Ring chromosome 21 in phenotypically apparently normal persons: report of two families from Switzerland and Italy.

If a ring 21, originating from breaks close to the telomere of 21q and anywhere in 21p, replaces a normal 21, it may be associated with an apparently normal phenotype. An apparently normal mother and son were ascertained by a prenatal chromosome study. A second mother, with a ring 21 but without gross anomalies, is short of stature, has epilepsy and has a low normal intelligence. He daughter is a mosaic: 46,XX/47,XX,+r(21) and has the Down's syndrome. None of these four persons was found to have mitoses with more than one ring 21 or with rings of double size.