DCG: Disorders with Complex Genetics 10.1 Introduction: Complex genetic disorders and disorders with complex genetics

In medicine and medical genetics, one often encounters the phrase complex genetic disorder to refer to many common syndromes like diabetes, cancers, and hypertension. The definition of a complex genetic disorder is quite simple—it is a disorder that has something genetic going on but no one has a clue as to what the genetics are. The term complex genetic disorder is actually a misnomer because it implies that the disorder is genetic whereas empirical research usually shows that the environment can play a large etiological role. To insure that we do not neglect the environment, the term disorder(s) with complex genetics (DCG) will be used in this text. There are two hallmarks for DCGs. We have already mentioned the first– all DCGs have an important environmental influence on them. The second hallmark is the prevalence of the disorder. The frequency of Mendelian disorders is one is several thousands of births. (Sickle cell anemia is a rare exception.) The frequency of DCG is one to two orders of magnitude higher. Indeed, if a disorder has prevalence of .1% or above, it is certain to be a DCG. In terms of human behavior, all forms of psychopathology (schizophrenia, affective disorder, the anxiety disorders, etc.) are DCGs. Indeed, all forms of psychopathology have important—albeit largely unknown—environmental factors intimately tied up in their etiologies. Do not let the term DCG engender pessimism about ever finding the causes and effective treatments for psychopathology (or any medical DCG for that matter). Indeed, research into DCG, although it is very difficult, is one of the exciting challenges to both the genetic and the epidemiological community. Finding and cloning the gene for a Mendelian disorder is much akin to following