A genetic disorder is a disease caused by a different form of a gene, called a variation, or a change in a gene, called a mutation. Genetic brain disorders specifically affect the development and function of the brain. Some genetic brain disorders are due to random gene mutations or mutations caused by environmental exposure, such as cigarette smoke. Other disorders are inherited, which means that a mutated gene or group of genes is passed down through a family. Still other disorders are due to a combination of genetic changes and other outside factors. Some examples of genetic brain disorders include leukodystrophies , phenylketonuria , Tay-Sachs disease , and Wilson disease . Many people with genetic brain disorders fail to produce enough of certain proteins that influence brain development and function. These brain disorders can cause serious problems that affect the nervous system. Some are life-threatening.