Familial dyslexia: genetic and medical findings in eleven three-generation families.

In addition to providing information on the inheritance of dyslexia, the present study of eleven three-generation families has provided a unique opportunity to compare affected and unaffected family members at all ages. The data presented here are based on pedigree information, a questionnaire administered to all participating family members in relation to sex ratio, handedness, the severity of dyslexia by sex, pre- and perinatal complications, medical complications, years of education and earning ability, and a battery of standardized tests to define the presence or absence of dyslexia. The pattern of inheritance was consistent with the postulated autosomal dominant mode of inheritance and penetrance was found to be > 90 percent. Of 73 individuals determined to have a gene leading to dyslexia, seven were classified as obligate carriers and six as compensated adults who had no current symptoms or diagnostic evidence of dyslexia. The sex ratio (1.06) was not different from the expected ratio of 1.04. Left-handedness, major pre- and perinatal complications, and autoimmune disorders and allergy were not more common in dyslexics than non-dyslexics. The number of years of education and average income were similar in affected and unaffected family members. Compensated adults and obligate carriers were similar to unaffected family members in each of these parameters.