Genetic Causes

Approximately one thousand genetic disorders are associated with mental
retardation, and the number increases regularly. If mental retardation is associated
with other conditions or features, it is syndromic; if it is the only primary
symptom, it is said to be nonspecific. In general, a genetic involvement is more likely to be found in severe forms of mental retardation than it is in
milder forms. A few examples of chromosomal and single-gene disorders
leading to mental retardation will be discussed as representative examples.