Author Topic: 1st cousins  (Read 650 times)

Offline Murrell

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1st cousins
« on: Monday 15 February 21 08:35 GMT (UK) »
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.
Power Ward Rooney  Southern Ireland

Offline majm

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Re: 1st cousins
« Reply #1 on: Monday 15 February 21 08:54 GMT (UK) »
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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Offline Gadget

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Re: 1st cousins
« Reply #2 on: Monday 15 February 21 09:20 GMT (UK) »
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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Offline Murrell

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Re: 1st cousins
« Reply #3 on: Monday 15 February 21 09:29 GMT (UK) »
Many thanks for your replies
Power Ward Rooney  Southern Ireland

Offline Gadget

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Re: 1st cousins
« Reply #4 on: Monday 15 February 21 09:57 GMT (UK) »
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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Offline Gadget

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Re: 1st cousins
« Reply #5 on: Monday 15 February 21 10:22 GMT (UK) »
Just checking, if this is what you are describing


                     
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Offline Murrell

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Re: 1st cousins
« Reply #7 on: Thursday 18 February 21 23:10 GMT (UK) »
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
Power Ward Rooney  Southern Ireland

Offline Gadget

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Re: 1st cousins
« Reply #8 on: Friday 19 February 21 00:42 GMT (UK) »
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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