Symptoms of the fatal single-gene disease answer include involuntary movements, mood swings, depression, irritability, and inability to remember facts.

The answer would be Huntington's disease.

Huntington's disease is a genetically inherited disease that was caused by a defect that will be inherited in autosomal dominant ways. That mean the children of the offspring to inherit the gene would be at least 50%. The symptoms would be personality disorder(mood swing, depression), decreased cognitive ability(forgetful), involuntary movement and unstable gait, slurred speech with difficulty in swallowing food.