This collection of genes is the body's blueprint and instruction manual. In such a complex process mistakes can sometimes be made and a faulty copy of the gene is made. These faults are called mutations.
Harmful mutations are very rare events and sometimes just happen spontaneously.
Sometimes they are caused by exposure to chemicals or radiation that damage the gene. Most often, in so called recessive gene disorders, individuals inherit faulty copies of genes from their parents and if neither parent contributes a correct gene then the offspring will suffer from a genetic disorder. There are about 5000 diseases caused by a gene containing an incorrect piece of information. There are many other diseases in which defective or absent genes are an important factor in the development of the disease but the presence of other factors such as an infection or exposure to certain chemicals triggers the disease. Examples are heart disease, some cancers, asthma, diabetes and Alzheimer's disease.