Here is an informative article comparing the number of genealogical ancestors you have at any generation with the number of genetic ancestors. The short answer is you get DNA from about 2 to the power n ancestors n generations back, up to 8 generations, then about 68 additional ancestors for each generation after that.
A section of Miscellaneous observations at the end of the article explains how it's possible for a child to share more DNA with a cousin than either of his parents have in common with that cousin. why it's much easier to not be inbred according to genetic ancestors than by genealogical ancestors.
via Tweet from Debbie Kennett
I remember Debbie Kennett making this point briefly during one of her presentations at the last Anglo-Celtic Conference in Ottawa. The charts help considerably, although my head is still spinning!
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