Dominant negative effect

In some conditions, the abnormal gene product not only loses
normal function but also interferes with the function of the
product from the normal allele. This type of mutation acts in a
dominant fashion and is referred to as having a dominant
negative effect. In type I osteogenesis imperfecta (OI), for
example, the causal mutations in the COL1A1 and COL1A2
genes produce an abnormal type I collagen that interferes with
normal triple helix formation, resulting in production of an
abnormal mature collagen responsible for the OI phenotype.