Dyslexia covers a group of symptoms, any or all of which may be seen in one individual, and which can be briefly summarized as: visual perceptual difficulties, directional confusion, sequencing problems, short term and long term memory problems, difficulties in naming, organizational problems, and general literacy skills.
The question of whether dyslexia is genetically determined has a long history, going back to 1900 when Hinshelwood, Morgan and Kerr noted that with two boys they saw who were intelligent but could not read, there were several cases in each family, which lead him to suggest the term 'congenital word blindness'. More recent research has supported the view that dyslexia runs in families, and is therefore at least partly genetically determined, it was reported that 88% of children seen had a family history of dyslexia (Hornsby, 1992). Another study coducted found that in 45% of cases, immediate family members of the dyslexic person were also retarded readers, confirming the strong tendency for dyslexia to run in families(Hornsby, 1992). However, it is important to consider whether one explanation for this apparent 'genetic' cause may be that those in families share the same environment. For example if one or both parents don't read very well, then it is not unexpected that their children have difficulties. .
To try and clear up this confusion, we turn to twin studies, of which many have been conducted. Twin studies are important because they allow comparison between monozygotic twins (who share 100% genes) and dizygotic twins (who share 50%), but where both have been brought up in the same environment, so that any difference between dizygotic twins can be reliably concluded to be based on genes rather than environment. Using this method, Herman (1959) found a 100% concordance for monozygotic twins, and 33% for dizygotic twins: this is a summary of three studies undertaken in Scandinavia in the 1950s.
