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Most genetic diseases are inherited from the parents, with different inheritance patterns depending on its dominance versus recessiveness and whether it is located on an autosome or the X chromosome. However, not all genetic diseases are inherited and a genetic condition without a family history is called "sporadic" rather than "inherited". Genetic diseases can also arise naturally via genetic mutations, which is presumably how the diseases arose in the first place. Hence, rare cases of genetic diseases are possible even if neither parent has any genetic history of the particular disease. In such cases, the risk of a reoccurrence in a second child is very low, but genetic testing is required to check whether the parents really do not have the genetic condition.
Read more about causes of Sporadic Genetic Diseases.
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