Every time an individual has a child, exactly half her DNA is copied into that child. Every time she has a grandchild, a quarter of her DNA on average goes into that grandchild. Unlike the first-generation offspring where the percentage contribution is exact, the figure for each grandchild is statistical. It could be more than a quarter, it could be less. Half your DNA comes from your father, half from your mother. In turn, when you make a child, you pass half of your DNA on to her. But which half do you give? For any piece of DNA (‘gene’), you are equally likely to pass on the version you
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