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I'm thinking about doing that DNA test on all my grandparents to know where their genes come from. They are getting very old and I don't want to let the opportunity pass to gather more knowledge about my heritage, something I find very interesting.

One of my grandparents died long ago, but he still has a brother and I was wondering if DNA testing him would reveal relevant information about my heritage from that branch of my family tree. Obviously all of his genes come from family of mine(his parents) but not all of his genes are the same as my grandparent's. Just how should I look at his tests to find information about my heritage?

By the way can I really trust these sort of tests?

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    $\begingroup$ if he is the brother of your grandparent then his DNA has some degree of probability that it is identical by descent to that of your grandparent - siblings on average share half of their DNA, though theoretically could share 0-100% of their DNA. Given how broad this is and that it borders on product/service recommendation, while seeing very little in the way of a proper question about biology, I am downvoting $\endgroup$ – rg255 May 20 '15 at 9:25
  • $\begingroup$ I can take the recommendation part off even the name of the company, but my question is pure biology $\endgroup$ – Yuri Borges May 20 '15 at 9:32
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    $\begingroup$ I disagree that this question is too broad. It is asking for a very specific case of the genetic tests which need to be carried out to determine the relationship of less closely related relatives. $\endgroup$ – March Ho May 20 '15 at 12:56
  • $\begingroup$ @rg255 you should develop a little and post your comment as an answer. $\endgroup$ – cagliari2005 May 21 '15 at 1:43
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As Rayoub's answer points, you are likely to share a random part of your genes with your grandparent's brother, and therefore analysing his DNA won't tell you where all your genes come from, since he has some genes you don't have and you have some genes he doesn't have.

However, I understand that you aren't that interested on your particular genes, but on what genetics can tell you on the history of your family - that is, to shed light on who were your ancestors and where they came from. Then, if your grandparent's brother was your grandparent's full brother, his ancestors are your grandparent's ancestors and therefore analysing his DNA can be as useful as analysing your grandparent's DNA.

In addition, since your grandparent and his brother doesn't share all the same DNA (likely just about 50% for full brothers) and each of them just have a 50% of each of their grandparent's DNA, if you could analyse DNA from both a grandparent and one or more siblings, you would be able to gather more information. In fact, you would be getting closer to get the same information you could have got from analysing your grandgrandparents'DNA.

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