Autosomal DNA

So I started thinking about what you have written.  First I’m assuming that the use of Y and mt DNA studies was of no value since you and Alex share none.  Right?

Then I calculate that you and Alex are 3^rd cousins with your MRCA being your 2^nd great grandparents. Right?  This means that you have 1/16^th of the DNA of each your 2nd ggmother and your 2^nd ggfather. (All this based on the fact that all men and women get ½ of their DNA from each of their parents.) And your 3^rd cousin, Alex, also has an equal share. So you and Alex ought to share about 1/8 of your autosomal DNA. The other 7/8 should be different. Of course they test only an extremely small percentage of the autosomes content. And complete misses are thus possible even though you share 1/8^th. So that’s why the test is said by some experts to be only about 80-90 % accurate. There is a 10-20 % chance of a false negative.  But false positives are very rare. If you are identified as a relative by the test, you can probably count on it, I think.  But if you are not found to be relatives, there is always the chance of a false negative. You might still be relatives and the test missed it.

Family Tree says straight out that they can locate all descendants of all 16 2^nd ggparents.  But this is a recent claim based on recent improvements in the technology (better chips for calculations). But they claim noting for cousins more remote than 3^rd cousins.  When did you have this 23andme test?  If it was more than a year ago or so, it was certainly pushing the technology to find all 3^rd cousins.

Also, as you say, there is a chance that there was some sort of break in the DNA chain. But no way would both the man and the woman ancestor not be who you think they are. So I don’t see this as being an explanation. So, if the test were done correctly today by Family Tree, the thing should have worked for you and Alex. Your son is more remote to Alex, 3^rd cousin once-removed. But you sisters should have scored the same as you did in the calculations. And you should have matched very closely to the autosomal DNA of your sisters.  Right? In fact… perfect matches.  But you should match only ½ the autosomal DNA of your son; the other half being that of his father.

Still, I hope you found the story interesting.  I wouldn’t share it with just anybody. Only with someone who might understand the complexities of family relations.  And you know a lot about that.

Stay in touch,