In persons who do not have fragile X syndrome , there is a section of the DNA in the FMR1 gene which is normally repeated a few times. Someone with the premutation has 55-200 repeated CGG's in that part of the gene. Someone with the full mutation has 200-800 CGG repeats. We have also learned that a variety of other diseases such as Huntington disease and Friedreich ataxia fall in this category of expansion diseases. As a result, research on any of these diseases contributes to the understanding of the others.
Most genes have a very low rate of mutation.
The majority of individuals who inherit a disease have at least one parent who is a carrier for that disease since new mutations are rare. Thus, there can be a family with no history of fragile X syndrome in which it suddenly appears in a number of offspring. Since the FMR1 gene is on the X chromosome, and since it has a tendency to mutate once it has become a premutation, the patterns of inheritance for fragile X syndrome are complex.