This bone disorder is usually present at birth as an inherited disease. Osteogenesis imperfecta (OI) is classified into four major types (and further subtypes).

All four types of OI are caused by defects in the amount or structure of Type 1 collagen, an important part of the bone matrix. The collagen problem usually results from a dominant genetic defect. This defect may be acquired by several different mechanisms:

• The defect may be inherited in an autosomal dominant pattern from an affected parent. This means that an affected parent, who carries a single gene for the disorder, has a 50% chance of having children with the disorder. Any child who inherits this gene will be affected.
• The defect may be acquired by a spontaneous mutation occurring in the individual egg or sperm that formed the child. In this case, neither parent carries a gene for the disorder or is affected by it. The parents, in this case, are no more at risk than the general population for having another child with the disorder.
• The defect may be acquired through a pattern of inheritance called mosaicism. This occurs when neither parent is affected, but one carries a percentage of sperm or eggs which contain the genetic defect. Therefore, though the parents are unaffected, some of their children may have the disorder and others will not. It is estimated that about 2% to 7% of unaffected parents who have had a child with OI will have another child with OI due to the phenomenon of mosaicism.