Complexity of the cognitive phenotype of an inherited form of learning disability.

An extended kindred with familial reading and spelling disorder was evaluated in a detailed genetic study. Comprehensive evaluations of cognitive functioning, verbal and non-verbal information-processing and academic achievement were obtained in order to arrive at a detailed phenotypic characterization of 'affected' and 'unaffected' family members. The pattern of major deficiencies in reading and spelling in this family was consistent with an autosomal dominant genetic determinant which had variable expression and incomplete penetrance. Although similar phonemic segmentation and sequencing difficulty appeared to characterize many of the affected family members, there was a high degree of phenotypic variability within this kindred. Caution is advised in interpreting the results of studies of subtypes of dyslexia which fail to take etiology into account, and it is argued that rigorous within-kindred phenotypic studies of dyslexia should be done before generalizations are made to unrelated families.