In three generations of the KE family in London, about half of the family members had a severe speech and language defect. The pattern of inheritance suggested that it was caused by a dominant autosomal allele form of a gene... The gene was identified as FOXP2 because of its similarity to a family of the forkhead box transcription factors that are involved in turning on and off the expression of other genes. FOXP2 is expressed in a variety of adult and fetal tissues in addition to the developing brain. The one mutation in that gene found in the KE family essentially prevented that copy of the gene from functioning. Since we each have two copies of every gene... the affected members of the KE family had only half the usual amount of the FOXP2 protein expressed.
The link of FOXP2 to vocalization is also found in birds, mice, and echolocating bats.... The expression of FOXP2 is upregulated during the post-hatch learning of birdsong (zebra finch) or during singing season (canary). Ultrasonic vocalization in infant mice is significantly decreased if one copy of FOXP2 is deleted; if both copies are deleted, there is no vocalization and premature death. Echolocating bats... have an unusually high number of non-synonymous mutations in FOXP2.
It turns out that FOXP2 is a highly conserved gene with no amino acid changes in the chimpanzee line going back some 90 million years to the common ancestor with the mouse. This resistance to change suggests that FOXP2 is extraordinarily important for vertebrate development and survival. But there are two mutations that change amino acids in FOXP2 in the 6 million years of the human line since the common ancestor with the chimpanzee.... One of the human-specific mutations is shared with wolves tigers, and all members of the order Carnivora. Of the two mutations, this is the one which Enard et al. suggest might make the most difference in protein function by providing a potential site for phosphorylation.