DNA and Kissin' Cousins (Double)

 I attended the virtual Rootstech this year. In one of the chat sessions the instructor referred someone with double cousins to DNA painter. I don't think that was good advice. Here's why, double cousins prior to 4th cousin will show more DNA the majority of the time than the relationship. They will seem a 0.5 to 1 generation closer, and DNA painter, doesn't address this. 

My father's father had a set of double cousins. His Uncle from his mom, married his Aunt from his Dad. Now, that isn't the only double cousins (and I mean sibling marriages, there are another type of double, where two lines intermarry, but DNA is less reliably inherited at a higher rate, it depends on the distance.) in his tree, but it's the only CLOSE one.

Let's look at a few of Dad's and my double cousin relationships. 

This cousin, is my father's double second cousin once removed. He shares 379 cM. Here is what DNA painter shows. Notice 0.61% chance.

And here is another double second cousin once removed, Dad shares 341 cM. Here is what DNA painter shows. Notice, 4% chance.

Dad has two 2nd cousins once removed from this same line that aren't of the double cousin marriage. He matches 151 cM and 146 cM with them. Here is the DNA painter for 151 cM. Notice, 2nd level of probability.

In fact, he matches 266 cM with the father of one of those matches, here is the DNA painter on that. notice, 1st level of probability.

 

 

You will notice in the last two examples, the probability matches the match at a higher degree. Now, I will admit, due to my father's family location, I often fall into the 2nd probable percentage. Simply because he often has a case where he is related to someone twice. Without triangulation though, I can't identify which DNA comes from which family.

 

Here is a final example. This matches grandparents were first cousins. The match she is matching to is the great granddaughter of  the first matches great grandfather, and the  gggrandniece of the first matches grandfather. (yes, my great great grandfather married his great niece and had children). They match 308 cM. Now there is clearly pedigree collapse. They are half 2nd cousins, and 2nd cousins once removed. Notice though, they do not come near the smallest tier of the probabilities, as the 2 double cousins do. 

Now, I have more examples, that can reliably show my 3rd cousin matches look closer but when I get to 4th cousin, it falls off. Some match higher, some match lower. Kissing cousins can be a challenge in southern trees. If you know they are a double cousin, expect a match to be higher than DNA painter, if you don't know, do the research, and look out for it as a possibility.