Fragile X Syndrome (FXS) is commonly indicated by what genetic anomaly?

Fragile X Syndrome (FXS) is primarily associated with a mutation in the FMR1 gene, which is located on the X chromosome. This genetic anomaly typically involves the expansion of a CGG triplet repeat in the 5' untranslated region of the gene. When there are more than 200 repetitions of the CGG sequence, the FMR1 gene is silenced, leading to a deficiency in the protein that it produces—fragile X mental retardation protein (FMRP). This deficiency is directly linked to the development of Fragile X Syndrome, which manifests as intellectual disabilities, developmental delays, and various behavioral issues.

The other options relate to different chromosomal abnormalities that do not connect to FXS. Excessive mutations on the Y chromosome do not pertain to FXS, and missing chromosome 21 or additional copies of chromosome 18 are associated with other conditions, such as Down syndrome, but not Fragile X Syndrome. Thus, the correct choice accurately reflects the genetic mechanism responsible for Fragile X Syndrome.