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The problem is that if you carry a rare trait for a genetic disorder(that is reccessive and so you need two genes for it to be noticiable), and most people carry some such trait or another with all their massive amounts of DNA, the odds of having a partner that has the exact same rare trait are incredibly low, thus giving you a very low chance of even having that 25% come up. Yet if they are your sibling and thus have very similar DNA as you, it is much much much more likely that they will happen to be a carrier for the same rare trait as you, and then there is 25% that the child will have the carrier gene from both parents and thus actually get the disease. Now, a normal person is usually the carrier for more than one rare genetic disorder, but they don't evolve this out of them, because it is so rare that the odds of meeting somebody else that has it too, is low. But if you mate your sister or brother, or somebody else related to you, there are very high odds that they are the carriers for the same genetic-disorders as you.
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