FRAGILE-X SYNDROME

a chromosomal disorder associated with a fragile site on the end of the X chromosome. The major symptom of the syndrome is mental retardation. The X chromosome is one of two sex chromosomes (the other being the Y chromosome). Males have one X and one Y chromosome; females have two X chromosomes. The male who receives the fragile-X chromosome will be affected by the syndrome, which is thought to be one of the major causes of mental retardation in males. About one-third of the females who receive one fragile-X chromosome show mild mental retardation; the remaining two-thirds, though intellectually normal, have a 50-50 risk of passing the defective chromosome on to each of their children.