Genetics of Dyslexia and Left Handedness

Dyslexia is strongly hereditary, and so is whether you are right or left handed. Furthermore dyslexia is said to be commoner in people who have weakly established lateralisation and are neither strongly right or left handed. So we have been using genetic linkage techniques in our dyslexic families to look for chromosomal sites that may be linked to left versus right hand skill as well as to reading. Ours was the first genetic screen covering all the human chromosomes that has been carried out for handedness.

But we did not find any special relationship between reading problems and handedness; relative hand skill was the same in our dyslexic families as in the population at large. There wasn’t a strong tendency to mixed handedness or non-right handedness. Nevertheless we did find a very strong genetic linkage of relative hand skill to a site on chromosome 2, although this was not strongly linked to reading ability.
Even though non right handedness did not seem to be strongly associated with reading problems, overall motor dexterity did correlate with reading ability. The faster they were at moving pegs from one set of holes to another with either hand, the better they were at reading. Thus accurate motor timing and coordination seem to be conducive to good reading.

Another condition which is thought to be associated with non-right handedness is schizophrenia, and both traits are often accompanied by lack of the normal brain asymmetry favouring the left side. A similar site on chromosome 2 to that which we found was linked to handedness, seems also to be linked to schizophrenia/schizoaffective disorder.
Just recently we’ve found that this shared linkage is only seen in chromosomal regions inherited from the father. Thus this site on Chromosome 2 inherited from the father may contain genes that affect the lateralisation of brain development. This may contribute to non- right handedness, weak language development and to schizophrenic traits, but only indirectly to reading.