Final answer:
The most visible symptom of Huntington's Disease is involuntary movements known as chorea. Chorea becomes more obvious as the disease progresses, distinguishing it from symptoms of other neurological disorders. If one parent has HD, there is a 50 percent chance of passing the allele to their children, with the disease usually manifesting in middle age.
Step-by-step explanation:
The most visible symptom of Huntington's Disease (HD) is involuntary writhing movements, also known as chorea. As HD progresses, these movements may become more pronounced and affect the patient's ability to perform daily activities. Unlike the symptoms for Parkinson's disease such as tremor or muscle rigidity, or the symptoms for other neurological issues such as double or blurred vision and slurred speech, chorea is particularly characteristic of HD and typically appears after the onset of other symptoms like cognitive disturbances or mood changes, which are less visible. In terms of inheritance, if one parent has Huntington's disease, there is a 50 percent chance that the HD allele will be passed on to their children, which can be illustrated using a Punnett square. It's worth noting that HD symptoms typically do not manifest until middle age, and thus the disease may be unknowingly transmitted to offspring before the parent is aware they carry the defective gene.