Fragile X syndrome, which is characterized by unstable portions of the X chromosome affects males twice as frequently as females (1 in 4000 for males, 1 in 8000 for females) because females have two copies of the X chromosome. Therefore, if there is an error on one copy of the gene for fragile X syndrome, normal genes on the other copy of the X chromosome that is present in females can be expressed. Symptoms include characteristic facial dysmorphology, mental retardation, and social impairments. Fragile X syndrome does not cause alterations in the phenotypical sex characteristics.