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Apparent monosomy 21 owing to a ring 21 chromosome: parental origin revealed by DNA analysis.
  1. R Dalgleish,
  2. D P Duckett,
  3. M Woodhouse,
  4. R S Shannon,
  5. I D Young
  1. Department of Genetics, University of Leicester.


    A three and a half year old mildly retarded boy is presented. Karyotyping showed monosomy 21 (45,XY,-21) in all 50 metaphase spreads examined from two lymphocyte cultures, and in 20% of cells examined from cultured fibroblasts; the remaining 80% of cells showed a ring 21 chromosome (46,XY,r(21)(p1q22]. Molecular studies using chromosome 21 specific DNA probes confirmed the monosomy in blood and showed that the ring 21 chromosome was paternal in origin. Parental karyotypes were normal.

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