Final answer:
Juvenile Huntington's disease is most often caused by an abnormal expansion of more than 60 CAG repeats in the huntingtin gene, leading to early-onset symptoms before age 20, with a 50% chance of inheritance.
Step-by-step explanation:
The length of repeat that invariably causes juvenile Huntington's disease (HD) is generally more than 40 CAG repeats in the huntingtin gene. In the case of juvenile HD, this number is often significantly higher, typically exceeding 60 repeats. The larger the number of repeats, the earlier the onset of the disease, with the juvenile form, also known as early-onset HD, occurring in individuals before the age of 20. Juvenile HD is characterized by its rapid progression and is inherited in an autosomal dominant pattern, meaning that if one parent carries the expanded allele, there is a 50% chance of passing the disorder on to their offspring.
Huntington's disease leads to the degeneration of nerve cells in the brain, resulting in a wide range of movement, cognitive, and psychiatric symptoms. The disease is caused by a mutation in the HTT gene which results in the abnormal expansion of a DNA segment known as a CAG trinucleotide repeat. This mutation leads to the production of an abnormal version of the huntingtin protein, which disrupts normal cell functions and ultimately causes cell death. Unfortunately, because symptoms of Huntington's disease typically appear at middle age, individuals might have already passed the mutated gene to their children before they are diagnosed.