Dyslexia is heterogeneous and genetically complex. However, recent studies have identified potential candidate genes for dyslexia and other language-related traits. Genetics plays a major role in 40-80% of dyslexia, with a maximum role in spelling (70%) and word reading (58%). A child with an affected parent has 40-60% chance of developing dyslexia, the risk further increases if there are other affected members in the family (Stephenson, 1907; Olson et al, 1994; Ziegler et al, 2005).
Twin studies indicate the role of genetic factors in the familial clustering of dyslexia (Plomin & Kovas, 2005). The first genetic evidence for the high familiarity of developmental dyslexia came in the 1980s from two key twin studies, the Colorado Twin Reading Study and the London Twin Study. In the Colorado Twin Reading Study, the genetic component of phonological processing (75%) was found to be higher when compared to that of orthographic processing (31%) (DeFries et al, 1987). In the London Twin Study, a strong association (75%) was observed for the heritability of spelling and a moderate association (44%) for reading ability (Stevenson et al, 1987). A high heritability for dyslexia was found for boys in a UK-based study (Harlaar N et al, 2005).
According to a review on twin studies of language disability there is a 75% concordance for monozygotic (MZ) twins and 43% for dizygotic (DZ) twins (Stromswold, 2001). The concordance rate for reading disability in MZ and DZ twins was found to be about 70% and 50%, respectively (Oliver et al, 2004).
Linkage studies have identified dyslexia susceptible genes on specific chromosomal loci. HUGO Gene Nomenclature Committee has recorded nine dyslexia susceptibility loci, DYX1 to DYX9. These include,
Besides HGNC-listed DYX1–DYX9 regions, linkage for word reading on chromosome 13q12 (Igo et al,2006), and for phonological decoding efficiency on chromosome 2q22 have been reported (Raskind et al, 2005). Bivariate linkage scan on families with reading ability and attention deficit/hyperactivity disorder (ADHD) has shown pleiotropic loci on chromosome regions 14q32, 13q32 and 20q11 (Gayan et al, 2005). In addition, evidence of linkage for reading ability in affected sibling pairs with ADHD has been found on chromosome 10q11, 16p12 and 17q22(Loo et al,2004).
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