A candidate phenotype for familial dyslexia.

The probative analysis of genotype-phenotype relations in familial dyslexia requires operationally defined psychobiological outcome variables that are not confounded by cultural differences of orthography or other factors that may influence the clinical ascertainment and diagnosis of dyslexia. Timing precision, as expressed in coordinated motor action, was used as an objective behavioral measure that can be mapped on current knowledge of central nervous system functions as well as on the most salient non-reading deficits in developmental dyslexia. Dyslexia families with four distinct pedigrees and a normally reading reference group were the study subjects. The results indicated that impaired timing precision in bimanual coordination and in motor speech were transmitted vertically in affected members of about half of dyslexia families. Motor coordination deficits were associated with a disposition to make dysphonemic spelling errors. It is proposed that impaired timing precision identifies a behavioral phenotype in some familial dyslexia subtypes. The detailed analysis of coarticulation in speech production may be one pathway by which impaired timing precision in motor action impinges on reading and writing deficits in developmental dyslexia.