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Ancestry DNA

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Ancestry DNA

Postby Bradshaw_John » Wed Dec 11, 2019 7:12 pm

I took the Ancestry DNA test twelve months ago and was delighted a couple of months ago to break down a brick wall and narrow my late maternal grandmother’s father down to two brothers.
I recently persuaded my 1st cousin (my mother’s sister’s daughter) to take the same test and was amazed to find that none of the relevant matches featured in her matches on Ancestry.
Does the fact that I’m male and she is female mean that we don’t necessarily share the same matches even though we have the same great grandfather?
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Re: Ancestry DNA

Postby AdrianB38 » Wed Dec 11, 2019 8:53 pm

It's all a bit rocket science to me, but if you have a look at then in the preamble to the diagram, you see that
whilst you inherit half of your DNA from each parent, and they got half of theirs from each of their parents, you do NOT inherit 25% from each grandparent - it could be as little 0% or as much as 50% (although the extremes are very unlikely).

Therefore your first cousin doesn't have the same DNA as you - not by a long way. You would have expected that for the contribution made by her father, of course, but it also applies to the contribution from your mutual grandparents. From the chart you see that the average amount of DNA that you have in common with her is 874 centiMorgans (there's one hand waving bit because other than knowing that cMs are measures of commonality, I can't explain them). It can vary from 553 to 1225 at the 99% percentile (99% percentile means there's a chance that the actual variance is even wider but it's pretty low...)

So, the people that you match, are not necessarily going to be the people that she matches simply because she has different DNA - even in the "common ancestry" part. (It's actually nothing to do with male & female though I suspect that probably throws a minor contribution into the mixture - X and Y chromosomes and other alphabetic contributions)

Now, if anyone cares to correct me or add to what I've said, please do! I have no objection to being shown up if I learn something!!!
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Re: Ancestry DNA

Postby meekhcs » Thu Dec 12, 2019 6:20 pm

I know very little about DNA but with help I recently helped someone identify their Father.
The match between them and a half sister was expected to be c. 1800 centimorgans but in fact the match was only 1200+.
Ancestry gave a whole range of possible relationships, none of which was actually possible in their cases except half siblings.
Subsequently through further DNA research we found instances of proven half siblings matches as low as 1000+ centimorgans!
As Adrian mentioned this is entirely due to the different DNA we receive from our parents. So I am not surprised that given the results we found that you have come up with the results within your Family.

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