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Although most genetic diseases are inherited from parents, this is not always the case. When a genetic disease occurs without any family history or genetic defects in the parents, the disease is called a sporadic genetic disease.
The cause of these diseases is usually a random mutations in gene in the DNA that occurred somewhere in the development of the fetus. This is presumably how the diseases arose in the first place throughout history.
It is more common for a sporadic disease to be a dominant genetic disease. The obvious reason is that a dominant disease requires only one random mutation, whereas two identically located mutations are required for recessive diseases. Hence, a random mutation is more likely to make a person a "carrier" of a recessive disease than actually give them the disease.
By this method of random DNA mutations, 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.
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