A Study was conducted to test the hypothesis proposed by J. S. Symmes and J. L. Rapoport that a sex-linked recessive gene might account for the good spatial ability found among dyslexic readers, the familial pattern of the disorder, and the frequently reported sex ratio of three affected males to one female. Spatial/reasoning ability scores were obtained from 125 dyslexic children, their biological parents, their siblings, and 125 matched control families. The results indicated that although males tended to perform better than females on tasks requiring spatial/reasoning ability, there was no reason to believe that ascertainment bias favoring dyslexic males with superior spatial ability could account for the 3:1 sex ratio. Tests for sex linkage also failed to confirm the Symmes-Rapoport hypothesis. With regard to familial patterns of spatial ability, spatial/reasoning scores obtained from biological siblings and parents of the dyslexic children confirmed a familial pattern of competence but not of superiority when compared to the respective members of the control families. Thus, a familial pattern of reading deficits did not appear to be etiologically related to spatial/reasoning ability. (FL)