Interval mapping of data from two independent samples of sibpairs, at least one member of whom was reading disabled, revealedevidence for a quantitative trait locus (QTL) on chromosome 6. Resultsobtained from analyses of reading performance from 114 sib pairsgenotyped for DNA markers localized the QTL to 6p21.3. Analyses ofcorresponding data from an independent sample of 50 dizygotic twinpairs provided evidence for linkage to the same region. Incombination, the replicate samples yielded a chi 2 value of 16.73 (P =0.0002). Examination of twin and kindred siblings with more extremedeficits in reading performance yielded even stronger evidence for aQTL (chi 2 = 27.35, P < 0.00001). The position of the QTL wasnarrowly defined with a 100:1 confidence interval to a 2-centimorganregion within the human leukocyte antigen complex.

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