Research approaches in the behavioral genetics of psychiatric disorders

Multi-factorial inheritance combines genetic and environmental effects. Each factor contributes a relatively small part to the overall phenotype. Multi-factorial multi-genetic illnesses are usually divided into two categories: 1. Continuous variation of the normal distribution--phenotypes that can be measured within the normal scale (i.e. blood pressure). A normal distribution is obtained and the mean is in the middle. 2. Multi-factorial threshold trait--a threshold separates between the existence or absence of the phenotype. The curve of distribution obtained only predicts sensitivity for the illness rather than it's existence, since the illness appears only beyond a specific threshold, which represents a specific combination of defected genes. Mental illnesses (psychoses, affective disorders and neuroses) are multi-factorial illnesses, affected at a level of approximately 50% by inheritance. These diseases are not inherited in a simple Mendelian way, and, most probably, many genes are involved. Today, there is still no biochemical or molecular marker for any of the mental illnesses, and diagnosis of patients is obtained according to behavioral accepted scales. The aim of behavioral genetics is to understand the interaction between genes and behavioral variability among individuals. Most behaviors are complex, and when a genetic influence is observed, we can not point at a single responsible gene. Behavioral genetics targets to identify multiple genes and to understand the contribution of each of them to a specific behavioral trait. Methods to study multi-gene diseases include linkage analysis, family studies, twin and adoption studies, search for candidate genes affected by drug therapy of the patients, analysis of candidate genes using knockout mice and SNP's, methods that identify the effect of specific genes on behavior by QTL, and gene expression using DNA differential display and DNA microarray chips. The present review summarizes and exemplifies these issues.