Osteogenisis Imperfecta (OI) is a genetic disorder characterized by bones that break easily, often from little or no apparent cause. There are at least four recognized types of the disorder, representing extreme variation in severity from one individual to another. For example, a person may have just a few or a as many as several hundred fractures in a lifetime. It is estimated that there are about 20,000 to 50,000 people with Osteogenesis Imperfecta in the United States. Osteogenesis Imperfecta is caused by a genetic defect that affects the body's production of collagen. Collagen is the major protein of the body's connective tissue and can be likened to the framework around which a building is constructed. In OI, a person has either less collagen than normal, or a poorer quality of collagen than normal--leading to weak bones that fracture easily. The characteristics features of Osteogenesis Imperfecta vary greatly from person to person--even among people with the same type of OI, and not all characteristics are evident in each case.    Clinical Features of Osteogenesis Imperfecta Type I  Most common and mildest type of Osteogenesis Imperfecta. Bones predisposed to fracture. Most fractures occur before puberty. Normal or near-normal stature. Loose joints and low muscle tone. Sclera (whites of the eyes) usually have a blue, purple, or gray tint. Triangular face. Tendency toward spinal curvature. Bone deformity absent or minimal. Brittle teeth possible. Hearing loss possible, often beginning in early 20s or 30s. Collagen structure is normal, but the amount is less than normal. Type II Most severe form of Osteogenesis Imperfecta. Frequently lethal at or shortly after birth, often due to respiratory problems. In recent years, some people with Type II have lived into young adulthood. Numerous fractures and severe bone deformity. Small stature with underdeveloped lungs. Collagen is improperly formed. Type III Bones fracture easily. Fractures often present at birth, and x-rays may reveal healed fractures that occurred before birth. Short stature. Sclera have a blue, purple, or gray tint. Loose joints and poor muscle development in arms and legs. Barrel-shaped rib cage. Triangular face. Spinal curvature. Respiratory problems possible. Bone deformity, often severe. Brittle teeth possible. Hearing loss possible. Collagen is improperly formed. Type IV (Between Type I and Type III in severity) Bones fracture easily, most before puberty. Shorter than average stature. Sclera are white or near-white (i.e., normal in color). Mild to moderate bone deformity. Tendency toward spinal curvature. Barrel-shaped rib cage. Triangular face. Brittle teeth possible. Hearing loss possible. Collagen is improperly formed. Inheritance Factors Most cases of Osteogenesis Imperfecta are caused by a dominant genetic defect. Some children with OI inherit the disorder from a parent. Other children are born with OI even though there is no family history of the disorder. In these children, the genetic defect occurred as a spontaneous mutation. Because the defect, whether inherited or due to a spontaneous mutation, is usually dominant, a person with Osteogenesis Imperfecta has a 50 percent chance of passing on the disorder to each of his or her children. Genetic counselors can help people with OI and their family members further understand OI genetics and the possibility of recurrence, and assist in prenatal diagnosis for those who wish to exercise that option.