Fragile Syndrome X

The first family with fragile X syndrome was described by J. Purdon Martin and Julia Bell in 1943. This family had eleven severely retarded males, and the inheritance pattern of the mental retardation appeared to be X-linked. X-linked traits are inherited on the X chromosome and are more common in males, who have only one X chromosome, than in females, who have two.

In 1969, in a different family, Herbert Lubs observed a constriction near the end of the long, or q, arm of the X chromosome in four mentally retarded males and two of their mentally normal female relatives. This constriction made the X chromosome appear to be broken. Hence the name "fragile X."

For years, little attention was paid to Lubs's finding. Renewed interest in the observation emerged in the late 1970s, when additional families were identified with mental retardation and the same chromosome abnormality, or fragile site. Moreover, in 1977 Grant Sutherland discovered that the ability to detect this fragile site was dependent on the chemicals used to study patients' chromosomes. Sutherland's crucial observation helped develop the The Sherman Paradox. The daughter (III) of an unaffected male carrier (II) is more likely to have affected offspring than the mother (I) of the male carrier is. first diagnostic test for fragile X syndrome. Using this knowledge, investigators reexamined chromosomes from the original fragile X family described in 1943 and demonstrated that, indeed, affected individuals in this family carried the characteristic fragile site.