Mental retardation and intellectual disability

Abstract

Cognitive or intellectual disability (ID) in humans is a common trait that has a genetic etiology in as many as one-half of cases. Genetic causes range from single-gene defects to trisomies. The discovery of mutations that lead to ID has led to improved diagnosis and the opportunity for reproductive decisions based on prenatal diagnosis. In addition, understanding the gene defects in ID has elucidated molecules and pathways important for normal cognition, assisting with efforts to understand brain function. This chapter touches on the definition and history of ID, then considers in detail several classifications of gene defects that lead to ID. These include unbalanced gene dosage, with attention to Down syndrome and Prader-Willi and Angelman syndromes, and single gene disorders, particularly X-linked disorders Fragile X and Rett syndromes. Each disorder illustrates important principles in human genetics. Future directions in gene discovery, mutation detection and treatment are discussed.