The gene mutation has usually been carried unknowingly for many generations. The premutation can expand to a larger full mutation when it is passed on to the next generation through a female. It is the full mutation that causes fragile X syndrome. Your child may be the first one in your family with behavior or development problems, or there may be other relatives who are affected by the fragile X mutation. Your doctor or a genetic counselor will explain how this gene mutation is passed on in your family and who should be tested for it.
Genes are the building blocks of chromosomes.
Each person has 23 pairs of chromosomes. One pair, called the sex or X and Y chromosomes, determine whether a person is male or female. Females have two X chromosomes and males have one X and one Y chromosome. In fragile X syndrome the gene mutation is on an X chromosome. The full mutation in the FMR1 gene causes the FMR1 gene to turn off.