We mentioned above that linkage analysis led to the identification of most genes causing Mendelian genetic disorders. Such disorders are caused by rare mutations that cause disease under a specific model (for example autosomal dominant) with a high penetrance (close to 100% of mutation carriers are affected) and with very rare phenocopies (almost everyone affected has the same genetic condition). Unfortunately these criteria are not met by the more common human disorders that, although usually milder than Mendelian disorders, represent overall a much bigger burden to our society. Psychiatric diseases, which are both common and complex, are often characterized by a relatively early age of onset and require lifelong treatment. As treatments are often only partially effective these diseases have devastating consequences for the quality of life of the patients and their caregivers.
