Mosaic tetrasomy 21 in a male child.

We report a three-year-old male child with dysmorphy, severe mental retardation and a mosaic tetrasomy 21 caused by an extra F-like chromosome composed of two long arms of chromosome 21. We have compared this unusual case with two other similar published case reports. Hypotheses to account for the origin of the abnormal chromosome and the mosaicism are presented and discussed.