tic disorder that is caused by a break or weakness on the long arm of
the X chromosome (females have two X chromosomes and males have one X and one Y). The
gene where the break or weakness has occurred has been named FMR1 (Fragile X Mental
Retardation-1). Because of this defect, the gene cannot manufacture the protein that it normally
makes, which causes the features and characteristics of the syndrome. “Fragile X is estimated to
occur in 1 in 1,200 males and 1 in 2,500 females” (2). It occurs in all racial, ethnic, and
socioeconomic groups. It is said to be one of the most common genetic diseases and is the most
common inherited cause of learning disabilities and mental retardation known to exist.
Fragile X is, as mentioned above, inherited. Fathers who are carriers can only pass the affected
gene on to their daughters. The reason for this is that for males, the father always gives a Y
chromosome and the mother always gives an X chromosome (if the father gives an X
chromosome instead of a Y, the child would be female). Therefore, in order for the father to pass
the fragile X gene, the recipient would have to be a female. “Mothers who are carriers have a
50% chance of passing on the fragile X mutation to their children” (1). Females tend to be more
mildly affected by the fragile X gene than males because females have a second X chromosome
to compensate for the defective gene on the affected X chromosome. “It is estimated that 1 in
259 females, in the general population, can carry the fragile X mutation either as premutation or
full mutation” (3). Those with premutations are generally unaffected intellectually and of those
who carry the full mutation, approximately 50% will display a learning disability, intellectual
deficit, or mental retardation.
The underlying cause of Fragile X is a change in a single gene, the Fragile X Mental Retardation 1 (FMR1) gene, which is found on the X...