DNA assumptions, segment overlap, and "in common with"

+7 votes
I have some general questions about assumptions that can be made based on DNA chromosome segments and where they overlap, using a chromosome browser. After a one-to-one comparison has been made to determine that the same chromosome (of the pair) is being compared:

1. Can one assume, if the length and position of the compared segments appear to begin and end at the same location and are about the same length, that they point to a single set of common ancestors?

2. What can be assumed if a shorter segment overlaps a much longer one?

3. What if, looking at 2-3 different matches on the same chromosome, one segment starts at about the center of another segment, and then continues a good distance beyond the end of the first? Can any conclusion be drawn from that?

4. If a DNA match has about 120 cM of shared DNA on two chromosomes -- one about 90cM, one about 30 cM -- are there any general assumptions that can be made about how far back in time the common ancestors are?

5. What exactly does sorting DNA matches by "in common with" achieve? After closer examination, I can tell it's not anything quite so helpful as sorting by people who share the same ancestral line.  What is it doing when I sort by "in common with"?

in Genealogy Help by R. Greenup G2G6 Mach 7 (72.5k points)

1 Answer

+6 votes

1-2. If you are talking about a single one-to-one match, it doesn't make any sense to talk about "overlapping" segments. You have segments in common and their location (based on some matching threshold) or none at all. It only makes sense to talk about "overlapping" segments when you are comparing multiple matches to one another for "shared segments."

In this case, a "shared segment" that is long enough normally will point to a single common ancestor (not a pair; although there may be a pair, and if a pair only had surviving children with one another, it may only be possible to associate a segment to pair rather than an individual).

What "long enough" means is subject to some wiggle room. Often 7 cM is considered significant. But bear in mind that especially given variables such as the number of SNPs, it's not set in stone. A "7 cM" segment could actually be a 3 cM segment from one shared ancestor and a 4 cM segment from another shared ancestor. Or an SNP-sparse, identical-by-state segments.

2-3. About the only conclusion one might draw is that the individual with the longer shared segment inherited more DNA from the common ancestor. The match with the longer segment might be from an older generation. That's not necessarily the case especially when you're talking about a single segment. Powerful evidence of the degree of relationship comes from comparing total cM count with many segments.

4. The best two links I can give you are these:

  1. Autosomal DNA Statistics
  2. Shared CM Project

120 cM is by far most consistent with "Second cousins once removed, half second cousins, first cousin three times removed, half first cousin twice removed" etc., but it's possible for it to be a degree closer or further than that.

5. Can you clarify which service you are using? Is this Family Tree DNA? How are you sorting people by ancestral line?

by Nathan Kennedy G2G6 Mach 4 (40.4k points)

Thank you for answering so quickly!  I was talking about comparing segments using GEDmatch, although I have also done comparisons using FamilyTree DNA.

I did mean that I was comparing myself to at least two other matches in the chromosome browser. (It confuses the heck out of me not to see my own "segment" visually represented-- but I guess my own chromosome would be continuous, and not a segment at all.)

I am not (yet) really sorting people by ancestral line, except by occasionally finding a tree with a common ancestor.  I was hoping I could use the "in common with" along with the kit of someone whose common ancestor I had figured out, and get a nice list of people who were related to both of us in the same way.  I can't figure out what it is doing when it gives me a list of people who are all "in common" with each other. To the extent I can analyze the results, the list often contains people who I know are connected to me on different ancestral lines.

I hope that wasn't a totally incomprehensible clarification, and thanks for the answers you have already given me.  (I haven't checked out the links you gave, but that's what I'll do next!)


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