What is the primary genetic cause of intellectual disability in developed countries?

The primary genetic cause of intellectual disability in developed countries is Fragile X syndrome. This condition is caused by a mutation in the FMR1 gene on the X chromosome, leading to a deficiency in the fragile X mental retardation protein, which is crucial for normal brain development and function. Fragile X syndrome is significant not only for its prevalence but also because it has an established genetic basis and is linked to additional features such as behavioral problems, anxiety, and social difficulties, in addition to cognitive impairment.

While Down syndrome and fetal alcohol syndrome can also lead to intellectual disability, they differ in their underlying causes. Down syndrome is a chromosomal disorder caused by trisomy 21, and although it is a common cause of developmental delay, it does not surpass Fragile X syndrome in genetic prevalence within developed countries. Fetal alcohol syndrome results from environmental factors, specifically alcohol exposure during pregnancy and is not a genetic condition. Glycogen storage diseases primarily affect metabolic processes and do not typically manifest as intellectual disability.

Thus, while several conditions can contribute to intellectual disabilities, the genetic nature and frequency of Fragile X syndrome make it the primary genetic cause in developed nations.