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Question answered:09/10/02 Warning! this question is over two years old.
The Osteogenesis Imperfecta Foundation, from the US National Institute for Health discuss the issue of genetics on their website. They offer the following as part of their genetics fact sheet:
"OI in Families
There are essentially three scenarios that occur to cause a child to be born with osteogenesis imperfecta.
- Direct Inheritance from a Parent
- A New Dominant Mutation
- Mosaicism
A person with OI has two genes for type 1 collagen--one gene is faulty, the other is normal. Each time that person conceives a child, he or she passes on one of the two genes to the child. Therefore, there is a 50 percent chance that his or her child will inherit the faulty gene. If the child inherits the faulty gene, he or she will have the same type of OI as the parent. However, the child may be affected in different ways than the parent (e.g., the child's number of fractures, level of mobility, stature, etc. may not be identical to his or her parent's).
If the parent with OI passes on his or her normal gene to a child, that child will not have OI and cannot pass on the disorder to his or her own children.
About 25 percent of children with OI are born into a family with no history of the disorder. That is, a child is born with a dominant genetic mutation that causes OI, yet neither parent has OI. This occurs when the child has a "new" or "spontaneous" dominant mutation. The gene spontaneously mutated in either the sperm or the egg before the child's conception. Now that the child has a dominant gene for OI, he or she has a 50 percent chance of passing the disorder on to his or her children, as explained above.
As far as we know, nothing the parents did caused a spontaneous mutation to occur. There are no known environmental, dietary, or behavioral triggers for this type of mutation.
In most cases, when a family with no history of OI has a child with OI, they are not at any greater risk than the general population for having a second child with OI. (For the exception to this rule, see "Mosaicism" below.) In addition, unaffected siblings of a person with OI are at no greater risk of having children with OI than the general population.
In studies of families into which infants with OI Type II (the perinatal lethal form) were born, it was found that most of the babies had a new dominant mutation in a collagen gene. However, in some of these families, more than one infant was born with OI. Previously, researchers had seen this recurrence as evidence of recessive inheritance of this form of OI. More recently, however, researchers have concluded that the rare recurrence of OI in a previously unaffected family is more likely due to a phenomenon called mosaicism.
Studies suggested that the mutation, instead of occurring in an individual sperm or egg, occurred in a percentage of the cells that give rise to a parent's multiple sperm or eggs. Thus, although the parent is not affected by the disorder, the mutation present in a percentage of his or her reproductive cells can result in more than one affected child. It is estimated that 2 to 4 percent of families into which an infant with OI Type II is born are at risk of having another affected child because of this problem with sperm or eggs." (1)
The OI foundation recommend that families seek counselling from a qualified physician or genetics clinic.
1. Osteogenesis Imperfecta Foundation. Genetics. http://oif.convio.com/site/PageServer?pagename=Genetics
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