Final answer:
Fragile X syndrome is caused by a mutation in the FMR1 gene on the X chromosome, where a DNA segment known as CGG triplet repeat is expanded, resulting in the gene being turned off and not producing a necessary protein for normal brain development.
Step-by-step explanation:
The gene variant that causes Fragile X syndrome is a mutation in the FMR1 gene on the X chromosome. This gene typically produces a protein necessary for normal brain development. In Fragile X syndrome, a specific DNA segment, known as CGG triplet repeat, is expanded within the FMR1 gene. The normal repeat size is between 5 to 40, while in Fragile X syndrome, it can be more than 200 repeats, leading to the gene being silenced (methylated) and not producing the required protein. This disruption can result in a variety of developmental problems including learning disabilities and cognitive impairment. Since the FMR1 gene is located on the X chromosome, Fragile X syndrome is considered an X-linked disorder. This partly explains why genetic disorders caused by abnormal numbers of chromosomes often involve the X chromosome - because females have two X chromosomes (making them more likely to have a normal copy of the gene), while males have only one X chromosome and are thus more susceptible if a mutation is present.