Final answer:
Several symptoms may develop 15-20 years after the clinical onset of Huntington's Disease (HD), including involuntary spastic movement, difficulty initiating and controlling voluntary movement, and cognitive decline. The disease causes the brain's cells to break down, leading to muscle spasms and personality changes.
Step-by-step explanation:
After the clinical onset of Huntington's Disease (HD), several symptoms may develop 15-20 years later. These symptoms include involuntary spastic movement, difficulty initiating and controlling voluntary movement, and cognitive decline. The disease causes the brain's cells to break down, leading to muscle spasms and personality changes. Additionally, individuals who are heterozygous for the dominant Huntington allele (Hh) may develop the disease around age 40, at which point they may have already passed the allele to 50 percent of their offspring.