Achondroplasia is a genetic disorder whose primary feature is dwarfism. In those with the condition, the arms and legs are short, while the torso is typical of normal length.  Those affected have an average adult height of 131 centimeters (4 ft 4 in) for males and 123 centimeters (4 ft) for females. Other features include an enlarged head and a prominent forehead. Complications can include sleep apnea or recurrent ear infections. The disorder does not generally affect intelligence. Achondroplasia is caused by a mutation in the fibroblast growth factor receptor 3 (FGFR3) gene that results in its protein being overactive. The disorder has an autosomal dominant mode of inheritance, meaning only one mutated copy of the gene is required for the condition to occur. About 80% of cases result from a new mutation, which originates in the father's sperm. The rest are inherited from a parent with the condition.