Scientists used to think of mutations as rare events. But in recent years they have gradually come to realize how many mutations happen. They are accumulated at the rate of about one hundred per genome per generation in mammals. That is, your children will have one hundred differences from you and your spouse in their genes as a result of random copying errors by your enzymes or as a result of mutations in your ovaries or testicles caused by cosmic rays. Of those one hundred, about ninety-nine will not matter: they will be so-called silent or neutral mutations that do not affect the sense of
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