Genetics, Behavior, and Behavioral
Geneticists and specialists working with individuals with intellectual disability now recognize that genetic syndromes may have characteristic physical phenotypes and behavioral features that may be linked to a specific genetic syndrome. These patterns of behavior are referred to as behavioral phenotypes. This chapter utilizes a developmental perspective to provide a definition and characterization of behavioral phenotypes in neurodevelopmental disorders and to discuss etiology, methodologies to understand underlying mechanisms, and the natural history of the disorder. Neurogenetic disorders with behavioral phenotypes include (1) Down syndrome, (2) Velocardiofacial syndrome, (3) Smith-Magenis syndrome, (4) Turner syndrome, (5) Rett's disorder, (6) Lesch-Nyhan syndrome, (7) Prader-Willi and Angelman syndromes, (8) fragile X syndrome, and (9) Williams syndrome. Each of these neurogenetic disorders involves a different genetic mechanism and provides a portal to understand neurodevelopment. A disorder that is environmentally induced, fetal alcohol syndrome, is also discussed and it, too, may provide a key to understanding aspects of the developing brain (Ikonomidou et al., 2000).
The first description of behavior associated with an intellectual disability syndrome was by Down (1887). In describing the syndrome that bears his name, Down observed that “They have considerable powers of imitation, even bordering on being mimics. Their humorousness and a lively sense of the ridiculous often color their mimicry.” Later, he added: “Several patients who have been under my care have been wont to convert their pillow cases into surplices (vestments) and to imitate, in tone and gesture, the clergymen or chaplain which