Several people have mentioned that with each generation, the consanguinity drops by 50%. This gives 50% genetic relationship to parents, 25% to grandparents, and so on.
However, just to nitpick, beyond parents this is statistical, not hard and fast. You have, on the average, a 25% genetic relationship with your grandfather, and 12.5% relationship with your great-grandfather, and so on, but you could, by sheer chance, have considerably more or less at each stage.
Over time it will tend to even out, but still, it is statistical, not guaranteed.
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