WHEN DEALING with possible genetic diseases it is important to remember that "familial is not always genetic and genetic is not always familial." Familial simply means more than 1 case of a disorder in a family. Familial instances of a disease can certainly have toxic or infectious causes as a result of common exposure. Alternatively, single cases of a disorder in a family can have a primary genetic cause. Such single cases are often referred to as isolated or sporadic. There are at least 5 explanations for the sporadic occurrence of a possible genetic disorder. The first possibility is that the cause is not genetic, but actually acquired or environmental, a so-called phenocopy. Second, autosomal recessive genetic diseases often occur only once in a family, especially in small sibships. Third, the case may represent a new mutation of an autosomal dominant disease. Fourth, other family members may also carry the dominant mutation, but not express obvious symptoms or signs of the disease. This is called reduced penetrance. Finally, the biological father may not be the social father as a result of adoption or false paternity. Note that in the first 2 examples there is a very low recurrence risk to children of the patient vs a high (50%) recurrence risk to children in the other 3 examples.
Bird TD. Sporadic Cases of Possible Genetic DiseasesTo Test or Not to Test?. Arch Neurol. 2000;57(3):309-310. doi:10.1001/archneur.57.3.309