Complex Multifactorial Genetic Diseases

A trait is often of interest by virtue of being either highly desirable, for example intelligence, or highly undesirable, for example cancer. Genetic influence over the trait can be established inmanyways including twin studies, clustering statistics and statistical modelling. Once the existence of a genetic component has been established, the next step is to locate the genes involved. The ease or difficulty of this is influenced by the complexity of the trait and the size of the genetic effect. In multifactorial traits, multiple pathways may lead to both genotypic and phenotypic heterogeneity, the genotype– phenotype relationship may be complicated (low or agedependent penetrances), phenocopies may exist, and the phenotype itself may be hard to measure.