FOXP2 is the only known
gene so far for human
language. It was isolated in recent years from the study of a family, variously called K, K(E), or KE, who suffer from defects in it and in whom the defects are inherited in a classic
Mendelian manner. Yet it is hard to say what deficits in language it causes, and even harder to say how it causes them.
There are of course numerous general conditions that affect, among other things, language: learning/intelligence impairments such as Down's Syndrome and Williams Syndrome have some linguistic effect, but it is not all that large. People with Down's Syndrome mostly use human languages in a pretty similar way to other humans, and Williams children are linguistically inventive. Also, of course, physical defects of the tongue or larynx or ears can prevent normal language being made or understood. But these are not specifically defects of the linguistic system.
From the other direction, there must be many specifically or largely linguistic genes, such as factors contributing to dyslexia, but so far none of these others have been isolated. We really have no idea at all how language is genetically encoded: a hundred genes, twenty, five hundred? And what effect do they have, individually or in concert? Who knows?
I don't know where the K(E) family come from or what their native language is: this private information is strictly embargoed. But they have been intensely studied by linguists as well as physical scientists. There are three generations. The grandmother suffers from the specific language impairment, and the grandfather does not. They have three daughters and two sons: one of the sons is normal and the other four children have the defect. All five children have families of their own. The eldest daughter has two affected sons and two unaffected daughters; the unaffected son has two unaffected children; the youngest daughter has nine children, of whom four are affected, including one of fraternal twins; and so on. The pattern is unmistakable: a single rare but dominant gene.
What is the impairment? It makes a major difference to these people's ability to use language, but it's hard to pin down what that difference is. First, it damages their ability to manipulate their tongue and lower jaw: it's physically much harder for affected family members to speak. But this isn't the main thing. Some major component of their understanding of grammar is missing. Roughly, very roughly, it looks like FOXP2 is a gene for morphology, and does not specifically affect syntax. Or perhaps it's a gene for government, a very specific part of syntax that links related words.
Here's what they get wrong: They can't make or understand plurals correctly; nor the tense, number, or person marking on verbs; nor derivational morphology (e.g. why 'friend' makes 'friendship' but 'friendly' makes 'friendliness'). They can't use the right pronouns to refer to someone/something previously mentioned (anaphora). More detailed problems can be added, and linguists can't put their finger on any one controlling aspect of it.
But here's what the affected (dysphasic) and unaffected (normal) members of the family agree on -- and the normal members have normal grammatical ability like you and me --: Reflexives, passives, possessives, gender, and judgements about who/what the subjects and objects of sentences are. Also in general they can hear sound differences and understand narrative structures equally well.
It's rather mysterious how FOXP2 could cause such specific harm, because it's an ancient gene: it occurs in fungi as well as mammals. It's a transcription factor: in general it controls the rate at which other genes work. So defects in this should show effects high up in the stream of causation and induce a range of physical and mental effects; but they don't. The physical effect on the K(E) family is restricted to the articulation problem, and they are cognitively close to normal. In fact on average their IQ is low, but with some overlap over the normal range. So it's a bit like Down's Syndrome, but less general: all the defects are related to language ability. The most striking is the severe damage to certain aspects of grammar.
The family were first studied from about 1990. In the late 1990s scientists homed in on the affected chromosome and in 1998 identified the mutation as occurring in region 7q31 of chromosome 7. In 2001 the precise mutation was discovered: a single change of G to A in the DNA. This was apparently unknown in any of the organisms that use FOXP2: it's an extremely important and highly conserved regulatory gene. Presumably mutations in it have been selected out of the genepool very fast.
This identification in fact brought about a name change. It had originally been called SPCH1. Then when its precise nature was identified, it was renamed FOXP2 because it was part of a known family of so-called forkhead genes (named after their striking effect on fruit flies when they go wrong).
Another interesting thing about this highly conserved gene is that mice differ from gorillas and chimpanzees in only one mutation in it: but humans differ from gorillas and chimpanzees in a further two mutations. These also seem on genetic evidence to be very recent (perhaps the last 100 000 years). This seems to be when human language evolved. Coincidence? I hope we find out some day soon.
Basic source: lecture notes from my psycholinguistics course last year, by Prof. Neil Smith of UCL.
Web source: I found this useful: http://www.evolutionpages.com/FOXP2_language.htm