The one which causes CMT probably should contain information for making a protein which is necessary for proper nerve function. When a single abnormal or mutant gene in an individual causes the clinical picture of the disease as in your family, the disorder is said to be dominantly inherited. It is true that sometimes a child will be born with such a disease when neither parent carries the mutant gene. In such a case the first mutation of that gene occurred spontaneously in the egg or sperm that form that individual. We do not believe this the case in your family, since we have found evidence of the gene in you and your mother.
Dominant disorders can vary widely in terms of the severity of the symptoms.
This is called expressivity. In some individuals there can be severe clinical problems while other affected members of the family may have hardly noticeable or even unnoticeable symptoms. For example, in dominantly inherited deafness there may be a severely deaf child with a parent who has a very small hearing loss detectable only on a sophisticated audiogram. The parent may be unaware of this small loss. Therefore, in some of these dominantly inherited disorders while the gene is inherited, the severity of the symptoms may vary widely.