DUPLICATION OF THE SHORT ARM OF THE X-CHROMOSOME IN MOTHER AND DAUGHTER

An 11-year-old girl with short stature, mental retardation, and mild d ysmorphic features was found to have an inverted duplication of most o f the short arm of the X chromosome [dic inv dup(X)(qter --> p22.3 = p 22.3 --> cen:)]. Her mother, who is also short and retarded, carries t he same duplication. Fluorescence in situ hybridization with an X chro mosome library, and with X centromere-specific alpha satellite and tel omere probes, was useful in characterizing the duplication. In most fe males with structurally abnormal X chromosomes, the abnormal chromosom e is inactivated. Although the duplicated X was consistently late repl icating in the mother, X chromosome inactivation studies in the proban d indicated that in 11 % of her lymphocytes the duplicated X was activ e.