The GBG model: is there more to consider than handedness?

Absence of linkage of phonological coding dyslexia to chromosome 6p23-p21.3 in a large family data set

Previous studies have suggested that a locus predisposing to specific reading disability (dyslexia) resides on chromosome 6p23-p21.3. We investigated 79 families having at least two siblings affected with phonological coding dyslexia, the most common form of reading disability (617 people genotyped, 294 affected), and we tested for linkage with the genetic markers reported to be linked to dyslexia in those studies. No evidence for linkage was found by LOD score analysis or affected-sib-pair methods. However, using the affected-pedigree-member (APM) method, we detected significant evidence for linkage and/or association with some markers when we used published allele frequencies with weighting of rarer alleles. APM results were not significant when we used marker allele frequencies estimated from parents. Furthermore, results were not significant with the more robust SIMIBD method using either published or parental marker frequencies. Finally, family-based association analysis using the AFBAC program showed no evidence for association with any marker. We conclude that the APM method should be used only with extreme caution, because it appears to have generated false-positive results. In summary, using a large data set with high power to detect linkage, we were unable to find evidence for linkage or association between phonological coding dyslexia and chromosome 6p markers.

Associations among familial sinistrality, allergies, and developmental language disorders

Here we report the results of a study that tested what has become known as the Geschwind hypothesis (Geschwind & Galaburda, 1987). This study involved 526 people who were tested by an allergist for IgE-mediated allergies and who filled out a 12-item handedness questionnaire that also included questions about eyedness, prevalence of left-handedness in the immediate family, and familial developmental language problems. A series of log-linear analyses revealed that if the definition of "anomalous" included left-handedness and having a first-order left-handed relative, one was indeed more likely to be allergic than were those not so classified. When the definition of anomalous simply included those classified as left-handed, the results were not significant. Parallel findings resulted when the criterion variable was the presence of developmental language disorders. No evidence was found for relations between allergies and developmental language problems.