genetic disorder characterized by an abnormality in the conversion of cartilage into bone. As a consequence, bones that depend on cartilage models for development, particularly long bones such as the femur and humerus, cannot grow. Achondroplasia is the most common cause of dwarfism. In those afflicted with the disorder, the limbs are very short (fingers reach only to the hips), but the trunk is almost normal in size. The head is enlarged because of some overgrowth of the vault bones following premature closure of sutures at the base of the skull. Other manifestations of achrondoplasia include a bulging forehead, saddle nose, protruding jaw, deeply incurved lower back with prominent buttocks, and a narrow chest; women with the disorder may have narrow pelvises and, subsequently, difficulty in childbirth. Achondroplasia is inherited as an autosomal dominant trait; about 80 percent of cases of the disorder result from new genetic mutations rather than from the parental transmission of defective genes. Affected individuals are of normal intelligence and have otherwise normal health.