fragile X chromosome

fragile X chromosome An X chromosome that has a fragile site and is associated with a common form of mental retardation. Fragile X chromosome is due to a mutation (a trinucleotide repeat) in the DNA at that spot. Not all people who inherit the fragile site have the fragile X syndrome, and it is not yet known what genetic or environmental factors cause the breaks or deletions at this fragile site. Also known as FRAXA (as is fragile X syndrome). See also fragile X syndrome.