Chromosomes

[strictlysnoopy]

   

    Hope somebody can help to clarify.    I have a 4th-6th cousin who has discovered on Gedmatch that he and I share two chromosomes from my maternal side of the family.   He has added them together  together and says that he is now a closer cousin than first thought.  Is he right?

         Secondly,  due to this new development he says that he is descended from my other great grandparents.  I have always thought that inheritance of chromosomes from parents is random and therefore we have inherited a chromosome that other cousins have not but from the same ancestors. Am I correct?

        Thanks

       

[phil57]

First of all, how do you know that he is a 4th - 6th cousin? If that is a statement relating to your match by the DNA testing company, it is a suggestion based on the most likely ranges of probability for the length of the match between you, nothing more. The actual match could be closer or more distant.

Presumably the length of the overall match between you is also given by the testing company? Adding two chromosomes together (whatever that means - presumably the combined length of the matches between you on those chromosomes) doesn't in itself prove anything, particularly unless you or he has sufficient information to compare them with a chromosome by chromosome breakdown of other matches on the same line of descent. You or he could also have inherited segments on other chromosomes to the same ancestor, which are not shared between the both of you.

The match between you proves that you are related, unless it is at a lower length of less than around 15 cM or so, where there could be room for doubt. The overall length of the match between you can suggest a range of likely relationships and the probabilities for each of them, using a tool such as DNA Painter.

But the only way to definitively confirm the exact relationship is to methodically and robustly prove the lineage of both of you back to a common ancestor by research and evidence from documented sources that link each generation to the previous generation, and so on, until you can confirm where the two lines meet. The probabilities for suggested likely relationships given by the DNA Painter tool or a similar relationship estimator can be used to guide that research along specific avenues, but the paper trail is going to be the definitive proof of how you may be related. The DNA match initself proves nothing more than that you are related, not how. 

[strictlysnoopy]

Thank you Phil,  you have confirmed my conclusions with a comprehensive and wonderful explanation.   We have well documented paper evidence that we have in common a Henry and Gwenllian Williams 2nd great grandparents for me and 3greatgrandparents for him.   Ancestry for both our DNA results.
   
      This latest fanciful theory of his annoyed me as it is ridiculous. Thank you again for confirmation.

[Biggles50]

Out of interest, what is Ancestry reporting as the cM and segment details

ie as per this

[strictlysnoopy]

     

       Ancestry results as follows

           23cMs   3 segments

           Unweighte shared DNA.  50cMs

           Longest segment.  21cMs

          It was Gedmatch that gave a two chromosome match between us. Sorry that is all I know as I find Gedmatch very difficult to use.   

      Thanks

         

[Rosinish]

I find Gedmatch very difficult to use.             

I'm not surprised, it's so complicated but you're ahead of me as I've yet to discover how to use it! 

Annie

[phil57]

We have well documented paper evidence that we have in common a Henry and Gwenllian Williams 2nd great grandparents for me and 3greatgrandparents for him.

So you are 3rd cousins, once removed.

The mean shared cM for that relationship according to DNA Painter is 48 cM, with a range between 0 and 192 cM over 4514 submissions. 1,686 of those submissions were between 26 and 50 cM, and 2,826 submissions were at 50 cM or less.

At a match of 50 cM unweighted, you are pretty much on the mean value for shared cM at that relationship level. That in itself is not proof of your relationship, but it corroborates your documentary evidence very well.

It is notable that from an unweighted match across 3 segments, Ancestry's Timber algorithm has effectively stripped out over half of the unweighted match length.

The unweighted CiM tool at DNA-sci.com allows comparison between weighted match lengths (after Ancestry's Timber calculation) and the unweighted length or comparable figures from other testing sites that don't attempt to strip IBD segments. The stripped match length of 23 cM and unweighted length both suggest that matches in the region of 3C to 3C1R grouping are the highest probability.

For Ancestry's stripped length of 23 cM in particular, the tool suggests the highest probability (30.7%) for a 3C1R, Half-3C, Half-2C2R or 2C3R match, so I would be entirely happy with that as corroboration of your documented 3C1R relationship based on the paper research.

[strictlysnoopy]

 

     Thank you Phil,    much appreciated.

[louisa maud]

Is there such a thing that determines maternal or paternal DNA,
Hope that isn't a daft question, have done a DNA   but am none the wiser
Louisa  Maud