Determining shared maternal or paternal lineage via shared DNA segments?

Question

If DNA cousins and I share the exact same segment of DNA, does it always mean we're related via the same side of the family?

Background: I have taken DNA tests with National Geographic, Ancestry, and 23&me. I really like the more complete, detailed information 23 provides. So, in an attempt to more fully understand my DNA and figure out shared lineage with cousins (whether I'm related to them via my father or mother's side, and perhaps even trace to which exact great or greats), I started a spreadsheet. On it, I have rows for each DNA strand (1-22, plus x) and columns for segments starting from the lowest numeral (1) to the highest, dividing them up in between according to clusters of shared DNA numbers I share with various cousins. Under each strand, I have a row for each cousin. The result is that I have found multiple cousins who line up in the same column because they share the same exact segment length (e.g., 1-5,679,900) for a particular strand. If I am able to connect with one of those cousins and figure out exactly how we're related (usually by using Ancestry trees), I conclude that I'm related via the same ancestor(s) to everyone else who shares those specific segments.

Then it occurred to me that my thinking might be completely off, since I'm so confused about DNA. Because half of each segment comes from the maternal side and half from the paternal side (I think), then my spreadsheet is worthless. Or is it? Please advise.

Answer 1

I would encourage you to use DNA Painter for this purpose, rather than trying to make your own spreadsheet. It is essentially doing what you are doing but has much greater functionality – it is a tool designed for this task. DNA Painter website allows you to map specific segments to certain ancestors, where known, and help visually identify patterns.

As you have found, just because you match two people at the same location does not necessarily mean that they both are related through the same common ancestry.

For example:

• You match Jane from position 1,000 to 1,000,000 on chromosome 5.
• You match Ann from position 1,000 to 1,000,000 on chromosome 5.

Based on this information alone, Jane could be a cousin on your father's side, and Ann could be a cousin on your mother's side, who just happen to be related to you at the same region on chromosome 5.

To test this, you need to perform a direct comparison between Jane and Ann. If Jane matches Ann from position 1,000 to 1,000,000 on chromosome 5, then there is a high likelihood that this segment matches because you all share common ancestry. This is the process of triangulation.

Also you have to keep in mind that things can become complicated if your parents were related; you could match the same person on both your maternal and paternal chromosomes.

Also be aware that it is tempting, but sometimes counterproductive, to over-analyse small segments. With anything less than 10-15 cM you could potentially match at a segment due to chance.

In trying to map your chromsomes the single best thing you can do is test close relatives (parents, aunts/uncles, first cousins), as this really helps narrow down your matches to a particular line.

Answer 2

This is my first attempt to answer a question regarding genetic genealogy -- although I've been doing it for nearly two years now. So this answer may be "amateurish".

My ancestors come from an endogamous group (cousins married cousins -- most times > 2nd cousins), this greatly complicates the process DNA analysis. Particularly because tested matches that you have most likely are related on both mother's and father's side. But, if you know, or have ruled out the possibility of your ancestors being endogamous, then you can safely assume that all you cousins that "line up" (both start and end locations) in a segment will be from one or the other side of your family. (But often this does not happen)

With that said, one way of validating that they are from your mother's or father's side is to compare the family trees of the cousins who share this segment. This is usually referred to as part of the process of "triangulation". When you find a common ancestor for two or more of your cousins that share the same segment of DNA, you have confidence that your ancestor is "valid".

Edit: I agree with Harry Vervet's answer... it's important to verify that each of your segment matches match each of the other cousins in your segment (something that GEDmatch's 3D chromosome browser does well). This is also part of the "triangulation" process.

Note: I personally attempt to find the common ancestor for all the cousins in a segment. For example if I have five cousins matching a segment, and I find three who share a common ancestor with me, I can contact the other two cousins and let them know my findings (which may help them in their research). Also, I have segments where I have over ten matching cousins, yet there is NO matching common ancestor, which can indicate a lot of incorrect family trees!