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General => Ancestral Family Tree DNA Testing => Topic started by: Murrell on Monday 15 February 21 08:35 GMT (UK)
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If 1st cousins marry how will relationships show up thru dna.
Any children from the marriage would have grandparents who were their aunt/uncle.
I'm puzzled.
Someone mentioned double cousins but l wasn't really understanding that either.
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Double first cousins.
Let's say that John marries Mary and Jack marries Jill.
Let's say that John and Jill are brother and sister.
So Johns children and Jills children are first cousins.
Lets also say that Mary and Jack are brother and sister.
So Mary's children and Jacks children are first cousins
So all the children of John and Mary are first cousins to all the children of Jack and Jill. So it's Doubling up.
So all those children from those two couples have only two sets of grandparents.
I hope that helps. There are other examples, e.g. John and Jack could be brothers marrying Mary and Jill who are sisters.
JM
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They would be double first cousins. First cousins share in the range of 396-1397 cMs, with an average of 866cM according to Painter:
https://dnapainter.com/tools/sharedcmv4
So double first cousins would (theoretically) be in the range of, say 792-2794 cMs
Gadget
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Many thanks for your replies
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Sorry - that would be the relationship between the parents, so the children would be double 1C1R - double 1st cousins once removed as well as siblings.
Add - Siblings - 1813-3488 cMs; 1C1R 102-980 cMs
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Just checking, if this is what you are describing
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and
Some refs:
https://genetics.thetech.org/ask-a-geneticist/can-you-marry-cousin
https://www.yourdnaguide.com/ydgblog/2019/2/23/endogamy
https://learn.familytreedna.com/autosomal-ancestry/universal-dna-matching/ancestors-married-relatives-affect-results/
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Hi Gadget
Many thanks for the reference links. Makes for interesting reading. So unless I'd totally misunderstood the cousins marrying would give a higher relationship reading for subsequent descendants. Hope that doesn't sound too wordy.
I found it interesting that some 1st cousins decentants of 1st cousins marriage could show up as 1/2 siblings
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Two of my second cousins married - they were 1st cousins to each other. I know that at least one of the children had physical disabilities. This particular trait was from the other line.
I do have a higher cM than expected with some of my known 3rd cousins. When I traced them back, I found that they were related to me on other lines as well. These lines were all from a small mining community.
Painter and other such guides don't show these relationships but you have to be aware of the possibility
Gadget
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unless I'd totally misunderstood cousins marrying would give a higher relationship reading for subsequent descendants.
I found it interesting that some 1st cousins decentants of 1st cousins marriage could show up as 1/2 siblings
As always...the info. is based on 'likelihood' i.e. 'predicted'...they cannot tell for certain what the relationship is as it's based on cMs etc...
This is a note from 23andme as to my brother's relationship with a 'predicted' 3rd cousin who I have been in contact with, received info. from her as to her parents & g/parents re dates/places which I've been able to match up as a definite 3rd cousin from my own paper trail along with the 3rd cousins limited info. on names/dates of her g/parents from the birth cert. of her father...
We do share 2 x g g/parents as 'predicted' although I was of an open mind it could be a generation further back.
Our ancestors were from a small Scottish Island & like many rural areas as well as close-knit communities not rural, many people married relations whether knowingly or not as I've found in my research over the years...
"You and (name removed) may share a set of great-great-grandparents. You could also be from different generations (removed cousins) or share only one ancestor (half cousins).
I hope this helps?
Annie