One mutated gene left half of a three-generation family unable to master grammar

The KE family passed down a severe speech and language disorder for three generations, affecting about half its members with a single dominant gene. In 2001 geneticists traced it to a point mutation in FOXP2, the first gene ever linked to human speech and language. Affected members struggled to sequence mouth movements for speech and to apply grammar rules like regular past tense, not just to pronounce words clearly.

— Cecilia S. L. Lai, Simon E. Fisher, Jane A. Hurst, Faraneh Vargha-Khadem, Anthony P. Monaco, A forkhead-domain gene is mutated in a severe speech and language disorder — Nature, vol. 413, pp. 519-523, 2001