Objective: --There is strong evidence that developmental dyslexia is both familial and heritable, but the mode of genetic transmission has remained unclear. In this article, we examine specific genetic hypotheses about the mode of transmission of developmental dyslexia by performing complex segregation analyses.
Design: --A family study method was applied, whereby the relatives of dyslexic probands were examined for dyslexia. The families studied represent four independently ascertained samples.
Setting: --The four samples of families were primarily from rural and suburban communities of Colorado, Washington State, and Iowa.
Participants: --A total of 204 families and 1698 individuals in the four samples combined.
Main outcome measures: --The complex segregation program, POINTER, was used to test competing genetic hypotheses of how a categorical trait (dyslexia) is transmitted in families.
Results: --The results were consistent with major locus transmission in three of four samples and with polygenic transmission in the fourth. In these three samples, the estimates of penetrance for the AA, Aa, and aa genotypes (where A is the abnormal allele) were, respectively, 1.000, 1.000, and 0.001 to 0.039 in males, and 0.560 to 1.000, 0.550 to 0.897, and 0.000 in females. The estimated gene frequency of the major locus was between 3% and 5%.
Conclusions: --Sex-influenced, additive, or dominant transmission occurs in a significant proportion of dyslexic families. Other evidence indicates, however, that dyslexia is etiologically heterogeneous and that there is genetic heterogeneity even among families selected for apparent dominant transmission. Thus, while no single major locus may account for all of dyslexia, it is important to pursue potential major loci for dyslexia using linkage techniques.