The jokes on me… heads up!
In my last post I noted that the Pro Tools cM relatedness was pretty accurate! Today I found two Matches who were 1C – their parents were brothers. But the SMOM said 1,637cM they had to be half siblings. I checked with DNAPainter – 1,637cM is 100% half siblings (for same generation relationship). Back to the drawing board… Did the two brother marry (or have children with) the same wife? Maybe one brother died, and the other married the widow… Nope. Checking some more – the two brothers married two sisters! They were double 1C! Not in the DNA Painter range of options, but spot on for twice the 1C cMs. All is OK, but it had me scratching my head for a few minutes.
[22CT] Segment-ology: Pro Tools Part 12 – The Jokes on Me by Jim Bartlett 20241028
segment-ology 
I’ve seen similar “double cousin” results in the descendants of the marriage of my grandmother’s uncle to my grandfather’s aunt.
LikeLiked by 1 person
I’ve seen similar “double cousin” results in the descendants of the marriage of my grandmother’s uncle to my grandfather’s aunt.
LikeLike
Jim… For me the “double cousin” issue is a subject of interest. On my paternal side my great-grandparents were double- first cousins. However, the more interesting problem is happening on my maternal side where a genealogy friend of several years has now with DNA testing become either my 2C or a double 3C. We are now trying to figure out where her adopted father falls into one of two families- in that a brother and sister of one family married a sister and brother of the other family (resulting in TWO sets of great-great grandparents in common).
ISOGG autosomal statistics has provided initial guidance how double cousin relationships overlap with other relationships as you indicated in your post (ex. double 2C overlap with 1C1R or double 3C overlap with 2C1R). Also, DNA-Science has a double cousin model, which I have had initial success with plugging in both known and unknown relationships. However, I am a bit bothered by the number of pair-wise comparisons needed to be imputed into the model. Currently, I am looking into the Banyan-DNA model which also can handle double cousin relationships with all DNA sharing imputes all at the same time. My question have you or any of your viewers had any experience/luck with this model? Thanks George…
LikeLike
George, See my older post on endogamy with a method to calculate shared cMs (works for double cousins and 3/4 cousins, etc.) https://segmentology.org/2015/12/02/endogamy-i/ The issue is that most of our calculations and data are from ancestral couples – and the DNA from both are counted. But by being careful to use the DNA from one parent at a time, it can be figured out. Jim
LikeLike
Jim… thanks for pointing me to your older post – you have made the calculations are clear…
LikeLike
My maternal grandparents both had siblings who also married each other. Plus, my maternal grandmother’s family was from the Toe River Valley in NC, where everyone married their cousins, it seemed! Consequently, my mother has many more DNA matches than my father.
LikeLike
skkidd Good feedback. In your case it will be even more important to watch out for multiple relationships. Jim
LikeLike
I have been having trouble with something for a few days until I have realised that I also had the two brothers marrying two sisters….all were first cousins. The two mothers were sisters.
The difference for mine was that the mothers were born in 1870’s and so the flow on is some years downstream
LikeLike
Gary, A good example. No matter where it happens, the average will be about twice the cMs… Jim
LikeLike