The cause of mental retardation due to a narrowing of parts of the X chromosome is

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Fragile X syndrome is caused by a mutation in a gene on the X chromosome called FMR1. This mutation causes the X chromosome to become constricted or "fragile" in certain areas, resulting in mental retardation and other symptoms. Down syndrome is caused by an extra copy of chromosome 21, Klinefelter syndrome is caused by an extra X chromosome in males, and Turner syndrome is caused by a missing or partially missing X chromosome in females.

The Cause of Mental Retardation Due to a Narrowing of Parts