Figure 6.2. Causes of disorders of complex genetics. This figure gives a hypothetical partitioning of the major causes for disorders with complex genetics. There are likely to be some Mendelizing forms (i.e., single genes with high penetrance that will cause the disorder irrespective to other known factors), but they will probably account for only a few percent of cases. There may be several major susceptibility loci like the ApoE locus and Alzheimer's disease; these are genes that have relatively large effects but are neither necessary nor sufficient for the development of the disorder. Most cases are assumed to be due to multifactorial causes. A significant proportion of cases may be phenocopies produced exclusively by an extreme environmental insult.