Fragile X-linked mental retardation of macro-orchidism.

A fragile site near the end of the long arm (q) of the X chromosome appears to be directly related to the gene responsible for the mental retardation found among males and some females who possess this variant X chromosome. Macro-orchidism is present in most males studied. Other mild phenotypic similarities may exist. The expression of the fragile X (fra[X]) chromosome is dependent on the concentration of folic acid and thymidine in the culture medium, which partly explains why fra(X) was not noted earlier and connected promptly with X-linked mental retardation. Whereas prenatal diagnosis is now possible, genetic counseling is complicated by recent reports of "intellectually normal" fra(X) males. Further studies are needed to form solid conclusions about intellectual deficits, learning behavior or personality characteristics of fra(X) persons.