Why can a new mutation cause a congenital disorder in a family with no previous history of that disorder?

A congenital disorder can appear in a child even when there is no family history of the condition because the underlying mutation may be a new, sporadic event rather than an inherited trait.12

This situation typically occurs through the following mechanisms:

• Germinal Mutations: A mutation can occur for the first time in the egg or sperm (germ cells) of a parent. Because this mutation is present in the gamete that forms the child, the child will possess the genetic abnormality even though the parents do not.132
• Post-conception Mutations: Genetic alterations can also occur spontaneously following conception, as the chromosomes from the egg and sperm combine and the embryo begins to develop.32
• Environmental Factors: While many mutations occur spontaneously, they can be stimulated by environmental factors such as radiation, certain chemicals, or carcinogens. Additionally, factors like advancing maternal age are known to increase the frequency of chromosomal abnormalities, such as Down syndrome, which often arise sporadically.134

Because these mutations arise de novo in the individual, the disorder is genetic but not necessarily familial or hereditary.2