I have had several of my DNA matches contact me because either I match both of their parents, or I match them and their spouse.
In many cases, they make what seems to be a logical conclusion, which is that somehow, despite their genealogy knowledge, somehow they are distantly related to their spouse, or their parents are distantly related, or something like that.
The conclusion is correct, but backwards. If I am a DNA match to you and I am also a DNA match to your spouse, then it is I … or one of my ancestors, that have related spouses. Given that my DNA contains markers that are 98.4% Ashkenazi Jewish, this should not be a huge suprise. The Ashkenazi Jews are an endogamous population. Despite efforts to avoid marrying too closely within the same family, it is inevitable that there will be some mating of related people. I speculate that the old tradition of having a matchmaker officiate over the selection of a mate was, in part to avoid marriages that were too close.
One thing that had me perplexed was that I ran both the Are Your Parents Related? test on GEDMatch, and also David Pike’s Run of Homozygosity utility and they both indicate that my parents are not related. Well, this is a head scratcher. Are both these utilities incorrect? If not, how can there be so many couples who are both share DNA with me. It seems unlikely, given that my parents come from different and distant parts of Germany’s pre-Holocaust Jewish population.
Tonight was the Eureka moment. I had tested whether my parents were related. What if one of their parents was related. Sure enough … I ran both tests against my mother’s DNA, and they show that her parents are distantly related. Well, that solves one mystery in my genetic past.