I recently ran in to this video from Ancestry and was surprised to see an improvement that they have made in their reports. It has always bugged me that they are the only company that doesn’t report the actual genetic match, but just the family tree. While it is very helpful to have the family trees available for comparison, it is discouraging not to have the actual genetic information. But now with the press of a button you can find out how large your match is with your cousins and see how they make the determination of what distance that cousin presents. It does not tell you where the match is, but it is a huge improvement.
This video should explain it pretty well. It also brought to my mind the reason why I have so many Reynolds matches in all of my DNA matches. A large number of the Reynolds/DeMoss family were Quakers. Quakers had to marry within their faith, and have their marriage approved by the church. After several generations it became more difficult to find unrelated Quakers, so they started marrying 1st cousins. This would also explain the high infant mortality.
We inherit 50% of each parent’s genetic make-up, decreasing in half for each generation. No two siblings, except identical twins will inherit the same 50% from each parent. But, if our ancestors marry a first cousin that has inherited 25% of anonymous DNA from the same grandparent, they will have a stronger DNA match than normal, allowing for more 4-6 cousin matches in our reports. A cousin may be reported as a 4th-5th in say, FTDNA or 23andme, because of the larger match, but when you compare your trees as in Ancestry you may find that they are actually a 5th or 6th cousin. The inner-marriage philosophy is mine alone, not backed up by any scientific research, but it sure makes sense to me.
On the other end of the spectrum, I have two cousin with the same shared great grandparents. One of them measures as a 2nd cousin in 23andme and the other, a known 3rd cousin, does not show a genetic match at all to me. They both have a strong 2nd cousin match to each other. Apparently, cousin #1 inherited one set of matching segments with me and cousin #2 inherited different genes from the same grandparents that I don’t share. She still has that family’s dna, but just different ones than I do. I’ve always had a strong interest in inheritance, without knowing anything about DNA. This really brings it all in to focus.