Why are some genetic diseases congenital but not familial?

Some genetic diseases are congenital (existing at birth) but not familial because they are not inherited from parents. While a familial disease is passed down through generations via germ cells, a non-familial genetic disorder often arises from a new mutation that occurs for the first time during the formation of the gametes or during the early development of the fetus.1

In these instances, the child possesses a genetic abnormality, but the parents do not carry the mutation in their own genetic makeup. Down syndrome, for example, frequently occurs sporadically due to errors in chromosome division, such as trisomy, rather than through inheritance from a parent. Because these mutations are spontaneous and unique to the affected individual, they do not follow the patterns of hereditary transmission seen in familial conditions.12