A genetic disorder is an illness caused by abnormalities in the genome. They are heritable, and are passed down from the parents' genes. If a genetic disorder is present from birth, it is a type of congenital defect. Some only show up in later life.
Most genetic disorders are quite rare and affect one person in every several thousands or millions. Sometimes they are relatively frequent in a population. Some types of recessive gene disorders confer an advantage in certain environments when only one copy of the gene is present. Sickle cell anaemia is an example.
The same disease, such as some forms of cancer, may be caused by an inherited genetic condition in some people, by new mutations in other people, and by nongenetic causes in still other people. A disease in only called a genetic disease when it is inherited.