DNA Sticky segments

Question

We have identified a group of over 40 people who match at the same place on chromosome 5. The matches usually range from 10-25 cMs with one match to me being 47. About half of the matches start at about 145 and run into the 160s. The shorter matches start around 152-156 and run to 166.

There is only one common known ancestor within the group (late 1700s in New York). Most of us have no idea how we might be related. At last two in the group are Australian and have only English and Scottish ancestry. After checking several GEDCOMS, I have found three that go back to two of my ancestral lines - the Seamans and Jacksons of Hempstead, Long Island in the late 1600s. These two families intermarried a lot and I don't think anyone has sorted them out.

Long Island or Connecticut seems to be the origin of the American branch but the Australians must stem from the English origins.

Isn't it unusual for so many people to share such a large segment so far removed from the common ancestor?

Answer 1

Having a lot of shared segments at the same place is a common occurrence, even for fairly large shared segments.

The matches will generally not start or end at the same location as you has observed. That is because crossover can occur on your side of the common ancestor down to yourself, or on the match's side of the common ancestor down to him/herself. Not only that, but smaller segments may come from more distant ancestors who may have caused additional crossover points.

But what you must be careful about is not to quickly jump to any conclusions about who the common ancestor is, especially when you are going as far back as the 1700s. Yes, you may have found a possibility, but you and the people you are comparing with likely have few lines researched back that far, and the common ancestor might be on another line you have not researched. There is no guarantee that just because you found one possible common ancestor, that this segment is from him/her.

In order to be conclusive about the common ancestor, you'll have to try to map your chromosomes to the ancestors they come from and do so at least in the area of Chromosome 5 that you are looking at. For both you and at least a few of your matches, you'll have to first determine if that segment is matching on your maternal or paternal chromosome. Then make sure that the sides would be correct for the hypothesis of that 1700s common ancestor. Next, try to go another generation back and determine which grandparent that segment came from for both you and your matches. Make sure that is still correct for the hypothesis. Once you prove the hypotheses wrong, you'll have to look instead on the ancestral lines you identified. But if the mapping keeps going in the right direction, then the farther back you can go, the more likely your hypothesis is correct.

Mapping your DNA to your ancestors is not something I can describe in a few words in this answer. I suggest you read the blog posts of Jim Bartlett at www.segmentology.org and check out the tool DNA Painter by Jonny Perl at www.dnapainter.com