C or half 3C?

[pughcd]

Hi everyone,

I have three DNA matches on Ancestry who all share a great grandparent couple. They are all from the same generation and two of them are brother & sister.

I believe I am related to their great grandfather. Indeed he attended my grandmother's christening in 1905 and acted as her sponsor. Now there is a possibility that the gentleman and my great grandmother might be half siblings rather than siblings so I wanted to test the theory using "The Shared cM Project 4.0 v4".

I have put my shared cM results below;

1) cousin X, 149cM, 3C = 17% probability, half 3C = 6% probability,

2) cousin Y (brother), 60cM, 3C = 22% probability, half 3C = 30% probability,

3) cousin Z (sister), 63cM, 3C =23% probability, half 3C = 32% probability.

So taking the brother (Y) and sister (Z) together one might assume we are half 3C  but the addition of the other cousin (X) gives a more balanced result. As these cousins are all from the same family, sharing a great grandparent, I am taking the view that the low result of 6% for cousin X with whom I get  the closest result (149cM) is telling me that 3C is the best overall description for all of the three cousins. This would indicate that my great grandmother and the great grandfather are full siblings and that my cousin matches are full 3C.

Does this sound reasonable?
Thanks in advance.
pughcd

[Biggles50]

Maybe.

I would not assume that X and the two siblings Y & Z are full 2C’s without further evidence.

You maybe a full 3C with X and a 1/2 3C with Y & Z.

Hence you could do with finding out how much cM they share with each other.

Also how other Shared Matches fit into the tree.

I would also look a bit closer into the ancestry of X along their lines that they do not share with Y & Z and if there is any prior linkage to yourself.  So what I would be looking for is a possibility of pedigree collapse in the line between you and the 149cM match.

[pughcd]

Thanks Biggles50,

Fortunately we know that X, Y and Z are all full 2C as they are part of a family group who have a fully sourced tree with photos and certificates. All their grandparents are accounted for and are known siblings within the tree. These are my closest South Asian cousins from the Punjab.

I share 11 segments with X (149cM), 5 segments with Z (63cM) and 3 segments with Y (60cM).

[Biggles50]

My prior post is still valid.

Additional information regarding the shared cM between each is still needed.

Using the segcM tool gives a much higher probability that your 143 match is in the 2C group.

At this stage keep all options on the table.

Personally I do not rely on documentation alone.

[pughcd]

Thanks for your thoughts Biggles50. I may have the  shared cM data from the closest cousin. At the end of the day I am trying to decide on a full or half cousin relationship and sometimes the data is not conclusive either way!

pughcd

[Petros]

I agree with Biggles. Your 149 cM match looks more likely to be a 2C than a 3C. The histogram on DNA painter shows the bulk of 3C matches fall at < 100 cM.

That is consistent with my own observations with 3C from a single set of great grandparents, only one of 5 exceeds 100 cM with most well below that

[Glen in Tinsel Kni]

My fully sourced and documented 1c looks great until you look at our shared cM. I could have bought a dozen  dna kits with the money I have spent on certs containing nothing but lies and false fathers.

[rsel]

Have you tried using the WATO tool on the DNApainter site ?  You can build the tree for X, Y & Z and then enter the cM sizes and use that to suggest the relationships back to you. The advantage over the straight shared cM relationships is it will base the best fit on all 3 match sizes rather than each individually.

Richard

[pughcd]

Have you tried using the WATO tool on the DNApainter site ?

Hi Richard,

Yes I have used the WATO tool and it has given me some slight confidence in choosing one option over another. Thanks!

pughcd