Why can Huntington disease be passed on before symptoms appear?

Huntington disease is a hereditary condition caused by a genetic mutation in the HTT gene, which is located on chromosome 4.1

The disease can be passed to offspring before symptoms appear because the genetic mutation is present from birth in every individual who inherits it. While symptoms typically do not manifest until middle age—usually between ages 35 and 50—the underlying DNA abnormality, characterized by expanded CAG trinucleotide repeats, is already established in the person's genetic code.1

Because the condition follows an autosomal dominant pattern of inheritance, a child of an affected parent has a 50 percent chance of inheriting the mutated gene. Consequently, an individual may unknowingly pass the mutation to their children long before they experience the involuntary movements or cognitive decline associated with the disease's progression.1