DNA Relationship Question

[BrianClaydon]

Good morning all,

I manage a DNA test for my father. He shares 42cM of his DNA with a recognised relative who we have established is his second cousin once removed. We believe they only shared a maternal ancestor, rather than both parents. This is suggested through documentary evidence and family recollection.

I'm still rather new to the DNA experience, but does the 42cM length corroborate that, suggest otherwise, or cannot be used to establish that? I know there may not be certitude, but are the results suggestive of a half or full relationship?

Thanks for any help you may be able to offer.

Kind Regards,

Brian

[LizzieL]

This is a useful site to calculate possible relationships

https://dnapainter.com/tools/sharedcmv4

42cM is a bit lower than average for a Half 2nd cousin once removed, but possible. A full 2C1R would be about twice as much as the half relationship.

[Biggles50]

When we are looking at a specific match we use the DNA Painter Shared cM tool, enter the cM in the cell and it presents the %age probabilities together with the Mean value and the Range of values in each potential relationship.

The 42 cM has an 11% probability of a 2C1R, the mean value in the 2C1R range being 66 cM.

There is also the dna-sci website where you enter the cM and Segment numbers and this gives the %age Probabilities.

https://dna-sci.com/tools/segcm/

With your 42cM match then looking at a 1/2 relationship using the DNA Painter results indicates closer to a 3C1R then to a 2C1R, but as you have the citation records and family knowledge your conclusions look right.

Personally I use the above sites as a guide as to where to place the match in the family tree and work out the route to the MRCA.

Do be aware that sites like DNA Painter use the data that users upload to them and the database and hence said database will be limited.

[BrianClaydon]

Thank you LizzieL and Biggles50,

for your responses and the excellent resources you have shared.

In summation then the 42cM shared is below average for a half 2C1R relationship, but in conjunction with documentary evidence and family oral history, it is more likely than a full 2C1R relationship. Thank you for the clarification.

Kind Regards,

Brian