I'm a 2nd cousin to someone, my brother is a distant cousin?

Question

My brother and I have both been tested on 23andme. There are many people that show up at different relationships and I'm trying to understand how there can be large differences in predicted relationships. We are of Ashkenazi heritage and I've heard that can make relationships seem closer than they actually are. My brother and I do show up as brothers (48% shared), so it's not that.

One example is a person is predicted to be a 2nd cousin to me (81cm, 10 segments) but a distant cousin to my brother (43cm, 6 segments). We both share the same 10.5cm segment with this other person but I also share segments of approx 9, 11, 10, 12 that my brother does not, plus 5 in the 5-6 range. My brother has a 8cm segment with this person I don't have then four of 5-6cm that do not match mine.

So given I don't share any long segments with this person and the total is on the low side for 2nd cousins, could it be that the smaller segments that seem to have pushed me into a closer predicted relationship are just IBS and the person is really farther from me than 2nd cousin? Or do you think we are 2nd cousins and my brother just didn't happen to inherit enough of the same DNA segments from our common ancestor?

We have a family tree that only goes back 2 generation for the most part but know the names of our grandparents siblings and their descendants. Unfortunately we don't recognize the names as belonging to anyone in the family tree we know about. However most of the people that match as 2nd cousins to me or my brother are listed as distant cousins to the other brother. I'm trying to figure out if the predicted relationship of 2nd cousin is real and there is some unexplained people in our genealogy or if they are really more distant relatives.

Answer 1

What you are observing is normal. There is enough randomness, even between siblings, that they may show up as different relationships at 23andme and other companies. 23andMe estimates your relationship with your matches using some criteria based on number of matching segments, total cM, and larger cM matches.

Second cousins have a large possible range and can share between 46 cM and 515 cM. See Blaine Bettinger's Shared cM Project.

I am of Ashkenazi heritage as well, and as an example, I match at 23andMe to a known second cousin (2.13%, 159 cM, 10 segments) and I match to his brother (2.00%, 149 cM, 8 segments).

In my case, you might think that does not seem very different, but take a look at the segment comparison diagram. We only overlap on a few segments and all the matching segments appear randomly placed. If one brother would have got that large segment 2 match from his other grandparent, he'd share 42 cM less than he does with me.

To stress the point more, one of my second cousin's daughters does not show up on the 23andMe match list at all to me, even though she is a 2nd cousin once removed. It turns out we share 0.73% (54 cM) and the people on my match list, many of whom are said to be 4th and even distant cousins, only goes down to 0.82% (59 cM).

If many of the people that match as 2nd cousins to you are listed as distant cousins to the other brother, it could be as simple as you having got by chance a bit more of two grandparents segments, whereas your brother got more of the other grandparents segments. This could actually help you in your research, because the people who are closer to you may be through different sets of grandparents than the ones who are closer to your brother.

There are no hard rules with this. These are all clues. You must use DNA to help develop your family tree, and also develop your family tree to help explain the DNA.