Genetics
Fragile X Syndrome occurs in the FMR1 gene. This is the fragile x mental retardation gene. This gene is inherited through the X chromosome, so it is inherited through an X linked inheritance pattern. The FMR1 genes main job is to make the FMRP protein. This stands for fragile x mental retardation protein. This is needed for normal brain development. If you have FXS, you don't make this protein, or don't make enough of it. The FMR1 gene starts with a repetition of the pattern C-G-G, which stands for cytosine, guanine, guanine. These are base pairs on DNA. The pattern is then repeated 6-44 times in humans with a regular FMR1 gene. This pattern of base pairs can be repeated between 55-200 times as well, and the brain can still develop normally. This is called a premutation. If you have a premutation, you still don't have FXS. If your C-G-G pattern repeats over 200 times, that is when you have fragile x syndrome. This results in the FMR1 gene completely turning off, so no protein can be made. Here is a good video explaining this, from the Center for Disease Control and Prevention.