Two separate groups of dyslexia families were ascertained through probands attending special schools for dyslexic students. An additional control group of families was ascertained through randomly selected students attending public schools. The 3 groups were interviewed by questionnaire about the family's demographic characteristics, and about the incidence of reading and spelling disorders in all first and second order relatives. One group of dyslexia families was also examined by standardized intelligence and academic achievement tests. Developmental dyslexia was found to aggregate in families; there were 4-5 times as many affected males as females among clinically identified students attending the special schools, but the sex ratio of affected relatives after probands had been excluded was approximately 1.4 males for every female. Sibs were at greater risk for reading difficulties when one parent was affected than when neither parent was affected. Sibs were also at greater risk for academic difficulties, and affected sibs were more severely impaired, when the father rather than the mother was the affected parent. In dyslexia families with 2 affected parents, the sibs were at greater risk, and the affected sibs were more severely impaired, than in families where only one parent was affected. Moreover, in families with 2 affected parents, both of the parents were more severely impaired in reading and spelling than parents of the same sex in families with one affected parent. Some indirect evidence is presented that assortative mating may codetermine patterns of affectedness in dyslexia families.

The motor control of bimanual coordination and motor speech was compared between first degree relatives from families with at least 2 dyslexic family members, and families where probands were the only affected family members. Half of affected relatives had motor coordination deficits; and they came from families in which probands also showed impaired motor coordination. By contrast, affected relatives without motor deficits came from dyslexia families where probands did not have motor deficits. Motor coordination deficits were more common and more severe among affected offspring in families where both parents were affected than among affected offspring in families where only one parent was affected. However, motor coordination deficits were also more common and more severe in affected parents when both parents were affected than among affected parents in families where only one parent was affected. We conclude that impaired temporal resolution in motor action identifies a behavioral phenotype in some subtypes of developmental dyslexia. The observed pattern of transmission for motor deficits and reading impairment in about half of dyslexia families was most congruent with a genetic model of dyslexia in which 2 codominant major genes cosegregate in dyslexia pedigrees where the proband is also motorically impaired.

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