Partial trisomy 21

Abstract

Five patients showing several stigmata of Down's syndrome and a partial trisomy of chromosome 21 are reported. Three patients with only a moderate degree of mental retardation had an additional deleted chromosome 21; the characteristic dark G‐band region of the long arm of 21 was missing. In another patient with a typical Down's syndrome the G‐band region and the distal portion of the long arm of 21 were present in excess and were translocated onto the short arm of 15. Clinical features suggestive of Down's syndrome were also seen in a mildly retarded girl with a mosaic chromosome constitution (one cell line had an isochromosome for the long arm of chromosome 21 and the other had a deletion of the short arm of chromosome 21). By correlating clinical and cytogenetic data on these patients, an attempt was made to analyse the phenotypic effects caused by the presence of excess amounts of different segments of chromosome 21.