In Parkinson's disease, a deficiency of a brain chemical called dopamine impairs the function of the part of the brain that controls movement. As a result, patients may have difficulty moving or they may have uncontrolled movements of their hands and fingers. Parkinson's disease usually occurs sporadically, with no known cause. In a few families, however, the disease seems to be inherited through a gene mutation (change). There is a 50-50 chance that a parent with the mutated gene will pass it on to a child. Children who do inherit the abnormal gene may or may not go on to actually develop Parkinson's disease-the relative chance of this happening is not known.