This paper investigates the etiology of developmental dysphasia and its linguistic properties. Data are presented that suggest that at least some cases of dysphasia are associated with an abnormality in a single dominant gene. The results of a series of tests on a large three-generation family, in which half of the members have dysphasia, are reported. These results show that abstract morphology is impaired in these subjects. It is argued further that the data are consistent with the hypothesis that the dysphasics learn the feature-marked lexical items of language as unanalyzed lexical items. They do not have the underlying capacity to learn language by constructing paradigms.