Learning Disabilities

The genetics of learning disabilities became of an area of significant interest beginning in the 1990s. Reading disabilities run in families and this familiarity may be due in part to genetic influences and in part to environment. These genetic influences are likely to have a direct impact on the development of the brain or a specific region of the brain that is probably involved in language.

Genetic influences appear to be more prominent in children with phonological coding deficits than in those with visual coding deficits (Pennington 1991). These studies have been generally involved identical and fraternal twins. Deficits in specific processes have been found in phonological coding (the ability to discriminate sounds in words) and phonemic analysis (the ability to sound out words) compared to in visual-spatial deficits (DeFries et al. 1991). The concordance of phonetically based learning problems was 71 percent for identical twins but only 49 percent for fraternal twins. Bruce Pennington and his colleagues (1991) found evidence of a major gene transmission in a large sample of families with reading disabilities linking a small set of genes that indirectly affect reading. Although chromosomes 6 and 15 have been linked to reading problems, it is likely that the difficulty is due to several genes that have not been fully evaluated (Smith, Kimberling, and Pennington 1991). Genetic analysis of children with mathematics or written expression disabilities is another area that requires study.

Environmental influences also impact the brain and culture may change the development of neurons in a specific manner (e.g., reading left to right rather than right to left). Arabic and Hebrew readers have been found to show differences in hemispheric activation on reading tasks—particularly tasks that involve orthographic processing (Eviatar 2000). In addition, preliminary studies have indicated that those readers that read right to left do not show the same right hemispheric preference for the processing of faces and emotion as do those who read left to right (Eviatar 1997; Vaid and Singh 1989). Genetics and neuro-imaging studies may provide more information about these differences.

Familial risk for learning disabilities is clearly significant and substantial in many of the research findings (Gilger, Pennington, and DeFries 1991). Environment may play a role in the development of reading disabilities but no difference has been found between preschool literacy rate in children with reading problems and those without reading problems (Scarborough 1991). What has been found that within the family, the child with a predisposition for a reading problem is less interested in reading and reading-like activities than those without such a predilection (Scarborough, Dobrich, and Hager 1991). Moreover, differences in amount of time being read to, looking at books, and listening to stories were found between siblings with and without later reading difficulties.