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A large number of human disorders—autism and cancer among them—display a confusing pattern of inheritance. In some cases, they are clearly genetic, with frequent occurrences in individual families. But in others, new cases will appear in families that were otherwise unaffected.

Initially, this pattern was assumed to result from diseases that had multiple causes. But with some diseases, we've come to recognize that mutations (whether inherited or not) play a much larger role than expected. The latest disease to join this category is schizophrenia.