genetic disease and fragile x syndrome

Fragile x syndrome is caused alteration of the FMR1 gene located on chromosome X.
Males have one X chromosome and one Y, in women has two copies of chromosome X, the disease occurs only when both copies of the gene is altered , in women homozygous recessive. Women who have a healthy gene on one X and one altered on X are called heterozygous not develop symptoms and are healthy carriers, because they can still transmit the mutation to their children. The name of the disease stems from the fact that the X chromosome has a characteristic shrinkage.