Osteogenesis imperfecta (OI) is a genetic disorder. Most cases (90 percent) are caused by a faulty gene that reduces either the amount or the quality of type 1 collagen throughout the body. These mutations are inherited in a dominant manner. The other 10 percent of cases are caused by mutations in other genes that are inherited in either a dominant or a recessive manner.
Children inherit two copies of each gene – one from each parent. When OI is caused by a dominant mutation only one copy of the OI gene mutation is necessary for the child to have OI. In the majority of cases, the gene is either inherited from a parent who has OI or results from a spontaneous new mutation occurring at the time of conception. In rare cases dominant OI can occur when a parent is mosaic for an OI mutation. This means that an OI causing mutation is present in a percentage of one parent’s cells, but does not cause any symptoms in the parent. For a child to inherit OI in a recessive manner, the gene mutation must come from both parents. In this situation, the parents do not have OI, but both carry the mutation in their genes.
A person who has dominant OI has a 50 percent chance of passing on the disorder to each of his or her children. An affected child will have the same mutation, and therefore the same type of OI, as the parent. However, the expression— the degree of severity, or number of fractures— may be different. Unaffected siblings of a child with dominant OI have no greater risk of having children with OI than anyone in the general population. Unaffected siblings of a child with recessive OI have a 67 percent chance of being a carrier for the recessive gene. Genetic testing is available for siblings.