Some beginners queries

[cardinalcanary]

My queries may have been raised before. If so please point me in the direction of earlier replies.

My questions are as follows;

1. I am getting a lot of matches on GEDmatch and Family Tree DNA on the same part of my 16th chromosome. 20 to 40 cM's. Is it likely that we all relate to each other via the same root or is the chromosome you match on random?

2. If you have 2 long string matches with someone on different chromosomes, does this mean you link via two different roots?

3. If you match someone through more than one root, could this mean a predictec 4th cousin could actually be a much more distant cousin?

If this sort of thing is simply described somewhere, please can you let me know where to look.

Many thanks

Stephen

[hurworth]

Have a look at this article.  It mentions known "pile up" regions on various chromosomes.  There is a segment of Chromosome 16 mentioned, but it's only 6.5cM long.  If you're matching on segments between 20cM and 40cM in length then it's unlikely to be random. 

https://dna-explained.com/2016/09/08/concepts-managing-autosomal-dna-matches-step-2-updating-match-spreadsheets-bucketed-family-finder-matches-and-pileups/

If you're matching people on the same part of the chromosome then you are very likely to have a common ancestor somewhere.  The common ancestor could be quite distant though.

If you match someone in more than two places they possibly could be related via two different roots, or you both could have inherited the same segments from the one mutual ancestor or couple.

The answer to 3 is yes.

This is worth a read too:
http://www.khazaria.com/genetics/excess-ibd-regions.html

[cardinalcanary]

Dear Hurworth

Thank-you for the info. Very useful.

I am trying to figure out why me, a guy in the UK born and bred has literally thousands of dna matches with people in the Southern States of the USA.

Many of them match on the 16th chromosome.

Looking at their trees, a lot of them claim roots in America back to the 1600's so to have Ancestry suggest they are 4th cousins seems way out.

I think we must match because of the same families intermarrying with each other several times.

[hurworth]

I think you're correct that it's largely due to intermarriage between colonial American families.  20-40cM are LONG segments though.  It would be interesting to know just how closely related the people in the US are to each other.
 

[Gardenshed]

I have about 1400 matches on FTDNA, no close relatives and most seem to be US based - all over the place within the US. Only a minority of my distant matches seem to have trees but those that I have looked at have US born ancestors going back to the end of the 18th century or v early 19th century. Little trace of the millions of UK and Irish and European immigrants that flooded in to the US from the mid 19th century and later, which is when I believe my own non direct
Line relatives are most likely to have gone over ( and in a couple of cases I know that relatives emigrated to the US in the late 1800s).

Is this just a peculiarity of my matches or are there factors about US research which encourage people to identify with earlier settlers?

I was also wondering, after reading the above two posts, given the scale and mobility of the US population from the 19th century onwards, would intermarriage in the 17th century still be showing up in DNA tests in 2017? If the answer is positive maybe I need to be less sceptical about my distant matches' trees.

Thanks.

[hurworth]

Good questions gardenshed.  I don't know the answer.

I just had quick looking at my relatives' kits on Gedmatch and they only share a 20cM or longer segment with about 50 matches each (and on a range of chromosomes).  Are you getting many more than that?  Sometime the longest segment shared with even a third cousin is less than 20cM. 

[Gardenshed]

Thanks  Hurworth. I haven't had the chance to count them up (will do so when I can log on to my laptop) but the answer is only a very small number i.e I think less than a dozen of those 1400 have matches of more than 20 centimorgans in length. Until recently the longest segment shared with any match was 22 cM. I now have one with a 44cM one - twice the length of the next longest one. I need more UK and Irish people to test!

I am still at the absolute beginner stage, so forgive the daft questions but from my reading of the explanatory material around I would have thought the length of the OPs matching segments suggests a much more recent connection than the 1600s?

[hurworth]

I need more UK and Irish people to test!

I am still at the absolute beginner stage, so forgive the daft questions but from my reading of the explanatory material around I would have thought the length of the OPs matching segments suggests a much more recent connection than the 1600s?

We ALL need more UK and Irish people to test!

The length of the segments does suggest a more recent connection but there seems to be SO many of them that I'm struggling for an explanation.  A more recent connection often (but not always) shares more than one segment.   

[Seaton Smithy]

1. I am getting a lot of matches on GEDmatch and Family Tree DNA on the same part of my 16th chromosome. 20 to 40 cM's. Is it likely that we all relate to each other via the same root ... ?

Not necessarily.  Every chromosome is actually a chromosome pair, with one half from your mother and the other half from your father.

For example, you could match with both Tom and Sally on what looks like a similar segment, but Tom may not match with Sally, because Tom is from your mother's side and Sally is from your father's side.

You need to make sure that Tom and Sally actually match each other - this is known as triangulation.  Easy to do in GEDMatch by comparing Sally's kit with Tom's.