Osteogenesis imperfecta is caused by a dominant allele but not all people with this allele actually suffer from the disease. What is this phenomenon called?

This phenomenon is called as Penetrance.

Explanation:

Sometimes a phenotype is not expressed even in the presence of dominant allele. Pentrance is the probability that the a particular trait would express. It is the extent to which a particular trait could be expressed in the phenotype. Extra digits on finger or toes and Osteogenesis imperfecta are examples of pentrance.