Cross-country, longitudinal twin studies provide strong evidence for both the biological and environmental basis of dyslexia, and the stability of genetic influences on reading and spelling, even when skills improve in response to instruction. Although DNA studies aimed at identifying gene candidates in dyslexia and related phenotypes (behavioral expression of underlying genotypes); and imaging studies of brain differences between individuals with and without dyslexia and the brain's response to instructional treatment are increasing, this review illustrates, with the findings of one multidisciplinary research center, an emerging trend to investigate the inter-relationships among genetic, brain and instructional treatment findings in the same sample, which are interpreted in reference to a working-memory architecture, for dyslexia (impaired decoding and spelling) and/or dysgraphia (impaired handwriting). General principles for diagnosis and treatment, based on research with children who failed to respond to the regular instructional program, are summarized for children meeting research criteria for having or being at risk for dyslexia or dysgraphia. Research documenting earlier emerging specific oral language impairment during preschool years associated with reading and writing disabilities during school years is also reviewed. Recent seminal advances and projected future trends are discussed for linking brain endophenotypes and gene candidates, identifying transchromosomal interactions, and exploring epigenetics (chemic al modifications of gene expression in response to developmental or environmental changes). Rather than providing final answers, this review highlights past, current and emerging issues in dyslexia research and practice.
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