Author Topic: How much DNA do I share with this cousin of allsorts?  (Read 787 times)

Offline cockney rebel

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How much DNA do I share with this cousin of allsorts?
« on: Tuesday 19 June 18 12:26 BST (UK) »
Hallo All

Can any one please work out this for me ?

My grandfather was twice married.
From the first marriage a daughter, from the second a son (my father).
The daughter married the second wife's brother (ie her step-mother's brother, my great-uncle)

How much percent DNA do the children of the daughter (my allsorts cousin) and my father (ie me) share ?
Is it more or less than 1st cousins (12,5%) ? 
We share three families but
we share only one grandparent, but her other grandparents were my great-grandparents.
I just don't know how to work it out.

I'm just wondering how it might reflect on my Ancestry results/matches.
Any help welcome!
Thanks
Rebel

Offline davidft

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Re: How much DNA do I share with this cousin of allsorts?
« Reply #1 on: Tuesday 19 June 18 13:49 BST (UK) »
Ah a quiz! Is there a prize?

My stab at it (sure there must be a website that could work it out for you somewhere)

GF Grandfather
W1 Wife 1
W2 Wife 2
BW2 Brother of W2
D1 Daughter of W1 + GF
R1 Son of W2 + GF
R1S R1’s spouse
CD1 Child of D1 + BW2
CR1 Child of R1 + R1S
FW2 Father of W2 and BW2
MW2 Mother of W2 and BW2

So CD1 inherits 50% D1  50% BW2
Which is 25% W1 25% GF 25% FW2 25% MW2

And

CR1 inherits    50% R1   50% R1S
Which is    25% W2 25% GF 50% R1S
Which is   12.5% FW2 12.5% MW2 25% GF 50% R1S

So CD1 and CR1 could have in common
25% GF 12.5% FW2 12.5% MW2,  Or up to 50% in common.

This places them much closer than 1st cousins but as DNA is inherited at random the actual percentage is almost certain to be lower i.e. the 25% CD1 inherits from FW2 could be completely different to the 12.5% CR1 inherits from FW2 in an extreme case for example.

The short answer is it could make ancestry matches look closer than they are

Anyone is open to critique my workings (famous last words  ::))
James Stott c1775-1850. James was born in Yorkshire but where? He was a stonemason and married Elizabeth Archer (nee Nicholson) in 1794 at Ripon. They lived thereafter in Masham. If anyone has any suggestions or leads as to his birthplace I would be interested to know. I have searched for it for years without success. Thank you.


Offline cockney rebel

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Re: How much DNA do I share with this cousin of allsorts?
« Reply #3 on: Tuesday 19 June 18 20:13 BST (UK) »
Ah! Now you see why I need other people's brains and logic!

@ Mart 'n' Al
Thanks for the two links, but I don't think they quite calculate the situation. More from an either/or aspect. Bit like " relationship calculators"....it's just too much to figure in, although I'm sure many families have  a similar situation.

@davidft
Oh I wish there were a prize ! But I like the logic of your answer, thankyou.
I've always thought it must be more than 1st cousins but only sharing one grandparent doesn't actually make us first cousins at all!

Many thanks for your input.
If any one has any other ideas please do add.
I just get lost .
Rebel


Offline brigidmac

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Re: How much DNA do I share with this cousin of allsorts?
« Reply #4 on: Tuesday 19 June 18 20:41 BST (UK) »
Sharing 1 grandparent makes you half first  cousins
 on Ancestry  our matches came up as "2nd cousins or nearer"

With your extra link of a set of mutual great grandparents  I would also expect that to come out as strong possibility 2nd cousins or closer .
We had one example where a grandson of half cousin came up as closer match than son of half cousin so.I'm guessing maybe there was another link on his mothers side .
Roberts,Fellman.Macdermid smith jones,Bloch,Irvine,Hallis Stevenson

Offline sugarfizzle

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Re: How much DNA do I share with this cousin of allsorts?
« Reply #5 on: Wednesday 20 June 18 20:56 BST (UK) »
To a certain extent, it doesn't really matter what we think the answer may be. :) :)

My first cousin is shown at Ancestry as -

Predicted relationship: 1st Cousins
Possible range: 1st - 2nd cousins
723 centimorgans shared across 32 DNA segments

You will share however much DNA has been passed down, which theoretically could be similar to or more than first cousins, but in practice, as davidft has already said, could be less.

Let us know when you get your results back, if this is a practical rather than theoretical question  ;)

Regards Margaret
STEER, mainly Surrey, Kent; PINNOCKS/HAINES, Gosport, Hants; BARKER, mainly Broadwater, Sussex; Gosport, Hampshire; LAVERSUCH, Micheldever, Hampshire; WESTALL, London, Reading, Berks; HYDE, Croydon, Surrey; BRIGDEN, Hadlow, Kent and London; TUTHILL/STEPHENS, London
WILKINSON, Leeds, Yorkshire and Liverpool; WILLIAMSON, Liverpool; BEARE, Yeovil, Somerset; ALLEN, Kent and London; GORST, Liverpool; HOYLE, mainly Leeds, Yorkshire

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