How can a new mutation cause a congenital disorder in a child with no family history of the condition?

A congenital disorder can occur in a child without a family history when a new mutation arises for the first time in the parents' reproductive cells or during early embryonic development. While many genetic conditions are inherited from parents, a disorder is not necessarily familial if the genetic alteration is spontaneous.12

This phenomenon occurs through several mechanisms:

• Germinal Mutations: A mutation may occur in the egg or sperm (germ cells) during the cell divisions that produce them. Because this mutation is present in the gamete, it is transmitted to the offspring at conception, even though the parents do not carry the mutation in their own body cells.342
• Post-conception Mutations: Genetic alterations can also occur following conception, as the chromosomes from the egg and sperm combine and the embryo begins to develop.4
• Environmental and Biological Factors: These mutations can happen spontaneously or be triggered by environmental factors such as radiation or chemical mutagens. Additionally, the frequency of such mutations can increase with parental age, such as through errors in DNA replication in men or chromosomal nondisjunction in women.341

Down syndrome is a well-known example of a genetic condition that often occurs sporadically in this manner rather than being inherited from parents.2