High Quality Content by WIKIPEDIA articles! X-Linked mental retardation refers to forms of mental retardation which are specifically associated with the X chromosome. Unlike many other types of mental retardation, the genetics of these conditions are relatively well understood. Mental retardation is a generalized disorder, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors with onset before the age of 18. Once focused almost entirely on cognition, the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environment.