Mosaic trisomy 21/monosomy 21 in a living female infant.

Many autosomal monosomies are presumed to end in arrested growth in the first few mitoses, prior even to the time of implantation, with possibly some proceeding to the stage of occult abortion. The single exception may be monosomy 21, although this has been questioned, with most earlier reports of monosomy 21 recently re-interpreted as being due to an unbalanced translocation involving chromosome 21. Here we report a female infant with a mosaic trisomy 21/monosomy 21 karyotype. While the karyotype 46,XX,i(21)(q10) is detected in all metaphases investigated in lymphocytes, mosaicism with the karyotype 46,XX,i(21)(q10)[31]/45,XX, -21[12] is seen in fibroblasts from a skin biopsy. Dysmorphic facial features and multiple malformations remarkably resemble cases of monosomy 21 that have been described in the literature. This suggests a dominant phenotypic effect of loss of one chromosome 21. Detailed clinical description, results of gene dosage studies, and cytogenetic analysis will be presented.