Praxic and nonverbal cognitive deficits in a large family with a genetically transmitted speech and language disorder.

A pronounced speech and language disorder affecting half of the 30 members of the four-generational KE family has been attributed by some researchers to a specific defect in the generation of morphosyntactic rules. The reported selectivity of the impairment has led to the view that the affected members suffer from a grammar-specific disorder. Our investigations of the same KE family indicate that the inherited disorder has a broad phenotype which transcends impaired generation of syntactical rules and includes a striking articulatory impairment as well as defects in intellectual, linguistic, and orofacial praxic functions generally. Although the evidence from this family thus provides no support for the existence of "grammar genes," their linguistic difficulties do constitute a prominent part of their phenotype. Investigations of the neural and genetic correlates of their disorder could therefore uncover important clues to some of the bases of the primary human faculties of speech and language.