Which well-known genetic syndrome is associated with an abnormality on the X chromosome?

Fragile X syndrome is indeed associated with an abnormality on the X chromosome, specifically a mutation in the FMR1 gene. This condition is characterized by a trinucleotide repeat expansion (CGG repeat) in the 5' untranslated region of the gene. In fragile X syndrome, when the number of repeats exceeds a certain threshold (typically over 200), it leads to hypermethylation and silencing of the FMR1 gene, resulting in the absence of the fragile X mental retardation protein (FMRP). This absence is crucial for normal neuronal development and synaptic function, leading to a range of cognitive impairments, behavioral issues, and physical features associated with the syndrome.

The relevance of this genetic basis not only establishes a clear link to the X chromosome but also highlights the inheritance pattern, as the syndrome is X-linked and affects males more severely than females due to the presence of one X chromosome in males versus two in females. This contributes to the phenotypic expression of the disorder.

In contrast, Turner syndrome is primarily a condition where there is a complete or partial absence of one of the X chromosomes but does not involve a mutation affecting a specific gene like FMR1. Marfan syndrome is caused by mutations in the