Are you talking about FOXP2? If so, absence of it foremost affects the motor control of speech organs. There also seems to be some effect on the actual brain fu

Are you talking about FOXP2? If so, absence of it foremost affects the motor control of speech organs. There also seems to be some effect on the actual brain functions that are usually associated with speech, but it is hard to say if this is really caused by the absence of the gene or by the developmental damage caused by its effects. It always seems tricky to me to pinpoint something as complex as language (and by extension speech) onto a single gene given the multitude of functions that are involved.