In the last 10 to 20 years, geneticists, neuroscientists, and others have made tantalizing discoveries about some basic features of mental retardation and developmental disabilities. For example, researchers have linked fragile X syndrome with excess or abnormal neuronal connections in the brain. They've found that adults who have velocardiofacial syndrome are very likely also to have psychosis. Another major finding is the difference in behavioral phenotypes in Prader-Willi syndrome (PWS) and Angelman syndrome (AS) because of genetic imprinting. The same region on chromosome 15 is deleted in both disorders, but a deletion inherited from the father results in PWS while a deletion inherited from the mother causes AS.
Voelker R. Putting Mental Retardation and Mental Illness on Health Care Professionals' Radar Screen. JAMA. 2002;288(4):433-435. doi:10.1001/jama.288.4.433