The basic unit of heredity. Genes can act alone or in combinations with other genes to determine the multitude of characteristics that determine an individual’s physical characteristics, from eye and hair color to the presence of or predisposition for certain diseases. Each individual gene, among the tens of thousands of them in the human body, carries just a small fragment of a hereditary message. Genes align in specific sequences on chromosomes, forming the cellular structures that determine the ways genetic information is manifested. The alignment follows the same sequence for each repetition of the same chromosome throughout the body. The consistency of the repetition contributes to a particular trait’s presentation.
At present, scientists believe that it is a combination of GENETIC PREDISPOSITION and ENVIRONMENTAL triggers that converges to allow Parkinson’s disease to develop. Although researchers have discovered a number of gene mutations in some people with Parkinson’s, the extent to which genes influence the development of Parkinson’s remains unknown. About 15 percent of people with Parkinson’s also have a close relative, such as a parent or sibling, who also has Parkinson’s. A few limited studies of identical twins in which one twin has Parkinson’s disease indicate that the other twin does not seem to have any greater tendency to development of the disease than the general population when the twins are older than age 50 but is more likely also to have Parkinson’s when the twins are younger than age 50. Most scientists believe that rather than a single causative factor or event, a convergence of genetic and environmental factors allows Parkinson’s disease to develop, although genetic factors may be the key factor in many young onset cases.
Although a couple of genetic mutations have been associated with Parkinson’s disease, no defect seems to reliably cause the disease: many people with the suspect genes never develop Parkinson’s. Genetic testing is generally available only in research settings, is expensive, and in people with Parkinson’s disease does not provide conclusive evidence of the disease’s presence or absence. As yet, genetic testing does not yield information that makes the course of treatment any different, nor provide information that is of much value for reproductive counseling.