A family of 30 members (known as the KE family) seems to be affected by a
genetic desease that is manifested in an autosomal dominant fashion, ie. it
doesn't skip a generation. They have detected the disorder in 4 consecutive
generations.
The affected members (15) suffer from specific impairment in grammar:they can
not generate syntactic rules related to tense, gender and number formation.
They have also problems with sound formation and articulation of words. Their
speech is not easy to understand.
Previous studies (Gopnik and Crago, Gopnik) suggested that the phenotype was
a specific grammar impairment and concluded that this inherited
grammar-specific disorder provided evidence for the existence of
"grammar-genes".
The present study includes 21 members of the KE family: 13 affected vs. 8
unaffected. A formal evaluation of intelligence was performed to determine
whether this problem was restricted to the verbal domain or whether non-verbal
aspects of intelligence were also affected. Verbal IQ and performance IQ were
assessed. The affected subjects scored 18-19 point on average below the
unaffected ones. The conclusion arrived to was that there was no evidence for
the existence of grammar-specific genes because the disorder doesn't affect
just morphosyntactic rule generation but it also affects intellectual,
linguistc and orofacial praxic functions. In other words, the genetic disorder
has a broader manifestation. It is not just confined to grammatical-verbal
problems but it has an impact in intellectual abilities in general.