Study of a patient with apparent monosomy 21 owing to translocation: 45,XX,21–,t(18q+)

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Abstract
A five-year-old girl with mild mental retardation and dyschondrosteosis was found to have 45 chromosomes. A G (21-22) chromosome was missing. The parents were chromosomally normal. Measurements confirmed the presence of an unusually long chromosome No. 17 or 18 in the girl’s karyotypes. Late labeling with 3H-thymidine and autoradiography showed it to be No. 18. By quinacrine fluoromicroscopy the apparently missing G chromosome proved to be No. 21. Thus, the apparent autosomal monosomy was the result of a de novo 18/21 translocation with loss of the short arm and centromere of No. 21 and of material from the long arms of No. 18 or No. 21 or both equivalent to two thirds of a G long arm. Although no evidence was found for deletion of a genetic marker, heterozygosity at five gene loci (MNSs, Jk, Hp, PGM, and AcP) excluded their deletion. We suggest that other instances of apparent autosomal monosomy may on careful study be found to be the result of similarly subtle chromosome rearrangements.

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