Linkage studies suggest a possible locus for developmental dyslexia on chromosome 1p

Eight extended dyslexic families with at least four affected individuals were genotyped with twelve genetic markers spanning the Rh (rhesus factor) locus. Eleven of these markers were located on the short arm and the other was on the long arm of chromosome 1. Five theoretically derived phenotypes were used in the linkage analyses: 1) phonemic awareness; 2) phonological decoding; 3) rapid automatized naming; 4) single word reading; and 5) vocabulary. In addition, a lifetime diagnosis of dyslexia was used as a phenotype. Both parametric and non-parametric genetic analyses were completed. The results supported the importance of a putative locus on 1p. In addition, two-locus analyses assuming the interaction between a 1p locus and a 6p locus, previously shown to be of interest for dyslexia, were conducted. As a result, the nonparametric linkage (NPL) scores for rapid automatized naming and phonological decoding were significantly increased. In particular, the NPL scores for rapid automatized naming exceeded 5.0 for certain markers. These results provide strong evidence for separate but jointly acting contributions of the 1p and 6p loci to the reading impairments associated with rapid naming and suggestive evidence for a similar mechanism involving phonological decoding.